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Accessing mp3 Meta-Data with Python

<p>What is the best way to retrieve mp3 metadata in python? I've seen a couple frameworks out there, but I'm unsure as to which would be the best to use.... Any ideas?</p>

2008-08-12T15:16:00Z

<p>Just additional information to you guys:</p> <p>take a look at the section "MP3 stuff and Metadata editors" in the page of <a href="http://wiki.python.org/moin/PythonInMusic" rel="nofollow">PythonInMusic</a>.</p>

2011-07-08T08:24:29Z

Accessing mp3 Meta-Data with Python

<p>What is the best way to retrieve mp3 metadata in python? I've seen a couple frameworks out there, but I'm unsure as to which would be the best to use.... Any ideas?</p>

2008-08-12T15:16:00Z

<p>After some initial research I thought songdetails might fit my use case, but it doesn't handle .m4b files. Mutagen does. Note that while some have (reasonably) taken issue with Mutagen's surfacing of format-native keys, that vary from format to format (TIT2 for mp3, title for ogg, \xa9nam for mp4, Title for WMA etc.), mutagen.File() has a (new?) easy=True parameter that provides EasyMP3/EasyID3 tags, which have a consistent, albeit limited, set of keys. I've only done limited testing so far, but the common keys, like album, artist, albumartist, genre, tracknumber, discnumber, etc. are all present and identical for .mb4 and .mp3 files when using easy=True, making it very convenient for my purposes.</p>

2015-07-12T23:17:59Z

Accessing mp3 Meta-Data with Python

<p>What is the best way to retrieve mp3 metadata in python? I've seen a couple frameworks out there, but I'm unsure as to which would be the best to use.... Any ideas?</p>

2008-08-12T15:16:00Z

<p>A problem with <code>eyed3</code> is that it will throw <code>NotImplementedError("Unable to write ID3 v2.2")</code> for common MP3 files.</p> <p>In my experience, the <code>mutagen</code> class <code>EasyID3</code> works more reliably. Example:</p> <pre><code>from mutagen.easyid3 import EasyID3 audio = EasyID3("example.mp3") audio['title'] = u"Example Title" audio['artist'] = u"Me" audio['album'] = u"My album" audio['composer'] = u"" # clear audio.save() </code></pre> <p>All other tags can be accessed this way and saved, which will serve most purposes. More information can be found in the <a href="https://mutagen.readthedocs.org/en/latest/tutorial.html#easy-id3" rel="nofollow">Mutagen Tutorial</a>.</p>

2016-01-24T00:07:24Z

Accessing mp3 Meta-Data with Python

<p>What is the best way to retrieve mp3 metadata in python? I've seen a couple frameworks out there, but I'm unsure as to which would be the best to use.... Any ideas?</p>

2008-08-12T15:16:00Z

<p>easiest method is <a href="https://github.com/Ciantic/songdetails" rel="nofollow">songdetails</a>..</p> <p>for read data</p> <pre><code>import songdetails song = songdetails.scan("blah.mp3") if song is not None: print song.artist </code></pre> <p>similarly for edit</p> <pre><code>import songdetails song = songdetails.scan("blah.mp3") if song is not None: song.artist = u"The Great Blah" song.save() </code></pre> <p>Don't forget to add <strong>u</strong> before name until you know chinese language.</p> <p>u can read and edit in bulk using python glob module</p> <p>ex.</p> <pre><code>import glob songs = glob.glob('*') // script should be in directory of songs. for song in songs: // do the above work. </code></pre>

2016-08-11T09:05:38Z

How do I treat an integer as an array of bytes in Python?

<p>I'm trying to decode the result of the Python os.wait() function. This returns, according to the Python docs:</p> <blockquote> <p>a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.</p> </blockquote> <p>How do I decode the exit status indication (which is an integer) to obtain the high and low byte? To be specific, how do I implement the decode function used in the following code snippet:</p> <pre><code>(pid,status) = os.wait() (exitstatus, signum) = decode(status) </code></pre>

2008-08-13T17:46:41Z

<p>This will do what you want:</p> <pre><code>signum = status &amp; 0xff exitstatus = (status &amp; 0xff00) &gt;&gt; 8 </code></pre>

2008-08-13T17:56:34Z

How do I treat an integer as an array of bytes in Python?

<p>I'm trying to decode the result of the Python os.wait() function. This returns, according to the Python docs:</p> <blockquote> <p>a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.</p> </blockquote> <p>How do I decode the exit status indication (which is an integer) to obtain the high and low byte? To be specific, how do I implement the decode function used in the following code snippet:</p> <pre><code>(pid,status) = os.wait() (exitstatus, signum) = decode(status) </code></pre>

2008-08-13T17:46:41Z

<p>You can unpack the status using <a href="http://docs.python.org/ref/shifting.html" rel="nofollow">bit-shifting</a> and <a href="http://docs.python.org/ref/bitwise.html" rel="nofollow">masking</a> operators.</p> <pre><code>low = status &amp; 0x00FF high = (status &amp; 0xFF00) &gt;&gt; 8 </code></pre> <p>I'm not a Python programmer, so I hope got the syntax correct.</p>

2008-08-13T17:58:27Z

How do I treat an integer as an array of bytes in Python?

<p>I'm trying to decode the result of the Python os.wait() function. This returns, according to the Python docs:</p> <blockquote> <p>a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.</p> </blockquote> <p>How do I decode the exit status indication (which is an integer) to obtain the high and low byte? To be specific, how do I implement the decode function used in the following code snippet:</p> <pre><code>(pid,status) = os.wait() (exitstatus, signum) = decode(status) </code></pre>

2008-08-13T17:46:41Z

<p>The folks before me've nailed it, but if you really want it on one line, you can do this:</p> <pre><code>(signum, exitstatus) = (status &amp; 0xFF, (status &gt;&gt; 8) &amp; 0xFF) </code></pre> <p>EDIT: Had it backwards.</p>

2008-08-13T18:14:10Z

How do I treat an integer as an array of bytes in Python?

<p>I'm trying to decode the result of the Python os.wait() function. This returns, according to the Python docs:</p> <blockquote> <p>a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.</p> </blockquote> <p>How do I decode the exit status indication (which is an integer) to obtain the high and low byte? To be specific, how do I implement the decode function used in the following code snippet:</p> <pre><code>(pid,status) = os.wait() (exitstatus, signum) = decode(status) </code></pre>

2008-08-13T17:46:41Z

<p>To answer your general question, you can use <a href="http://en.wikipedia.org/wiki/Bit_twiddling" rel="nofollow">bit manipulation</a> techniques:</p> <pre><code>pid, status = os.wait() exitstatus, signum = status &amp; 0xFF, (status &amp; 0xFF00) &gt;&gt; 8 </code></pre> <p>However, there are also <a href="http://docs.python.org/lib/os-process.html#l2h-2780" rel="nofollow">built-in functions</a> for interpreting exit status values:</p> <pre><code>pid, status = os.wait() exitstatus, signum = os.WEXITSTATUS( status ), os.WTERMSIG( status ) </code></pre> <p>See also:</p> <ul> <li>os.WCOREDUMP()</li> <li>os.WIFCONTINUED()</li> <li>os.WIFSTOPPED()</li> <li>os.WIFSIGNALED()</li> <li>os.WIFEXITED()</li> <li>os.WSTOPSIG()</li> </ul>

2008-08-13T18:52:36Z

How do I treat an integer as an array of bytes in Python?

<p>I'm trying to decode the result of the Python os.wait() function. This returns, according to the Python docs:</p> <blockquote> <p>a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.</p> </blockquote> <p>How do I decode the exit status indication (which is an integer) to obtain the high and low byte? To be specific, how do I implement the decode function used in the following code snippet:</p> <pre><code>(pid,status) = os.wait() (exitstatus, signum) = decode(status) </code></pre>

2008-08-13T17:46:41Z

<p>You can get break your int into a string of unsigned bytes with the <a href="http://docs.python.org/library/struct.html" rel="nofollow">struct</a> module:</p> <pre><code>import struct i = 3235830701 # 0xC0DEDBAD s = struct.pack("&gt;L", i) # "&gt;" = Big-endian, "&lt;" = Little-endian print s # '\xc0\xde\xdb\xad' print s[0] # '\xc0' print ord(s[0]) # 192 (which is 0xC0) </code></pre> <p>If you couple this with the <a href="http://docs.python.org/library/array.html" rel="nofollow">array</a> module you can do this more conveniently:</p> <pre><code>import struct i = 3235830701 # 0xC0DEDBAD s = struct.pack("&gt;L", i) # "&gt;" = Big-endian, "&lt;" = Little-endian import array a = array.array("B") # B: Unsigned bytes a.fromstring(s) print a # array('B', [192, 222, 219, 173]) </code></pre>

2009-03-26T14:52:42Z

How do I treat an integer as an array of bytes in Python?

<p>I'm trying to decode the result of the Python os.wait() function. This returns, according to the Python docs:</p> <blockquote> <p>a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.</p> </blockquote> <p>How do I decode the exit status indication (which is an integer) to obtain the high and low byte? To be specific, how do I implement the decode function used in the following code snippet:</p> <pre><code>(pid,status) = os.wait() (exitstatus, signum) = decode(status) </code></pre>

2008-08-13T17:46:41Z

<pre><code>exitstatus, signum= divmod(status, 256) </code></pre>

2009-03-26T23:48:20Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>I started writing up a summary of my experience with my own code generator, then went back and re-read your question and found you had already touched upon the same issues yourself, focus on the execution results instead of the code layout/look.</p> <p>Problem is, this is hard to test, the generated code might not be suited to actually run in the environment of the unit test system, and how do you encode the expected results?</p> <p>I've found that you need to break down the code generator into smaller pieces and unit test those. Unit testing a full code generator is more like integration testing than unit testing if you ask me.</p>

2008-08-14T14:04:25Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>Yes, results are the ONLY thing that matters. The real chore is writing a framework that allows your generated code to run independently... spend your time there.</p>

2008-08-14T14:38:19Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>If you are running on *nux you might consider dumping the unittest framework in favor of a bash script or makefile. on windows you might consider building a shell app/function that runs the generator and then uses the code (as another process) and unittest that.</p> <p>A third option would be to generate the code and then build an app from it that includes nothing but a unittest. Again you would need a shell script or whatnot to run this for each input. As to how to encode the expected behavior, it occurs to me that it could be done in much the same way as you would for the C++ code just using the generated interface rather than the C++ one.</p>

2008-08-14T15:46:01Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>Recall that "unit testing" is only one kind of testing. You should be able to unit test the <strong>internal</strong> pieces of your code generator. What you're really looking at here is system level testing (a.k.a. regression testing). It's not just semantics... there are different mindsets, approaches, expectations, etc. It's certainly more work, but you probably need to bite the bullet and set up an end-to-end regression test suite: fixed C++ files -> SWIG interfaces -> python modules -> known output. You really want to check the known input (fixed C++ code) against expected output (what comes out of the final Python program). Checking the code generator results directly would be like diffing object files...</p>

2008-08-14T18:15:42Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>Just wanted to point out that you can still achieve fine-grained testing while verifying the results: you can test individual chunks of code by nesting them inside some setup and verification code:</p> <pre><code>int x = 0; GENERATED_CODE assert(x == 100); </code></pre> <p>Provided you have your generated code assembled from smaller chunks, and the chunks do not change frequently, you can exercise more conditions and test a little better, and hopefully avoid having all your tests break when you change specifics of one chunk.</p>

2008-09-16T09:41:03Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>Unit testing is just that testing a specific unit. So if you are writing a specification for class A, it is ideal if class A does not have the real concrete versions of class B and C.</p> <p>Ok I noticed afterwards the tag for this question includes C++ / Python, but the principles are the same:</p> <pre><code> public class A : InterfaceA { InterfaceB b; InterfaceC c; public A(InterfaceB b, InterfaceC c) { this._b = b; this._c = c; } public string SomeOperation(string input) { return this._b.SomeOtherOperation(input) + this._c.EvenAnotherOperation(input); } } </code></pre> <p>Because the above System A injects interfaces to systems B and C, you can unit test just system A, without having real functionality being executed by any other system. This is unit testing.</p> <p>Here is a clever manner for approaching a System from creation to completion, with a different When specification for each piece of behaviour:</p> <pre><code>public class When_system_A_has_some_operation_called_with_valid_input : SystemASpecification { private string _actualString; private string _expectedString; private string _input; private string _returnB; private string _returnC; [It] public void Should_return_the_expected_string() { _actualString.Should().Be.EqualTo(this._expectedString); } public override void GivenThat() { var randomGenerator = new RandomGenerator(); this._input = randomGenerator.Generate&lt;string&gt;(); this._returnB = randomGenerator.Generate&lt;string&gt;(); this._returnC = randomGenerator.Generate&lt;string&gt;(); Dep&lt;InterfaceB&gt;().Stub(b =&gt; b.SomeOtherOperation(_input)) .Return(this._returnB); Dep&lt;InterfaceC&gt;().Stub(c =&gt; c.EvenAnotherOperation(_input)) .Return(this._returnC); this._expectedString = this._returnB + this._returnC; } public override void WhenIRun() { this._actualString = Sut.SomeOperation(this._input); } } </code></pre> <p>So in conclusion, a single unit / specification can have multiple behaviours, and the specification grows as you develop the unit / system; and if your system under test depends on other concrete systems within it, watch out.</p>

2010-05-19T23:17:11Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>My recommendation would be to figure out a set of known input-output results, such as some simpler cases that you already have in place, and <em>unit test the code that is produced</em>. It's entirely possible that as you change the generator that the exact string that is produced may be slightly different... but what you really care is whether it is interpreted in the same way. Thus, if you test the results as you would test that code if it were your feature, you will find out if it succeeds in the ways you want.</p> <p>Basically, what you really want to know is whether your generator will produce what you expect without physically testing every possible combination (also: impossible). By ensuring that your generator is consistent in the ways you expect, you can feel better that the generator will succeed in ever-more-complex situations.</p> <p>In this way, you can also build up a suite of regression tests (unit tests that need to keep working correctly). This will help you make sure that changes to your generator aren't breaking other forms of code. When you encounter a bug that your unit tests didn't catch, you may want to include it to prevent similar breakage.</p>

2010-07-25T23:56:06Z

How should I unit test a code-generator?

<p>This is a difficult and open-ended question I know, but I thought I'd throw it to the floor and see if anyone had any interesting suggestions.</p> <p>I have developed a code-generator that takes our python interface to our C++ code (generated via SWIG) and generates code needed to expose this as WebServices. When I developed this code I did it using TDD, but I've found my tests to be brittle as hell. Because each test essentially wanted to verify that for a given bit of input code (which happens to be a C++ header) I'd get a given bit of outputted code I wrote a small engine that reads test definitions from XML input files and generates test cases from these expectations.</p> <p>The problem is I dread going in to modify the code at all. That and the fact that the unit tests themselves are a: complex, and b: brittle.</p> <p>So I'm trying to think of alternative approaches to this problem, and it strikes me I'm perhaps tackling it the wrong way. Maybe I need to focus more on the outcome, IE: does the code I generate actually run and do what I want it to, rather than, does the code look the way I want it to.</p> <p>Has anyone got any experiences of something similar to this they would care to share?</p>

2008-08-14T13:59:21Z

<p>I find that you need to test what you're generating more than how you generate it.</p> <p>In my case, the program generates many types of code (C#, HTML, SCSS, JS, etc.) that compile into a web application. The best way I've found to reduce regression bugs overall is to test the web application itself, not by testing the generator. </p> <p>Don't get me wrong, there are still unit tests checking out some of the generator code, but our biggest bang for our buck has been UI tests on the generated app itself.</p> <p>Since we're generating it we also generate a nice abstraction in JS we can use to programatically test the app. We followed some ideas outlined here: <a href="http://code.tutsplus.com/articles/maintainable-automated-ui-tests--net-35089" rel="nofollow">http://code.tutsplus.com/articles/maintainable-automated-ui-tests--net-35089</a></p> <p>The great part is that it really tests your system end-to-end, from code generation out to what you're actually generating. Once a test fails, its easy to track it back to where the generator broke.</p> <p>It's pretty sweet.</p> <p>Good luck!</p>

2015-07-20T04:10:25Z

Using an XML catalog with Python's lxml?

<p>Is there a way, when I parse an XML document using lxml, to validate that document against its DTD using an external catalog file? I need to be able to work the fixed attributes defined in a document’s DTD.</p>

2008-08-15T18:42:20Z

<p>Can you give an example? According to the <a href="http://codespeak.net/lxml/validation.html" rel="nofollow">lxml validation docs</a>, lxml can handle DTD validation (specified in the XML doc or externally in code) and system catalogs, which covers most cases I can think of.</p> <pre><code>f = StringIO("&lt;!ELEMENT b EMPTY&gt;") dtd = etree.DTD(f) dtd = etree.DTD(external_id = "-//OASIS//DTD DocBook XML V4.2//EN") </code></pre>

2008-08-16T07:57:53Z

Using an XML catalog with Python's lxml?

<p>Is there a way, when I parse an XML document using lxml, to validate that document against its DTD using an external catalog file? I need to be able to work the fixed attributes defined in a document’s DTD.</p>

2008-08-15T18:42:20Z

<p>It seems that lxml does not expose this libxml2 feature, grepping the source only turns up some #defines for the error handling:</p> <pre><code>C:\Dev&gt;grep -ir --include=*.px[id] catalog lxml-2.1.1/src | sed -r "s/\s+/ /g" lxml-2.1.1/src/lxml/dtd.pxi: catalog. lxml-2.1.1/src/lxml/xmlerror.pxd: XML_FROM_CATALOG = 20 # The Catalog module lxml-2.1.1/src/lxml/xmlerror.pxd: XML_WAR_CATALOG_PI = 93 # 93 lxml-2.1.1/src/lxml/xmlerror.pxd: XML_CATALOG_MISSING_ATTR = 1650 lxml-2.1.1/src/lxml/xmlerror.pxd: XML_CATALOG_ENTRY_BROKEN = 1651 # 1651 lxml-2.1.1/src/lxml/xmlerror.pxd: XML_CATALOG_PREFER_VALUE = 1652 # 1652 lxml-2.1.1/src/lxml/xmlerror.pxd: XML_CATALOG_NOT_CATALOG = 1653 # 1653 lxml-2.1.1/src/lxml/xmlerror.pxd: XML_CATALOG_RECURSION = 1654 # 1654 lxml-2.1.1/src/lxml/xmlerror.pxi:CATALOG=20 lxml-2.1.1/src/lxml/xmlerror.pxi:WAR_CATALOG_PI=93 lxml-2.1.1/src/lxml/xmlerror.pxi:CATALOG_MISSING_ATTR=1650 lxml-2.1.1/src/lxml/xmlerror.pxi:CATALOG_ENTRY_BROKEN=1651 lxml-2.1.1/src/lxml/xmlerror.pxi:CATALOG_PREFER_VALUE=1652 lxml-2.1.1/src/lxml/xmlerror.pxi:CATALOG_NOT_CATALOG=1653 lxml-2.1.1/src/lxml/xmlerror.pxi:CATALOG_RECURSION=1654 </code></pre> <p>From the <a href="http://xmlsoft.org/catalog.html" rel="nofollow">catalog implementation in libxml2 page</a> it seems possible that the 'transparent' handling through installation in /etc/xml/catalog may still work in lxml, but if you need more than that you can always abandon lxml and use the default python bindings, which do expose the catalog functions.</p>

2008-08-30T18:10:33Z

Using an XML catalog with Python's lxml?

<p>Is there a way, when I parse an XML document using lxml, to validate that document against its DTD using an external catalog file? I need to be able to work the fixed attributes defined in a document’s DTD.</p>

2008-08-15T18:42:20Z

<p>You can add the catalog to the <code>XML_CATALOG_FILES</code> environment variable:</p> <pre><code>os.environ['XML_CATALOG_FILES'] = 'file:///to/my/catalog.xml' </code></pre> <p>See <a href="http://thread.gmane.org/gmane.comp.python.lxml.devel/5907" rel="nofollow">this thread</a>. Note that entries in <code>XML_CATALOG_FILES</code> are space-separated URLs. You can use Python's <code>pathname2url</code> and <code>urljoin</code> (with <code>file:</code>) to generate the URL from a pathname.</p>

2011-12-05T20:54:36Z

Can you check that an exception is thrown with doctest in Python?

<p>Is it possible to write a doctest unit test that will check that an exception is raised?<br> For example, if I have a function <code>foo(x)</code> that is supposed to raise an exception if <code>x&lt;0</code>, how would I write the doctest for that? </p>

2008-08-15T18:43:17Z

<p>Yes. You can do it. The <a href="https://docs.python.org/3/library/doctest.html">doctest module documentation</a> and Wikipedia has an <a href="http://en.wikipedia.org/wiki/Doctest#Example_2%3a_doctests_embedded_in_a_README.txt_file">example</a> of it.</p> <pre><code> &gt;&gt;&gt; x Traceback (most recent call last): ... NameError: name 'x' is not defined </code></pre>

2008-08-15T19:04:13Z

Can you check that an exception is thrown with doctest in Python?

<p>Is it possible to write a doctest unit test that will check that an exception is raised?<br> For example, if I have a function <code>foo(x)</code> that is supposed to raise an exception if <code>x&lt;0</code>, how would I write the doctest for that? </p>

2008-08-15T18:43:17Z

<pre><code>&gt;&gt;&gt; import math &gt;&gt;&gt; math.log(-2) Traceback (most recent call last): ... ValueError: math domain error </code></pre> <p>ellipsis flag <em># doctest: +ELLIPSIS</em> is not required to use ... in Traceback doctest </p>

2015-11-18T18:22:25Z

Replacement for for... if array iteration

<p>I love list comprehensions in Python, because they concisely represent a transformation of a list.</p> <p>However, in other languages, I frequently find myself writing something along the lines of:</p> <pre><code>foreach (int x in intArray) if (x &gt; 3) //generic condition on x x++ //do other processing </code></pre> <p>This example is in C#, where I'm under the impression LINQ can help with this, but is there some common programming construct which can replace this slightly less-than-elegant solution? Perhaps a data structure I'm not considering?</p>

2008-08-16T22:28:39Z

<p>Depends on the language and what you need to do, a "map" as it's called in many languages could be what you're looking for. I don't know C#, but according to <a href="http://blogs.msdn.com/devdev/archive/2006/06/30/652802.aspx" rel="nofollow">this</a> page, .NET 2.0 calls map "ConvertAll".</p> <p>The meaning of "map" is pretty simple - take a list, and apply a function to each element of it, returning a new list. You may also be looking for "filter", which would give you a list of items that satisfy a predicate in another list.</p>

2008-08-16T22:37:06Z

Replacement for for... if array iteration

<p>I love list comprehensions in Python, because they concisely represent a transformation of a list.</p> <p>However, in other languages, I frequently find myself writing something along the lines of:</p> <pre><code>foreach (int x in intArray) if (x &gt; 3) //generic condition on x x++ //do other processing </code></pre> <p>This example is in C#, where I'm under the impression LINQ can help with this, but is there some common programming construct which can replace this slightly less-than-elegant solution? Perhaps a data structure I'm not considering?</p>

2008-08-16T22:28:39Z

<p>in Ruby:</p> <pre><code>intArray.select { |x| x &gt; 3 }.each do |x| # do other processing end </code></pre> <p>or if "other processing" is a short one-liner:</p> <pre><code>intArray.select { |x| x &gt; 3 }.each { |x| something_that_uses x } </code></pre> <p>lastly, if you want to return a new array containing the results of the processing of those elements greater than 3:</p> <pre><code>intArray.select { |x| x &gt; 3 }.map { |x| do_something_to x } </code></pre>

2008-08-16T22:47:53Z

Replacement for for... if array iteration

<p>I love list comprehensions in Python, because they concisely represent a transformation of a list.</p> <p>However, in other languages, I frequently find myself writing something along the lines of:</p> <pre><code>foreach (int x in intArray) if (x &gt; 3) //generic condition on x x++ //do other processing </code></pre> <p>This example is in C#, where I'm under the impression LINQ can help with this, but is there some common programming construct which can replace this slightly less-than-elegant solution? Perhaps a data structure I'm not considering?</p>

2008-08-16T22:28:39Z

<p>In Python, you have <a href="http://docs.python.org/tut/node7.html#SECTION007130000000000000000" rel="nofollow" title="Human Interface Guidelines">filter and map</a>, which can so what you want:</p> <pre><code>map(lambda x: foo(x + 1) filter(lambda x: x &gt; 3, intArray)) </code></pre> <p>There's also <a href="http://docs.python.org/tut/node7.html#SECTION007140000000000000000" rel="nofollow">list comprehensions</a> which can do both in one easy statement:</p> <pre><code>[f(x + 1) for x in intArray if x &gt; 3] </code></pre>

2008-08-16T22:56:30Z

Replacement for for... if array iteration

<p>I love list comprehensions in Python, because they concisely represent a transformation of a list.</p> <p>However, in other languages, I frequently find myself writing something along the lines of:</p> <pre><code>foreach (int x in intArray) if (x &gt; 3) //generic condition on x x++ //do other processing </code></pre> <p>This example is in C#, where I'm under the impression LINQ can help with this, but is there some common programming construct which can replace this slightly less-than-elegant solution? Perhaps a data structure I'm not considering?</p>

2008-08-16T22:28:39Z

<p>In C# you can apply selective processing on anything that lives inside an IEnumerable like this: </p> <pre><code>intArray.Where(i =&gt; i &gt; 3).ConvertAll(); DoStuff(intArray.Where(i =&gt; i 3)); </code></pre> <p>Etc..</p>

2008-08-16T23:06:52Z

Replacement for for... if array iteration

<p>I love list comprehensions in Python, because they concisely represent a transformation of a list.</p> <p>However, in other languages, I frequently find myself writing something along the lines of:</p> <pre><code>foreach (int x in intArray) if (x &gt; 3) //generic condition on x x++ //do other processing </code></pre> <p>This example is in C#, where I'm under the impression LINQ can help with this, but is there some common programming construct which can replace this slightly less-than-elegant solution? Perhaps a data structure I'm not considering?</p>

2008-08-16T22:28:39Z

<p>The increment in the original <code>foreach</code> loop will not affect the contents of the array, the only way to do this remains a <code>for</code> loop:</p> <pre><code>for(int i = 0; i &lt; intArray.Length; ++i) { if(intArray[i] &gt; 3) ++intArray[i]; } </code></pre> <p>Linq is not intended to modify existing collections or sequences. It creates new sequences based on existing ones. It is possible to achieve the above code using Linq, though it is slightly against its purposes:</p> <pre><code>var newArray1 = from i in intArray select ((i &gt; 3) ? (i + 1) : (i)); var newArray2 = intArray.Select(i =&gt; (i &gt; 3) ? (i + 1) : (i)); </code></pre> <p>Using <code>where</code> (or equivalent), as shown in some of the other answers, will exclude any values less than or equal to 3 from the resulting sequence.</p> <pre><code>var intArray = new int[] { 10, 1, 20, 2 }; var newArray = from i in intArray where i &gt; 3 select i + 1; // newArray == { 11, 21 } </code></pre> <p>There is a <code>ForEach</code> method on arrays that will allow you to use a lambda function instead of a <code>foreach</code> block, though for anything more than a method call I would stick with <code>foreach</code>.</p> <pre><code>intArray.ForEach(i =&gt; DoSomething(i)); </code></pre>

2008-08-17T00:44:57Z

Replacement for for... if array iteration

<p>I love list comprehensions in Python, because they concisely represent a transformation of a list.</p> <p>However, in other languages, I frequently find myself writing something along the lines of:</p> <pre><code>foreach (int x in intArray) if (x &gt; 3) //generic condition on x x++ //do other processing </code></pre> <p>This example is in C#, where I'm under the impression LINQ can help with this, but is there some common programming construct which can replace this slightly less-than-elegant solution? Perhaps a data structure I'm not considering?</p>

2008-08-16T22:28:39Z

<pre><code>map(lambda x: test(x + 1) filter(lambda x: x &gt; 3, arr)) </code></pre>

2011-06-25T11:28:54Z

Python version of PHP's stripslashes

<p>I wrote a piece of code to convert PHP's striplashes into valid Python [backslash] escapes:</p> <pre><code>cleaned = stringwithslashes cleaned = cleaned.replace('\\n', '\n') cleaned = cleaned.replace('\\r', '\n') cleaned = cleaned.replace('\\', '') </code></pre> <p>How can I condense it? </p>

2008-08-17T01:23:50Z

<p>You can obviously concatenate everything together:</p> <pre><code>cleaned = stringwithslashes.replace("\\n","\n").replace("\\r","\n").replace("\\","") </code></pre> <p>Is that what you were after? Or were you hoping for something more terse?</p>

2008-08-17T01:26:52Z

Python version of PHP's stripslashes

<p>I wrote a piece of code to convert PHP's striplashes into valid Python [backslash] escapes:</p> <pre><code>cleaned = stringwithslashes cleaned = cleaned.replace('\\n', '\n') cleaned = cleaned.replace('\\r', '\n') cleaned = cleaned.replace('\\', '') </code></pre> <p>How can I condense it? </p>

2008-08-17T01:23:50Z

<p>Python has a built-in escape() function analogous to PHP's addslashes, but no unescape() function (stripslashes), which in my mind is kind of ridiculous.</p> <p>Regular expressions to the rescue (code not tested):</p> <pre><code>p = re.compile( '\\(\\\S)') p.sub('\1',escapedstring) </code></pre> <p>In theory that takes anything of the form \\(not whitespace) and returns \(same char)</p> <p>edit: Upon further inspection, Python regular expressions are broken as all hell;</p> <pre><code>&gt;&gt;&gt; escapedstring 'This is a \\n\\n\\n test' &gt;&gt;&gt; p = re.compile( r'\\(\S)' ) &gt;&gt;&gt; p.sub(r"\1",escapedstring) 'This is a nnn test' &gt;&gt;&gt; p.sub(r"\\1",escapedstring) 'This is a \\1\\1\\1 test' &gt;&gt;&gt; p.sub(r"\\\1",escapedstring) 'This is a \\n\\n\\n test' &gt;&gt;&gt; p.sub(r"\(\1)",escapedstring) 'This is a \\(n)\\(n)\\(n) test' </code></pre> <p>In conclusion, what the hell, Python.</p>

2008-08-17T10:28:00Z

Python version of PHP's stripslashes

<p>I wrote a piece of code to convert PHP's striplashes into valid Python [backslash] escapes:</p> <pre><code>cleaned = stringwithslashes cleaned = cleaned.replace('\\n', '\n') cleaned = cleaned.replace('\\r', '\n') cleaned = cleaned.replace('\\', '') </code></pre> <p>How can I condense it? </p>

2008-08-17T01:23:50Z

<p>Not totally sure this is what you want, but..</p> <pre><code>cleaned = stringwithslashes.decode('string_escape') </code></pre>

2008-08-17T12:15:13Z

Python version of PHP's stripslashes

<p>I wrote a piece of code to convert PHP's striplashes into valid Python [backslash] escapes:</p> <pre><code>cleaned = stringwithslashes cleaned = cleaned.replace('\\n', '\n') cleaned = cleaned.replace('\\r', '\n') cleaned = cleaned.replace('\\', '') </code></pre> <p>How can I condense it? </p>

2008-08-17T01:23:50Z

<p>It sounds like what you want could be reasonably efficiently handled through regular expressions:</p> <pre><code>import re def stripslashes(s): r = re.sub(r"\\(n|r)", "\n", s) r = re.sub(r"\\", "", r) return r cleaned = stripslashes(stringwithslashes) </code></pre>

2008-08-17T12:55:25Z

Python version of PHP's stripslashes

<p>I wrote a piece of code to convert PHP's striplashes into valid Python [backslash] escapes:</p> <pre><code>cleaned = stringwithslashes cleaned = cleaned.replace('\\n', '\n') cleaned = cleaned.replace('\\r', '\n') cleaned = cleaned.replace('\\', '') </code></pre> <p>How can I condense it? </p>

2008-08-17T01:23:50Z

<p>use <code>decode('string_escape')</code></p> <pre><code>cleaned = stringwithslashes.decode('string_escape') </code></pre> <p>Using </p> <blockquote> <p><strong>string_escape</strong> : Produce a string that is suitable as string literal in Python source code</p> </blockquote> <p>or concatenate the replace() like Wilson´s answer.</p> <pre><code>cleaned = stringwithslashes.replace("\\","").replace("\\n","\n").replace("\\r","\n") </code></pre>

2014-02-18T19:02:19Z

Python Regular Expressions to implement string unescaping

<p>I'm trying to implement string unescaping with Python regex and backreferences, and it doesn't seem to want to work very well. I'm sure it's something I'm doing wrong but I can't figure out what...</p> <pre><code>&gt;&gt;&gt; import re &gt;&gt;&gt; mystring = r"This is \n a test \r" &gt;&gt;&gt; p = re.compile( "\\\\(\\S)" ) &gt;&gt;&gt; p.sub( "\\1", mystring ) 'This is n a test r' &gt;&gt;&gt; p.sub( "\\\\\\1", mystring ) 'This is \\n a test \\r' &gt;&gt;&gt; p.sub( "\\\\1", mystring ) 'This is \\1 a test \\1' </code></pre> <p>I'd like to replace \\[char] with \[char], but backreferences in Python don't appear to follow the same rules they do in every other implementation I've ever used. Could someone shed some light?</p>

2008-08-17T18:20:21Z

<p>Well, I think you might have missed the r or miscounted the backslashes...</p> <pre><code>"\\n" == r"\n" &gt;&gt;&gt; import re &gt;&gt;&gt; mystring = r"This is \\n a test \\r" &gt;&gt;&gt; p = re.compile( r"[\\][\\](.)" ) &gt;&gt;&gt; print p.sub( r"\\\1", mystring ) This is \n a test \r &gt;&gt;&gt; </code></pre> <p>Which, if I understood is what was requested.</p> <p>I suspect the more common request is this:</p> <pre><code>&gt;&gt;&gt; d = {'n':'\n', 'r':'\r', 'f':'\f'} &gt;&gt;&gt; p = re.compile(r"[\\]([nrfv])") &gt;&gt;&gt; print p.sub(lambda mo: d[mo.group(1)], mystring) This is \ a test \ &gt;&gt;&gt; </code></pre> <p>The interested student should also read Ken Thompson's <a href="http://cm.bell-labs.com/who/ken/trust.html" rel="nofollow" title="Cherry G80-11900">Reflections on Trusting Trust"</a>, wherein our hero uses a similar example to explain the perils of trusting compilers you haven't bootstrapped from machine code yourself.</p>

2008-08-17T19:01:02Z

Python Regular Expressions to implement string unescaping

<p>I'm trying to implement string unescaping with Python regex and backreferences, and it doesn't seem to want to work very well. I'm sure it's something I'm doing wrong but I can't figure out what...</p> <pre><code>&gt;&gt;&gt; import re &gt;&gt;&gt; mystring = r"This is \n a test \r" &gt;&gt;&gt; p = re.compile( "\\\\(\\S)" ) &gt;&gt;&gt; p.sub( "\\1", mystring ) 'This is n a test r' &gt;&gt;&gt; p.sub( "\\\\\\1", mystring ) 'This is \\n a test \\r' &gt;&gt;&gt; p.sub( "\\\\1", mystring ) 'This is \\1 a test \\1' </code></pre> <p>I'd like to replace \\[char] with \[char], but backreferences in Python don't appear to follow the same rules they do in every other implementation I've ever used. Could someone shed some light?</p>

2008-08-17T18:20:21Z

<p>You are being tricked by Python's representation of the result string. The Python expression:</p> <pre><code>'This is \\n a test \\r' </code></pre> <p>represents the string</p> <pre><code>This is \n a test \r </code></pre> <p>which is I think what you wanted. Try adding 'print' in front of each of your p.sub() calls to print the actual string returned instead of a Python representation of the string.</p> <pre><code>&gt;&gt;&gt; mystring = r"This is \n a test \r" &gt;&gt;&gt; mystring 'This is \\n a test \\r' &gt;&gt;&gt; print mystring This is \n a test \r </code></pre>

2008-08-17T19:26:33Z

Python Regular Expressions to implement string unescaping

<p>I'm trying to implement string unescaping with Python regex and backreferences, and it doesn't seem to want to work very well. I'm sure it's something I'm doing wrong but I can't figure out what...</p> <pre><code>&gt;&gt;&gt; import re &gt;&gt;&gt; mystring = r"This is \n a test \r" &gt;&gt;&gt; p = re.compile( "\\\\(\\S)" ) &gt;&gt;&gt; p.sub( "\\1", mystring ) 'This is n a test r' &gt;&gt;&gt; p.sub( "\\\\\\1", mystring ) 'This is \\n a test \\r' &gt;&gt;&gt; p.sub( "\\\\1", mystring ) 'This is \\1 a test \\1' </code></pre> <p>I'd like to replace \\[char] with \[char], but backreferences in Python don't appear to follow the same rules they do in every other implementation I've ever used. Could someone shed some light?</p>

2008-08-17T18:20:21Z

<p>The idea is that I'll read in an escaped string, and unescape it (a feature notably lacking from Python, which you shouldn't need to resort to regular expressions for in the first place). Unfortunately I'm not being tricked by the backslashes...</p> <p>Another illustrative example:</p> <pre><code>&gt;&gt;&gt; mystring = r"This is \n ridiculous" &gt;&gt;&gt; print mystring This is \n ridiculous &gt;&gt;&gt; p = re.compile( r"\\(\S)" ) &gt;&gt;&gt; print p.sub( 'bloody', mystring ) This is bloody ridiculous &gt;&gt;&gt; print p.sub( r'\1', mystring ) This is n ridiculous &gt;&gt;&gt; print p.sub( r'\\1', mystring ) This is \1 ridiculous &gt;&gt;&gt; print p.sub( r'\\\1', mystring ) This is \n ridiculous </code></pre> <p>What I'd like it to print is</p> <pre><code>This is ridiculous </code></pre>

2008-08-17T19:40:49Z

Python Regular Expressions to implement string unescaping

<p>I'm trying to implement string unescaping with Python regex and backreferences, and it doesn't seem to want to work very well. I'm sure it's something I'm doing wrong but I can't figure out what...</p> <pre><code>&gt;&gt;&gt; import re &gt;&gt;&gt; mystring = r"This is \n a test \r" &gt;&gt;&gt; p = re.compile( "\\\\(\\S)" ) &gt;&gt;&gt; p.sub( "\\1", mystring ) 'This is n a test r' &gt;&gt;&gt; p.sub( "\\\\\\1", mystring ) 'This is \\n a test \\r' &gt;&gt;&gt; p.sub( "\\\\1", mystring ) 'This is \\1 a test \\1' </code></pre> <p>I'd like to replace \\[char] with \[char], but backreferences in Python don't appear to follow the same rules they do in every other implementation I've ever used. Could someone shed some light?</p>

2008-08-17T18:20:21Z

<p>Isn't that what <a href="#13844">Anders' second example</a> does?</p> <p>In 2.5 there's also a <code>string-escape</code> encoding you can apply:</p> <pre><code>&gt;&gt;&gt; mystring = r"This is \n a test \r" &gt;&gt;&gt; mystring.decode('string-escape') 'This is \n a test \r' &gt;&gt;&gt; print mystring.decode('string-escape') This is a test &gt;&gt;&gt; </code></pre>

2008-08-17T21:36:32Z

Python Regular Expressions to implement string unescaping

<p>I'm trying to implement string unescaping with Python regex and backreferences, and it doesn't seem to want to work very well. I'm sure it's something I'm doing wrong but I can't figure out what...</p> <pre><code>&gt;&gt;&gt; import re &gt;&gt;&gt; mystring = r"This is \n a test \r" &gt;&gt;&gt; p = re.compile( "\\\\(\\S)" ) &gt;&gt;&gt; p.sub( "\\1", mystring ) 'This is n a test r' &gt;&gt;&gt; p.sub( "\\\\\\1", mystring ) 'This is \\n a test \\r' &gt;&gt;&gt; p.sub( "\\\\1", mystring ) 'This is \\1 a test \\1' </code></pre> <p>I'd like to replace \\[char] with \[char], but backreferences in Python don't appear to follow the same rules they do in every other implementation I've ever used. Could someone shed some light?</p>

2008-08-17T18:20:21Z

<p>Mark; his second example requires every escaped character thrown into an array initially, which generates a KeyError if the escape sequence happens not to be in the array. It will die on anything but the three characters provided (give \v a try), and enumerating every possible escape sequence every time you want to unescape a string (or keeping a global array) is a really bad solution. Analogous to PHP, that's using <code>preg_replace_callback()</code> with a lambda instead of <code>preg_replace()</code>, which is utterly unnecessary in this situation.</p> <p>I'm sorry if I'm coming off as a dick about it, I'm just utterly frustrated with Python. This is supported by every other regular expression engine I've ever used, and I can't understand why this wouldn't work.</p> <p>Thank you for responding; the <code>string.decode('string-escape')</code> function is precisely what i was looking for initially. If someone has a general solution to the regex backreference problem, feel free to post it and I'll accept that as an answer as well.</p>

2008-08-17T21:55:54Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>It's simple: A function that references variables from a containing scope, potentially after flow-of-control has left that scope. That last bit is very useful:</p> <pre><code>&gt;&gt;&gt; def makeConstantAdder(x): ... constant = x ... def adder(y): ... return y + constant ... return adder ... &gt;&gt;&gt; f = makeConstantAdder(12) &gt;&gt;&gt; f(3) 15 &gt;&gt;&gt; g = makeConstantAdder(4) &gt;&gt;&gt; g(3) 7 </code></pre> <p>Note that 12 and 4 have "disappeared" inside f and g, respectively, this feature is what make f and g proper closures.</p>

2008-08-17T19:32:33Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>I've never heard of transactions being used in the same context as explaining what a closure is and there really aren't any transaction semantics here.</p> <p>It's called a closure because it "closes over" the outside variable (constant)--i.e., it's not just a function but an enclosure of the environment where the function was created. </p> <p>In the following example, calling the closure g after changing x will also change the value of x within g, since g closes over x:</p> <pre><code>x = 0 def f(): def g(): return x * 2 return g closure = f() print(closure()) # 0 x = 2 print(closure()) # 4 </code></pre>

2008-08-17T20:20:20Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>I like <a href="http://effbot.org/zone/closure.htm">this rough, succinct definition</a>:</p> <blockquote> <p>A function that can refer to environments that are no longer active.</p> </blockquote> <p>I'd add</p> <blockquote> <p>A closure allows you to bind variables into a function <em>without passing them as parameters</em>.</p> </blockquote> <p>Decorators which accept parameters are a common use for closures. Closures are a common implementation mechanism for that sort of "function factory". I frequently choose to use closures in the <a href="http://c2.com/cgi/wiki?StrategyPattern">Strategy Pattern</a> when the strategy is modified by data at run-time.</p> <p>In a language that allows anonymous block definition -- e.g., Ruby, C# -- closures can be used to implement (what amount to) novel new control structures. The lack of anonymous blocks is among <a href="http://ivan.truemesh.com/archives/000411.html">the limitations of closures in Python</a>.</p>

2008-08-17T20:25:18Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>To be honest, I understand closures perfectly well except I've never been clear about what exactly is the thing which is the "closure" and what's so "closure" about it. I recommend you give up looking for any logic behind the choice of term.</p> <p>Anyway, here's my explanation:</p> <pre><code>def foo(): x = 3 def bar(): print x x = 5 return bar bar = foo() bar() # print 5 </code></pre> <p>A key idea here is that the function object returned from foo retains a hook to the local var 'x' even though 'x' has gone out of scope and should be defunct. This hook is to the var itself, not just the value that var had at the time, so when bar is called, it prints 5, not 3.</p> <p>Also be clear that Python 2.x has limited closure: there's no way I can modify 'x' inside 'bar' because writing 'x = bla' would declare a local 'x' in bar, not assign to 'x' of foo. This is a side-effect of Python's assignment=declaration. To get around this, Python 3.0 introduces the nonlocal keyword:</p> <pre><code>def foo(): x = 3 def bar(): print x def ack(): nonlocal x x = 7 x = 5 return (bar, ack) bar, ack = foo() ack() # modify x of the call to foo bar() # print 7 </code></pre>

2008-08-23T07:43:18Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>The best explanation I ever saw of a closure was to explain the mechanism. It went something like this:</p> <p>Imagine your program stack as a degenerate tree where each node has only one child and the single leaf node is the context of your currently executing procedure.</p> <p>Now relax the constraint that each node can have only one child.</p> <p>If you do this, you can have a construct ('yield') that can return from a procedure without discarding the local context (i.e. it doesn't pop it off the stack when you return). The next time the procedure is invoked, the invocation picks up the old stack (tree) frame and continues executing where it left off.</p>

2008-09-18T17:10:42Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p><a href="http://mrevelle.blogspot.com/2006/10/closure-on-closures.html">Closure on closures</a></p> <blockquote> <p>Objects are data with methods attached, closures are functions with data attached.</p> </blockquote> <pre><code>def make_counter(): i = 0 def counter(): # counter() is a closure nonlocal i i += 1 return i return counter c1 = make_counter() c2 = make_counter() print (c1(), c1(), c2(), c2()) # -&gt; 1 2 1 2 </code></pre>

2008-09-26T19:28:32Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>Here's a typical use case for closures - callbacks for GUI elements (this would be an alternative to subclassing the button class). For example, you can construct a function that will be called in response to a button press, and "close" over the relevant variables in the parent scope that are necessary for processing the click. This way you can wire up pretty complicated interfaces from the same initialization function, building all the dependencies into the closure.</p>

2009-01-23T16:13:53Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>For me, "closures" are functions which are capable to remember the environment they were created. This functionality, allows you to use variables or methods within the closure wich, in other way,you wouldn't be able to use either because they don't exist anymore or they are out of reach due to scope. Let's look at this code in ruby:</p> <pre><code>def makefunction (x) def multiply (a,b) puts a*b end return lambda {|n| multiply(n,x)} # =&gt; returning a closure end func = makefunction(2) # =&gt; we capture the closure func.call(6) # =&gt; Result equal "12" </code></pre> <p>it works even when both, "multiply" method and "x" variable,not longer exist. All because the closure capability to remember.</p>

2013-09-20T13:39:53Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>In Python, a closure is an instance of a function that has variables bound to it immutably.</p> <p>In fact, the <a href="https://docs.python.org/3.3/reference/datamodel.html#the-standard-type-hierarchy" rel="nofollow">data model explains this</a> in its description of functions' <code>__closure__</code> attribute: </p> <blockquote> <p>None or a <strong>tuple of cells</strong> that contain bindings for the function’s free variables. Read-only</p> </blockquote> <p>To demonstrate this:</p> <pre><code>def enclosure(foo): def closure(bar): print(foo, bar) return closure closure_instance = enclosure('foo') </code></pre> <p>Clearly, we know that we now have a function pointed at from the variable name <code>closure_instance</code>. Ostensibly, if we call it with an object, <code>bar</code>, it should print the string, <code>'foo'</code> and whatever the string representation of <code>bar</code> is.</p> <p>In fact, the string 'foo' <em>is</em> bound to the instance of the function, and we can directly read it here, by accessing the <code>cell_contents</code> attribute of the first (and only) cell in the tuple of the <code>__closure__</code> attribute:</p> <pre><code>&gt;&gt;&gt; closure_instance.__closure__[0].cell_contents 'foo' </code></pre> <p>As an aside, cell objects are described in the C API documentation:</p> <blockquote> <p><a href="https://docs.python.org/2/c-api/cell.html" rel="nofollow">"Cell" objects are used to implement variables referenced by multiple scopes</a></p> </blockquote> <p>And we can demonstrate our closure's usage, noting that <code>'foo'</code> is stuck in the function and doesn't change:</p> <pre><code>&gt;&gt;&gt; closure_instance('bar') foo bar &gt;&gt;&gt; closure_instance('baz') foo baz &gt;&gt;&gt; closure_instance('quux') foo quux </code></pre> <p>And nothing can change it:</p> <pre><code>&gt;&gt;&gt; closure_instance.__closure__ = None Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; TypeError: readonly attribute </code></pre> <h3>Partial Functions</h3> <p>The example given uses the closure as a partial function, but if this is our only goal, the same goal can be accomplished with <code>functools.partial</code></p> <pre><code>&gt;&gt;&gt; from __future__ import print_function # use this if you're in Python 2. &gt;&gt;&gt; partial_function = functools.partial(print, 'foo') &gt;&gt;&gt; partial_function('bar') foo bar &gt;&gt;&gt; partial_function('baz') foo baz &gt;&gt;&gt; partial_function('quux') foo quux </code></pre> <p>There are more complicated closures as well that would not fit the partial function example, and I'll demonstrate them further as time allows.</p>

2014-07-18T03:33:13Z

Can you explain closures (as they relate to Python)?

<p>I've been reading a lot about closures and I think I understand them, but without clouding the picture for myself and others, I am hoping someone can explain closures as succinctly and clearly as possible. I'm looking for a simple explanation that might help me understand where and why I would want to use them.</p>

2008-08-17T19:14:30Z

<p>Here is an example of Python3 closures </p> <pre><code>def closure(x): def counter(): nonlocal x x += 1 return x return counter; counter1 = closure(100); counter2 = closure(200); print("i from closure 1 " + str(counter1())) print("i from closure 1 " + str(counter1())) print("i from closure 2 " + str(counter2())) print("i from closure 1 " + str(counter1())) print("i from closure 1 " + str(counter1())) print("i from closure 1 " + str(counter1())) print("i from closure 2 " + str(counter2())) # result i from closure 1 101 i from closure 1 102 i from closure 2 201 i from closure 1 103 i from closure 1 104 i from closure 1 105 i from closure 2 202 </code></pre>

2015-09-22T20:16:22Z

Python Sound ("Bell")

<p>I'd like to have a python program alert me when it has completed its task by making a beep noise. Currently, I use <code>import os</code> and then use a command line speech program to say "Process complete." I much rather it be a simple "bell."</p> <p>I know that there's a function that can be used in Cocoa apps, NSBeep, but I don't think that has much anything to do with this.</p> <p>I've also tried <code>print(\a)</code> but that didn't work.</p> <p>I'm using a Mac, if you couldn't tell by my Cocoa comment, so that may help.</p> <p>Thanks!</p>

2008-08-17T21:33:39Z

<p>Have you tried :</p> <pre><code>import sys sys.stdout.write('\a') sys.stdout.flush() </code></pre> <p>That works for me here on Mac OS 10.5</p> <p>Actually, I think your original attempt works also with a little modification:</p> <pre><code>print('\a') </code></pre> <p>(You just need the single quotes around the character sequence).</p>

2008-08-17T21:46:02Z

Python Sound ("Bell")

<p>I'd like to have a python program alert me when it has completed its task by making a beep noise. Currently, I use <code>import os</code> and then use a command line speech program to say "Process complete." I much rather it be a simple "bell."</p> <p>I know that there's a function that can be used in Cocoa apps, NSBeep, but I don't think that has much anything to do with this.</p> <p>I've also tried <code>print(\a)</code> but that didn't work.</p> <p>I'm using a Mac, if you couldn't tell by my Cocoa comment, so that may help.</p> <p>Thanks!</p>

2008-08-17T21:33:39Z

<p>I had to turn off the "Silence terminal bell" option in my active Terminal Profile in iTerm for <code>print('\a')</code> to work. It seemed to work fine by default in Terminal.</p> <p>You can also use the Mac module <code>Carbon.Snd</code> to play the system beep:</p> <pre><code>&gt;&gt;&gt; import Carbon.Snd &gt;&gt;&gt; Carbon.Snd.SysBeep(1) &gt;&gt;&gt; </code></pre> <p>The Carbon modules don't have any documentation, so I had to use <code>help(Carbon.Snd)</code> to see what functions were available. It seems to be a direct interface onto Carbon, so the docs on Apple Developer Connection probably help.</p>

2008-08-17T21:56:10Z

Python Sound ("Bell")

<p>I'd like to have a python program alert me when it has completed its task by making a beep noise. Currently, I use <code>import os</code> and then use a command line speech program to say "Process complete." I much rather it be a simple "bell."</p> <p>I know that there's a function that can be used in Cocoa apps, NSBeep, but I don't think that has much anything to do with this.</p> <p>I've also tried <code>print(\a)</code> but that didn't work.</p> <p>I'm using a Mac, if you couldn't tell by my Cocoa comment, so that may help.</p> <p>Thanks!</p>

2008-08-17T21:33:39Z

<p>If you have PyObjC (the Python - Objective-C bridge) installed or are running on OS X 10.5's system python (which ships with PyObjC), you can do</p> <pre><code>from AppKit import NSBeep NSBeep() </code></pre> <p>to play the system alert.</p>

2008-08-29T15:47:05Z

Python Sound ("Bell")

<p>I'd like to have a python program alert me when it has completed its task by making a beep noise. Currently, I use <code>import os</code> and then use a command line speech program to say "Process complete." I much rather it be a simple "bell."</p> <p>I know that there's a function that can be used in Cocoa apps, NSBeep, but I don't think that has much anything to do with this.</p> <p>I've also tried <code>print(\a)</code> but that didn't work.</p> <p>I'm using a Mac, if you couldn't tell by my Cocoa comment, so that may help.</p> <p>Thanks!</p>

2008-08-17T21:33:39Z

<p>I tried the mixer from the pygame module, and it works fine. First install the module:</p> <pre><code>$ sudo apt-get install python-pygame </code></pre> <p>Then in the program, write this:</p> <pre><code>from pygame import mixer mixer.init() #you must initialize the mixer alert=mixer.Sound('bell.wav') alert.play() </code></pre> <p>With pygame you have a lot of customization options, which you may additionally experiment with.</p>

2011-05-24T12:14:54Z

Is there a python module for regex matching in zip files

<p>I have over a million text files compressed into 40 zip files. I also have a list of about 500 model names of phones. I want to find out the number of times a particular model was mentioned in the text files. </p> <p>Is there any python module which can do a regex match on the files without unzipping it. Is there a simple way to solve this problem without unzipping?</p>

2008-08-18T07:41:09Z

<p>You could loop through the zip files, reading individual files using the zipfile module and running your regex on those, eliminating to unzip all the files at once. </p> <p>I'm fairly certain that you can't run a regex over the zipped data, at least not meaningfully.</p>

2008-08-18T08:06:30Z

Is there a python module for regex matching in zip files

<p>I have over a million text files compressed into 40 zip files. I also have a list of about 500 model names of phones. I want to find out the number of times a particular model was mentioned in the text files. </p> <p>Is there any python module which can do a regex match on the files without unzipping it. Is there a simple way to solve this problem without unzipping?</p>

2008-08-18T07:41:09Z

<p>To access the contents of a zip file you have to unzip it, although the zipfile package makes this fairly easy, as you can unzip each file within an archive individually.</p> <p><a href="http://docs.python.org/lib/module-zipfile.html" rel="nofollow">Python zipfile module</a></p>

2008-08-18T08:10:57Z

Is there a python module for regex matching in zip files

<p>I have over a million text files compressed into 40 zip files. I also have a list of about 500 model names of phones. I want to find out the number of times a particular model was mentioned in the text files. </p> <p>Is there any python module which can do a regex match on the files without unzipping it. Is there a simple way to solve this problem without unzipping?</p>

2008-08-18T07:41:09Z

<p>There's nothing that will automatically do what you want.</p> <p>However, there is a python zipfile module that will make this easy to do. Here's how to iterate over the lines in the file.</p> <pre><code>#!/usr/bin/python import zipfile f = zipfile.ZipFile('myfile.zip') for subfile in f.namelist(): print subfile data = f.read(subfile) for line in data.split('\n'): print line </code></pre>

2008-08-18T08:19:06Z

Is there a python module for regex matching in zip files

<p>I have over a million text files compressed into 40 zip files. I also have a list of about 500 model names of phones. I want to find out the number of times a particular model was mentioned in the text files. </p> <p>Is there any python module which can do a regex match on the files without unzipping it. Is there a simple way to solve this problem without unzipping?</p>

2008-08-18T07:41:09Z

<p>Isn't it (at least theoretically) possible, to read in the ZIP's Huffman coding and then translate the regexp into the Huffman code? Might this be more efficient than first de-compressing the data, then running the regexp?</p> <p>(Note: I know it wouldn't be quite that simple: you'd also have to deal with other aspects of the ZIP coding&mdash;file layout, block structures, back-references&mdash;but one imagines this could be fairly lightweight.)</p> <p>EDIT: Also note that it's probably much more sensible to just use the <code>zipfile</code> solution.</p>

2008-09-03T14:42:10Z

Regex and unicode

<p>I have a script that parses the filenames of TV episodes (show.name.s01e02.avi for example), grabs the episode name (from the www.thetvdb.com API) and automatically renames them into something nicer (Show Name - [01x02].avi)</p> <p>The script works fine, that is until you try and use it on files that have Unicode show-names (something I never really thought about, since all the files I have are English, so mostly pretty-much all fall within <code>[a-zA-Z0-9'\-]</code>)</p> <p>How can I allow the regular expressions to match accented characters and the likes? Currently the regex's config section looks like..</p> <pre><code>config['valid_filename_chars'] = """0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@£$%^&amp;*()_+=-[]{}"'.,&lt;&gt;`~? """ config['valid_filename_chars_regex'] = re.escape(config['valid_filename_chars']) config['name_parse'] = [ # foo_[s01]_[e01] re.compile('''^([%s]+?)[ \._\-]\[[Ss]([0-9]+?)\]_\[[Ee]([0-9]+?)\]?[^\\/]*$'''% (config['valid_filename_chars_regex'])), # foo.1x09* re.compile('''^([%s]+?)[ \._\-]\[?([0-9]+)x([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.s01.e01, foo.s01_e01 re.compile('''^([%s]+?)[ \._\-][Ss]([0-9]+)[\.\- ]?[Ee]([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.103* re.compile('''^([%s]+)[ \._\-]([0-9]{1})([0-9]{2})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.0103* re.compile('''^([%s]+)[ \._\-]([0-9]{2})([0-9]{2,3})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), ] </code></pre>

2008-08-18T09:41:14Z

<p>Use a subrange of [\u0000-\uFFFF] for what you want.</p> <p>You can also use the re.UNICODE compile flag. <a href="http://docs.python.org/lib/re-syntax.html">The docs</a> say that if UNICODE is set, \w will match the characters [0-9_] plus whatever is classified as alphanumeric in the Unicode character properties database. </p> <p>See also <a href="http://coding.derkeiler.com/Archive/Python/comp.lang.python/2004-05/2560.html">http://coding.derkeiler.com/Archive/Python/comp.lang.python/2004-05/2560.html</a>.</p>

2008-08-18T09:43:10Z

Regex and unicode

<p>I have a script that parses the filenames of TV episodes (show.name.s01e02.avi for example), grabs the episode name (from the www.thetvdb.com API) and automatically renames them into something nicer (Show Name - [01x02].avi)</p> <p>The script works fine, that is until you try and use it on files that have Unicode show-names (something I never really thought about, since all the files I have are English, so mostly pretty-much all fall within <code>[a-zA-Z0-9'\-]</code>)</p> <p>How can I allow the regular expressions to match accented characters and the likes? Currently the regex's config section looks like..</p> <pre><code>config['valid_filename_chars'] = """0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@£$%^&amp;*()_+=-[]{}"'.,&lt;&gt;`~? """ config['valid_filename_chars_regex'] = re.escape(config['valid_filename_chars']) config['name_parse'] = [ # foo_[s01]_[e01] re.compile('''^([%s]+?)[ \._\-]\[[Ss]([0-9]+?)\]_\[[Ee]([0-9]+?)\]?[^\\/]*$'''% (config['valid_filename_chars_regex'])), # foo.1x09* re.compile('''^([%s]+?)[ \._\-]\[?([0-9]+)x([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.s01.e01, foo.s01_e01 re.compile('''^([%s]+?)[ \._\-][Ss]([0-9]+)[\.\- ]?[Ee]([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.103* re.compile('''^([%s]+)[ \._\-]([0-9]{1})([0-9]{2})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.0103* re.compile('''^([%s]+)[ \._\-]([0-9]{2})([0-9]{2,3})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), ] </code></pre>

2008-08-18T09:41:14Z

<p>\X seems to be available as a generic word-character in some languages, it allows you to match a single character disregarding of how many bytes it takes up. Might be useful.</p>

2008-08-18T09:53:13Z

Regex and unicode

<p>I have a script that parses the filenames of TV episodes (show.name.s01e02.avi for example), grabs the episode name (from the www.thetvdb.com API) and automatically renames them into something nicer (Show Name - [01x02].avi)</p> <p>The script works fine, that is until you try and use it on files that have Unicode show-names (something I never really thought about, since all the files I have are English, so mostly pretty-much all fall within <code>[a-zA-Z0-9'\-]</code>)</p> <p>How can I allow the regular expressions to match accented characters and the likes? Currently the regex's config section looks like..</p> <pre><code>config['valid_filename_chars'] = """0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@£$%^&amp;*()_+=-[]{}"'.,&lt;&gt;`~? """ config['valid_filename_chars_regex'] = re.escape(config['valid_filename_chars']) config['name_parse'] = [ # foo_[s01]_[e01] re.compile('''^([%s]+?)[ \._\-]\[[Ss]([0-9]+?)\]_\[[Ee]([0-9]+?)\]?[^\\/]*$'''% (config['valid_filename_chars_regex'])), # foo.1x09* re.compile('''^([%s]+?)[ \._\-]\[?([0-9]+)x([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.s01.e01, foo.s01_e01 re.compile('''^([%s]+?)[ \._\-][Ss]([0-9]+)[\.\- ]?[Ee]([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.103* re.compile('''^([%s]+)[ \._\-]([0-9]{1})([0-9]{2})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.0103* re.compile('''^([%s]+)[ \._\-]([0-9]{2})([0-9]{2,3})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), ] </code></pre>

2008-08-18T09:41:14Z

<p>In Mastering Regular Expressions from Jeffrey Friedl (great book) it is mentioned that you could use \p{Letter} which will match unicode stuff that is considered a letter.</p>

2008-08-18T10:17:35Z

Regex and unicode

<p>I have a script that parses the filenames of TV episodes (show.name.s01e02.avi for example), grabs the episode name (from the www.thetvdb.com API) and automatically renames them into something nicer (Show Name - [01x02].avi)</p> <p>The script works fine, that is until you try and use it on files that have Unicode show-names (something I never really thought about, since all the files I have are English, so mostly pretty-much all fall within <code>[a-zA-Z0-9'\-]</code>)</p> <p>How can I allow the regular expressions to match accented characters and the likes? Currently the regex's config section looks like..</p> <pre><code>config['valid_filename_chars'] = """0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@£$%^&amp;*()_+=-[]{}"'.,&lt;&gt;`~? """ config['valid_filename_chars_regex'] = re.escape(config['valid_filename_chars']) config['name_parse'] = [ # foo_[s01]_[e01] re.compile('''^([%s]+?)[ \._\-]\[[Ss]([0-9]+?)\]_\[[Ee]([0-9]+?)\]?[^\\/]*$'''% (config['valid_filename_chars_regex'])), # foo.1x09* re.compile('''^([%s]+?)[ \._\-]\[?([0-9]+)x([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.s01.e01, foo.s01_e01 re.compile('''^([%s]+?)[ \._\-][Ss]([0-9]+)[\.\- ]?[Ee]([0-9]+)[^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.103* re.compile('''^([%s]+)[ \._\-]([0-9]{1})([0-9]{2})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), # foo.0103* re.compile('''^([%s]+)[ \._\-]([0-9]{2})([0-9]{2,3})[\._ -][^\\/]*$''' % (config['valid_filename_chars_regex'])), ] </code></pre>

2008-08-18T09:41:14Z

<p>Python's re module doesn't support \p{Letter} or \X. However, the <a href="http://pypi.python.org/pypi/regex" rel="nofollow">new regex implementation on PyPI</a> does.</p>

2011-04-01T23:19:55Z

How do I validate xml against a DTD file in Python

<p>I need to validate an XML string (and not a file) against a DTD description file. </p> <p>How can that be done in <code>python</code>?</p>

2008-08-19T06:24:54Z

<p>Another good option is <a href="http://lxml.de/validation.html" rel="nofollow">lxml's validation</a> which I find quite pleasant to use.</p> <p>A simple example taken from the lxml site:</p> <pre><code>from StringIO import StringIO from lxml import etree dtd = etree.DTD(StringIO("""&lt;!ELEMENT foo EMPTY&gt;""")) root = etree.XML("&lt;foo/&gt;") print(dtd.validate(root)) # True root = etree.XML("&lt;foo&gt;bar&lt;/foo&gt;") print(dtd.validate(root)) # False print(dtd.error_log.filter_from_errors()) # &lt;string&gt;:1:0:ERROR:VALID:DTD_NOT_EMPTY: Element foo was declared EMPTY this one has content </code></pre>

2008-08-19T09:39:56Z

How do I validate xml against a DTD file in Python

<p>I need to validate an XML string (and not a file) against a DTD description file. </p> <p>How can that be done in <code>python</code>?</p>

2008-08-19T06:24:54Z

<p>from the examples directory in the libxml2 python bindings:</p> <pre><code>#!/usr/bin/python -u import libxml2 import sys # Memory debug specific libxml2.debugMemory(1) dtd="""&lt;!ELEMENT foo EMPTY&gt;""" instance="""&lt;?xml version="1.0"?&gt; &lt;foo&gt;&lt;/foo&gt;""" dtd = libxml2.parseDTD(None, 'test.dtd') ctxt = libxml2.newValidCtxt() doc = libxml2.parseDoc(instance) ret = doc.validateDtd(ctxt, dtd) if ret != 1: print "error doing DTD validation" sys.exit(1) doc.freeDoc() dtd.freeDtd() del dtd del ctxt </code></pre>

2008-11-06T22:17:48Z

Prototyping with Python code before compiling

<p>I have been mulling over writing a peak fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement some core routines in a compiled language eventually.</p> <p>IIRC, one of Python's original remits was as a prototyping language, however Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> <p>What should I know about designing functions/classes which I envisage will have to interface into the compiled language? And how much of these potential problems are dealt with by libraries such as cTypes, bgen, <a href="http://www.swig.org/">SWIG</a>, <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html">Boost.Python</a>, <a href="http://cython.org/">Cython</a> or <a href="http://www.riverbankcomputing.co.uk/software/sip/intro">Python SIP</a>?</p> <p>For this particular use case, (a fitting library) I imagine allowing users to define mathematical functions (Guassian, Lorentzian etc.) as Python functions which can then to be passed an interpreted by the compiled code fitting library. Passing and returning arrays is also essential.</p>

2008-08-19T12:32:38Z

<p>In my experience, there are two easy ways to call into C code from Python code. There are other approaches, all of which are more annoying and/or verbose.</p> <p>The first and easiest is to compile a bunch of C code as a separate shared library and then call functions in that library using ctypes. Unfortunately, passing anything other than basic data types is non-trivial.</p> <p>The second easiest way is to write a Python module in C and then call functions in that module. You can pass anything you want to these C functions without having to jump through any hoops. And it's easy to call Python functions or methods from these C functions, as described here: <a href="https://docs.python.org/extending/extending.html#calling-python-functions-from-c" rel="nofollow">https://docs.python.org/extending/extending.html#calling-python-functions-from-c</a></p> <p>I don't have enough experience with SWIG to offer intelligent commentary. And while it is possible to do things like pass custom Python objects to C functions through ctypes, or to define new Python classes in C, these things are annoying and verbose and I recommend taking one of the two approaches described above.</p>

2008-08-19T13:52:23Z

Prototyping with Python code before compiling

<p>I have been mulling over writing a peak fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement some core routines in a compiled language eventually.</p> <p>IIRC, one of Python's original remits was as a prototyping language, however Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> <p>What should I know about designing functions/classes which I envisage will have to interface into the compiled language? And how much of these potential problems are dealt with by libraries such as cTypes, bgen, <a href="http://www.swig.org/">SWIG</a>, <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html">Boost.Python</a>, <a href="http://cython.org/">Cython</a> or <a href="http://www.riverbankcomputing.co.uk/software/sip/intro">Python SIP</a>?</p> <p>For this particular use case, (a fitting library) I imagine allowing users to define mathematical functions (Guassian, Lorentzian etc.) as Python functions which can then to be passed an interpreted by the compiled code fitting library. Passing and returning arrays is also essential.</p>

2008-08-19T12:32:38Z

<p>The best way to plan for an eventual transition to compiled code is to write the performance sensitive portions as a module of simple functions in a <a href="http://en.wikipedia.org/wiki/Functional_programming" rel="nofollow">functional style</a> (stateless and without side effects), which accept and return basic data types.</p> <p>This will provide a one-to-one mapping from your Python prototype code to the eventual compiled code, and will let you use <a href="https://docs.python.org/library/ctypes.html" rel="nofollow">ctypes</a> easily and avoid a whole bunch of headaches.</p> <p>For peak fitting, you'll almost certainly need to use arrays, which will complicate things a little, but is still very doable with ctypes.</p> <p>If you really want to use more complicated data structures, or modify the passed arguments, <a href="http://www.swig.org/" rel="nofollow">SWIG</a> or <a href="https://docs.python.org/extending/" rel="nofollow">Python's standard C-extension interface</a> will let you do what you want, but with some amount of hassle.</p> <p>For what you're doing, you may also want to check out <a href="http://numpy.scipy.org/" rel="nofollow">NumPy</a>, which might do some of the work you would want to push to C, as well as offering <a href="http://projects.scipy.org/scipy/numpy/wiki/NumPyCAPI" rel="nofollow">some additional help in moving data back and forth between Python and C</a>.</p>

2008-08-20T01:45:05Z

Prototyping with Python code before compiling

<p>I have been mulling over writing a peak fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement some core routines in a compiled language eventually.</p> <p>IIRC, one of Python's original remits was as a prototyping language, however Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> <p>What should I know about designing functions/classes which I envisage will have to interface into the compiled language? And how much of these potential problems are dealt with by libraries such as cTypes, bgen, <a href="http://www.swig.org/">SWIG</a>, <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html">Boost.Python</a>, <a href="http://cython.org/">Cython</a> or <a href="http://www.riverbankcomputing.co.uk/software/sip/intro">Python SIP</a>?</p> <p>For this particular use case, (a fitting library) I imagine allowing users to define mathematical functions (Guassian, Lorentzian etc.) as Python functions which can then to be passed an interpreted by the compiled code fitting library. Passing and returning arrays is also essential.</p>

2008-08-19T12:32:38Z

<p>I haven't used SWIG or SIP, but I find writing Python wrappers with <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html" rel="nofollow">boost.python</a> to be very powerful and relatively easy to use.</p> <p>I'm not clear on what your requirements are for passing types between C/C++ and python, but you can do that easily by either exposing a C++ type to python, or by using a generic <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/v2/object.html" rel="nofollow">boost::python::object</a> argument to your C++ API. You can also register converters to automatically convert python types to C++ types and vice versa.</p> <p>If you plan use boost.python, the <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/tutorial/doc/html/index.html" rel="nofollow">tutorial</a> is a good place to start.</p> <p>I have implemented something somewhat similar to what you need. I have a C++ function that accepts a python function and an image as arguments, and applies the python function to each pixel in the image.</p> <pre><code>Image* unary(boost::python::object op, Image&amp; im) { Image* out = new Image(im.width(), im.height(), im.channels()); for(unsigned int i=0; i&lt;im.size(); i++) { (*out)[i] == extract&lt;float&gt;(op(im[i])); } return out; } </code></pre> <p>In this case, Image is a C++ object exposed to python (an image with float pixels), and op is a python defined function (or really any python object with a &#95;&#95;call&#95;&#95; attribute). You can then use this function as follows (assuming unary is located in the called image that also contains Image and a load function):</p> <pre><code>import image im = image.load('somefile.tiff') double_im = image.unary(lambda x: 2.0*x, im) </code></pre> <p>As for using arrays with boost, I personally haven't done this, but I know the functionality to expose arrays to python using boost is available - <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/v2/faq.html#question2" rel="nofollow">this</a> might be helpful.</p>

2008-08-26T15:58:08Z

Prototyping with Python code before compiling

<p>I have been mulling over writing a peak fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement some core routines in a compiled language eventually.</p> <p>IIRC, one of Python's original remits was as a prototyping language, however Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> <p>What should I know about designing functions/classes which I envisage will have to interface into the compiled language? And how much of these potential problems are dealt with by libraries such as cTypes, bgen, <a href="http://www.swig.org/">SWIG</a>, <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html">Boost.Python</a>, <a href="http://cython.org/">Cython</a> or <a href="http://www.riverbankcomputing.co.uk/software/sip/intro">Python SIP</a>?</p> <p>For this particular use case, (a fitting library) I imagine allowing users to define mathematical functions (Guassian, Lorentzian etc.) as Python functions which can then to be passed an interpreted by the compiled code fitting library. Passing and returning arrays is also essential.</p>

2008-08-19T12:32:38Z

<p><a href="http://cens.ioc.ee/projects/f2py2e/usersguide/index.html#the-quick-and-smart-way" rel="nofollow">f2py</a> (part of <code>numpy</code>) is a simpler alternative to SWIG and boost.python for wrapping C/Fortran number-crunching code.</p>

2008-09-29T22:30:59Z

Prototyping with Python code before compiling

<p>I have been mulling over writing a peak fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement some core routines in a compiled language eventually.</p> <p>IIRC, one of Python's original remits was as a prototyping language, however Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> <p>What should I know about designing functions/classes which I envisage will have to interface into the compiled language? And how much of these potential problems are dealt with by libraries such as cTypes, bgen, <a href="http://www.swig.org/">SWIG</a>, <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html">Boost.Python</a>, <a href="http://cython.org/">Cython</a> or <a href="http://www.riverbankcomputing.co.uk/software/sip/intro">Python SIP</a>?</p> <p>For this particular use case, (a fitting library) I imagine allowing users to define mathematical functions (Guassian, Lorentzian etc.) as Python functions which can then to be passed an interpreted by the compiled code fitting library. Passing and returning arrays is also essential.</p>

2008-08-19T12:32:38Z

<blockquote> <p>Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> </blockquote> <p>In C you cannot pass a function as an argument to a function but you can pass a function pointer which is just as good a function.</p> <p>I don't know how much that would help when you are trying to integrate C and Python code but I just wanted to clear up one misconception.</p>

2008-09-29T22:52:19Z

Prototyping with Python code before compiling

<p>I have been mulling over writing a peak fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement some core routines in a compiled language eventually.</p> <p>IIRC, one of Python's original remits was as a prototyping language, however Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> <p>What should I know about designing functions/classes which I envisage will have to interface into the compiled language? And how much of these potential problems are dealt with by libraries such as cTypes, bgen, <a href="http://www.swig.org/">SWIG</a>, <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html">Boost.Python</a>, <a href="http://cython.org/">Cython</a> or <a href="http://www.riverbankcomputing.co.uk/software/sip/intro">Python SIP</a>?</p> <p>For this particular use case, (a fitting library) I imagine allowing users to define mathematical functions (Guassian, Lorentzian etc.) as Python functions which can then to be passed an interpreted by the compiled code fitting library. Passing and returning arrays is also essential.</p>

2008-08-19T12:32:38Z

<p>In addition to the tools above, I can recommend using <a href="http://www.cosc.canterbury.ac.nz/greg.ewing/python/Pyrex/" rel="nofollow" title="Pyrex">Pyrex</a> (for creating Python extension modules) or <a href="http://psyco.sourceforge.net/" rel="nofollow" title="Psyco">Psyco</a> (as JIT compiler for Python).</p>

2009-05-16T15:08:53Z

Prototyping with Python code before compiling

<p>I have been mulling over writing a peak fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement some core routines in a compiled language eventually.</p> <p>IIRC, one of Python's original remits was as a prototyping language, however Python is pretty liberal in allowing functions, functors, objects to be passed to functions and methods, whereas I suspect the same is not true of say C or Fortran.</p> <p>What should I know about designing functions/classes which I envisage will have to interface into the compiled language? And how much of these potential problems are dealt with by libraries such as cTypes, bgen, <a href="http://www.swig.org/">SWIG</a>, <a href="http://www.boost.org/doc/libs/1_35_0/libs/python/doc/index.html">Boost.Python</a>, <a href="http://cython.org/">Cython</a> or <a href="http://www.riverbankcomputing.co.uk/software/sip/intro">Python SIP</a>?</p> <p>For this particular use case, (a fitting library) I imagine allowing users to define mathematical functions (Guassian, Lorentzian etc.) as Python functions which can then to be passed an interpreted by the compiled code fitting library. Passing and returning arrays is also essential.</p>

2008-08-19T12:32:38Z

<p>Finally a question that I can really put a value answer to :). </p> <p>I have investigated f2py, boost.python, swig, cython and pyrex for my work (PhD in optical measurement techniques). I used swig extensively, boost.python some and pyrex and cython a lot. I also used ctypes. This is my breakdown:</p> <p><strong>Disclaimer</strong>: This is my personal experience. I am not involved with any of these projects. </p> <p><strong>swig:</strong> does not play well with c++. It should, but name mangling problems in the linking step was a major headache for me on linux &amp; Mac OS X. If you have C code and want it interfaced to python, it is a good solution. I wrapped the GTS for my needs and needed to write basically a C shared library which I could connect to. I would not recommend it.</p> <p><strong>Ctypes:</strong> I wrote a libdc1394 (IEEE Camera library) wrapper using ctypes and it was a very straigtforward experience. You can find the code on <a href="https://launchpad.net/pydc1394">https://launchpad.net/pydc1394</a>. It is a lot of work to convert headers to python code, but then everything works reliably. This is a good way if you want to interface an external library. Ctypes is also in the stdlib of python, so everyone can use your code right away. This is also a good way to play around with a new lib in python quickly. I can recommend it to interface to external libs. </p> <p><strong>Boost.Python</strong>: Very enjoyable. If you already have C++ code of your own that you want to use in python, go for this. It is very easy to translate c++ class structures into python class structures this way. I recommend it if you have c++ code that you need in python. </p> <p><strong>Pyrex/Cython:</strong> Use Cython, not Pyrex. Period. Cython is more advanced and more enjoyable to use. Nowadays, I do everything with cython that i used to do with SWIG or Ctypes. It is also the best way if you have python code that runs too slow. The process is absolutely fantastic: you convert your python modules into cython modules, build them and keep profiling and optimizing like it still was python (no change of tools needed). You can then apply as much (or as little) C code mixed with your python code. This is by far faster then having to rewrite whole parts of your application in C; you only rewrite the inner loop. </p> <p><strong>Timings</strong>: ctypes has the highest call overhead (~700ns), followed by boost.python (322ns), then directly by swig (290ns). Cython has the lowest call overhead (124ns) and the best feedback where it spends time on (cProfile support!). The numbers are from my box calling a trivial function that returns an integer from an interactive shell; module import overhead is therefore not timed, only function call overhead is. It is therefore easiest and most productive to get python code fast by profiling and using cython.</p> <p><strong>Summary</strong>: For your problem, use Cython ;). I hope this rundown will be useful for some people. I'll gladly answer any remaining question.</p> <p><hr /></p> <p><strong>Edit</strong>: I forget to mention: for numerical purposes (that is, connection to NumPy) use Cython; they have support for it (because they basically develop cython for this purpose). So this should be another +1 for your decision. </p>

2009-11-02T13:16:20Z

Sanitising user input using Python

<p>What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to prevent an <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow">XSS</a> or SQL injection attack?</p>

2008-08-19T20:18:31Z

<p>If you are using a framework like <a href="http://www.djangoproject.com/" rel="nofollow">django</a>, the framework can easily do this for you using standard filters. In fact, I'm pretty sure django automatically does it unless you tell it not to.</p> <p>Otherwise, I would recommend using some sort of regex validation before accepting inputs from forms. I don't think there's a silver bullet for your problem, but using the re module, you should be able to construct what you need.</p>

2008-08-19T20:24:18Z

Sanitising user input using Python

<p>What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to prevent an <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow">XSS</a> or SQL injection attack?</p>

2008-08-19T20:18:31Z

<p>Jeff Atwood himself described how StackOverflow.com sanitizes user input (in non-language-specific terms) on the Stack Overflow blog: <a href="http://blog.stackoverflow.com/2008/06/safe-html-and-xss/" rel="nofollow">http://blog.stackoverflow.com/2008/06/safe-html-and-xss/</a></p> <p>However, as Justin points out, if you use Django templates or something similar then they probably sanitize your HTML output anyway.</p> <p>SQL injection also shouldn't be a concern. All of Python's database libraries (MySQLdb, cx_Oracle, etc) always sanitize the parameters you pass. These libraries are used by all of Python's object-relational mappers (such as Django models), so you don't need to worry about sanitation there either.</p>

2008-08-19T20:51:39Z

Sanitising user input using Python

<p>What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to prevent an <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow">XSS</a> or SQL injection attack?</p>

2008-08-19T20:18:31Z

<p>Here is a snippet that will remove all tags not on the white list, and all tag attributes not on the attribues whitelist (so you can't use <code>onclick</code>).</p> <p>It is a modified version of <a href="http://www.djangosnippets.org/snippets/205/">http://www.djangosnippets.org/snippets/205/</a>, with the regex on the attribute values to prevent people from using <code>href="javascript:..."</code>, and other cases described at <a href="http://ha.ckers.org/xss.html">http://ha.ckers.org/xss.html</a>.<br> (e.g. <code>&lt;a href="ja&amp;#x09;vascript:alert('hi')"&gt;</code> or <code>&lt;a href="ja vascript:alert('hi')"&gt;</code>, etc.)</p> <p>As you can see, it uses the (awesome) <a href="http://www.crummy.com/software/BeautifulSoup/">BeautifulSoup</a> library.</p> <pre><code>import re from urlparse import urljoin from BeautifulSoup import BeautifulSoup, Comment def sanitizeHtml(value, base_url=None): rjs = r'[\s]*(&amp;#x.{1,7})?'.join(list('javascript:')) rvb = r'[\s]*(&amp;#x.{1,7})?'.join(list('vbscript:')) re_scripts = re.compile('(%s)|(%s)' % (rjs, rvb), re.IGNORECASE) validTags = 'p i strong b u a h1 h2 h3 pre br img'.split() validAttrs = 'href src width height'.split() urlAttrs = 'href src'.split() # Attributes which should have a URL soup = BeautifulSoup(value) for comment in soup.findAll(text=lambda text: isinstance(text, Comment)): # Get rid of comments comment.extract() for tag in soup.findAll(True): if tag.name not in validTags: tag.hidden = True attrs = tag.attrs tag.attrs = [] for attr, val in attrs: if attr in validAttrs: val = re_scripts.sub('', val) # Remove scripts (vbs &amp; js) if attr in urlAttrs: val = urljoin(base_url, val) # Calculate the absolute url tag.attrs.append((attr, val)) return soup.renderContents().decode('utf8') </code></pre> <p>As the other posters have said, pretty much all Python db libraries take care of SQL injection, so this should pretty much cover you.</p>

2008-08-24T16:08:37Z

Sanitising user input using Python

<p>What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to prevent an <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow">XSS</a> or SQL injection attack?</p>

2008-08-19T20:18:31Z

<p>I don't do web development much any longer, but when I did, I did something like so:</p> <p>When no parsing is supposed to happen, I usually just escape the data to not interfere with the database when I store it, and escape everything I read up from the database to not interfere with html when I display it (cgi.escape() in python).</p> <p>Chances are, if someone tried to input html characters or stuff, they actually wanted that to be displayed as text anyway. If they didn't, well tough :)</p> <p>In short always escape what can affect the current target for the data.</p> <p>When I did need some parsing (markup or whatever) I usually tried to keep that language in a non-intersecting set with html so I could still just store it suitably escaped (after validating for syntax errors) and parse it to html when displaying without having to worry about the data the user put in there interfering with your html.</p> <p>See also <a href="http://wiki.python.org/moin/EscapingHtml" rel="nofollow">Escaping HTML</a></p>

2008-08-24T16:23:13Z

Sanitising user input using Python

<p>What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to prevent an <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow">XSS</a> or SQL injection attack?</p>

2008-08-19T20:18:31Z

<p>The best way to prevent XSS is not to try and filter everything, but rather to simply do HTML Entity encoding. For example, automatically turn &lt; into &amp;lt;. This is the ideal solution assuming you don't need to accept any html input (outside of forum/comment areas where it is used as markup, it should be pretty rare to need to accept HTML); there are so many permutations via alternate encodings that anything but an ultra-restrictive whitelist (a-z,A-Z,0-9 for example) is going to let something through.</p> <p>SQL Injection, contrary to other opinion, is still possible, if you are just building out a query string. For example, if you are just concatenating an incoming parameter onto a query string, you will have SQL Injection. The best way to protect against this is also not filtering, but rather to religiously use parameterized queries and NEVER concatenate user input.</p> <p>This is not to say that filtering isn't still a best practice, but in terms of SQL Injection and XSS, you will be far more protected if you religiously use Parameterize Queries and HTML Entity Encoding.</p>

2008-09-18T15:56:09Z

Sanitising user input using Python

<p>What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to prevent an <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow">XSS</a> or SQL injection attack?</p>

2008-08-19T20:18:31Z

<p><strong>Edit</strong>: <a href="https://github.com/jsocol/bleach">bleach</a> is a wrapper around html5lib which makes it even easier to use as a whitelist-based sanitiser.</p> <p><a href="http://code.google.com/p/html5lib/"><code>html5lib</code></a> comes with a whitelist-based HTML sanitiser - it's easy to subclass it to restrict the tags and attributes users are allowed to use on your site, and it even attempts to sanitise CSS if you're allowing use of the <code>style</code> attribute.</p> <p>Here's now I'm using it in my Stack Overflow clone's <code>sanitize_html</code> utility function:</p> <p><a href="http://code.google.com/p/soclone/source/browse/trunk/soclone/utils/html.py">http://code.google.com/p/soclone/source/browse/trunk/soclone/utils/html.py</a></p> <p>I've thrown all the attacks listed in <a href="http://ha.ckers.org/xss.html">ha.ckers.org's XSS Cheatsheet</a> (which are handily <a href="http://ha.ckers.org/xssAttacks.xml">available in XML format</a> at it after performing Markdown to HTML conversion using <a href="http://code.google.com/p/python-markdown2/">python-markdown2</a> and it seems to have held up ok.</p> <p>The WMD editor component which Stackoverflow currently uses is a problem, though - I actually had to disable JavaScript in order to test the XSS Cheatsheet attacks, as pasting them all into WMD ended up giving me alert boxes and blanking out the page.</p>

2008-10-30T00:41:40Z

Sanitising user input using Python

<p>What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to prevent an <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow">XSS</a> or SQL injection attack?</p>

2008-08-19T20:18:31Z

<p>To sanitize a string input which you want to store to the database (for example a customer name) you need either to escape it or plainly remove any quotes (', ") from it. This effectively prevents classical SQL injection which can happen if you are assembling an SQL query from strings passed by the user.</p> <p>For example (if it is acceptable to remove quotes completely):</p> <pre><code>datasetName = datasetName.replace("'","").replace('"',"") </code></pre>

2009-10-01T12:21:45Z

Create an encrypted ZIP file in Python

<p>I'm creating an ZIP file with ZipFile in Python 2.5, it works ok so far:</p> <pre><code>import zipfile, os locfile = "test.txt" loczip = os.path.splitext (locfile)[0] + ".zip" zip = zipfile.ZipFile (loczip, "w") zip.write (locfile) zip.close() </code></pre> <p>but I couldn't find how to encrypt the files in the ZIP file. I could use system and call PKZIP -s, but I suppose there must be a more "Pythonic" way. I'm looking for an open source solution.</p>

2008-08-20T00:16:40Z

<p>You can use the <a href="http://www.chilkatsoft.com/python.asp" rel="nofollow">Chilkat</a> library. It's commercial, but has a free evaluation and seems pretty nice.</p> <p>Here's an example I got from <a href="http://www.example-code.com/python/zip.asp" rel="nofollow">here</a>:</p> <pre><code>import chilkat # Demonstrates how to create a WinZip-compatible 128-bit AES strong encrypted zip zip = chilkat.CkZip() zip.UnlockComponent("anything for 30-day trial") zip.NewZip("strongEncrypted.zip") # Set the Encryption property = 4, which indicates WinZip compatible AES encryption. zip.put_Encryption(4) # The key length can be 128, 192, or 256. zip.put_EncryptKeyLength(128) zip.SetPassword("secret") zip.AppendFiles("exampleData/*",True) zip.WriteZip() </code></pre>

2008-08-20T01:20:48Z

Create an encrypted ZIP file in Python

<p>I'm creating an ZIP file with ZipFile in Python 2.5, it works ok so far:</p> <pre><code>import zipfile, os locfile = "test.txt" loczip = os.path.splitext (locfile)[0] + ".zip" zip = zipfile.ZipFile (loczip, "w") zip.write (locfile) zip.close() </code></pre> <p>but I couldn't find how to encrypt the files in the ZIP file. I could use system and call PKZIP -s, but I suppose there must be a more "Pythonic" way. I'm looking for an open source solution.</p>

2008-08-20T00:16:40Z

<p>I created a simple library to create a password encrypted zip file in python. - <a href="https://github.com/smihica/pyminizip"><strong>here</strong></a></p> <pre><code>import pyminizip compression_level = 5 # 1-9 pyminizip.compress("src.txt", "dst.zip", "password", compression_level) </code></pre> <p><strong>The library requires zlib.</strong></p> <p>I have checked that the file can be extracted in WINDOWS/MAC.</p>

2013-04-17T01:39:00Z

Create an encrypted ZIP file in Python

<p>I'm creating an ZIP file with ZipFile in Python 2.5, it works ok so far:</p> <pre><code>import zipfile, os locfile = "test.txt" loczip = os.path.splitext (locfile)[0] + ".zip" zip = zipfile.ZipFile (loczip, "w") zip.write (locfile) zip.close() </code></pre> <p>but I couldn't find how to encrypt the files in the ZIP file. I could use system and call PKZIP -s, but I suppose there must be a more "Pythonic" way. I'm looking for an open source solution.</p>

2008-08-20T00:16:40Z

<p>The duplicate question <a href="http://stackoverflow.com/a/2366917/874188">http://stackoverflow.com/a/2366917/874188</a> recommends using <code>7z</code> instead of <code>zip</code>. My experience bears this out.</p> <p>Copy/pasting the answer by @JFSebastian here too for completeness:</p> <p>To create encrypted zip archive (named <code>'myarchive.zip'</code>) using open-source <a href="http://www.7-zip.org/" rel="nofollow"><code>7-Zip</code></a> utility:</p> <pre><code>rc = subprocess.call(['7z', 'a', '-pP4$$W0rd', '-y', 'myarchive.zip'] + ['first_file.txt', 'second.file']) </code></pre> <p>To install 7-Zip, type:</p> <pre><code>$ sudo apt-get install p7zip-full </code></pre> <p>To unzip by hand (to demonstrate compatibility with zip utitity), type:</p> <pre><code>$ unzip myarchive.zip </code></pre> <p>And enter <code>P4$$W0rd</code> at the prompt.</p> <p>Or the same in Python 2.6+:</p> <pre><code>&gt;&gt;&gt; zipfile.ZipFile('myarchive.zip').extractall(pwd='P4$$W0rd') </code></pre>

2014-12-12T12:27:59Z

What's the best way to distribute python command-line tools?

<p>My current setup.py script works okay, but it installs tvnamer.py (the tool) as "tvnamer.py" into site-packages or somewhere similar..</p> <p>Can I make setup.py install tvnamer.py as tvnamer, and/or is there a better way of installing command-line applications?</p>

2008-08-20T13:07:25Z

<p>Try the <code>entry_points.console_scripts</code> parameter in the setup() call. As described in the <a href="http://peak.telecommunity.com/DevCenter/setuptools#automatic-script-creation">setuptools docs</a>, this should do what I think you want.</p> <p>To reproduce here:</p> <pre><code>from setuptools import setup setup( # other arguments here... entry_points = { 'console_scripts': [ 'foo = package.module:func', 'bar = othermodule:somefunc', ], } ) </code></pre>

2008-08-20T13:25:23Z

How to check set of files conform to a naming scheme

<p>I have a bunch of files (TV episodes, although that is fairly arbitrary) that I want to check match a specific naming/organisation scheme..</p> <p>Currently: I have three arrays of regex, one for valid filenames, one for files missing an episode name, and one for valid paths.</p> <p>Then, I loop though each valid-filename regex, if it matches, append it to a "valid" dict, if not, do the same with the missing-ep-name regexs, if it matches this I append it to an "invalid" dict with an error code (2:'missing epsiode name'), if it matches neither, it gets added to invalid with the 'malformed name' error code.</p> <p>The current code can be found <a href="http://github.com/dbr/checktveps/tree/8a6dc68ad61e684c8d8f0ca1dc37a22d1c51aa82/2checkTvEps.py" rel="nofollow">here</a></p> <p>I want to add a rule that checks for the presence of a folder.jpg file in each directory, but to add this would make the code substantially more messy in it's current state.. </p> <p>How could I write this system in a more expandable way?</p> <p>The rules it needs to check would be..</p> <ul> <li>File is in the format <code>Show Name - [01x23] - Episode Name.avi</code> or <code>Show Name - [01xSpecial02] - Special Name.avi</code> or <code>Show Name - [01xExtra01] - Extra Name.avi</code></li> <li>If filename is in the format <code>Show Name - [01x23].avi</code> display it a 'missing episode name' section of the output</li> <li>The path should be in the format <code>Show Name/season 2/the_file.avi</code> (where season 2 should be the correct season number in the filename)</li> <li>each <code>Show Name/season 1/</code> folder should contain "folder.jpg"</li> </ul> <p>.any ideas? While I'm trying to check TV episodes, this concept/code should be able to apply to many things..</p> <p>The only thought I had was a list of dicts in the format:</p> <pre><code>checker = [ { 'name':'valid files', 'type':'file', 'function':check_valid(), # runs check_valid() on all files 'status':0 # if it returns True, this is the status the file gets } </code></pre>

2008-08-20T22:50:55Z

<p>maybe you should take the approach of defaulting to: "the filename is correct" and work from there to disprove that statement:</p> <p>with the fact that you only allow filenames with: 'show name', 'season number x episode number' and 'episode name', you know for certain that these items should be separated by a "-" (dash) so you have to have 2 of those for a filename to be correct.<br> if that checks out, you can use your code to check that the show name matches the show name as seen in the parent's parent folder (case insensitive i assume), the season number matches the parents folder numeric value (with or without an extra 0 prepended).</p> <p>if however you don't see the correct amount of dashes you instantly know that there is something wrong and stop before the rest of the tests etc.</p> <p>and separately you can check if the file <code>folder.jpg</code> exists and take the necessary actions. <strong>or</strong> do that first and filter that file from the rest of the files in that folder.</p>

2008-08-21T05:59:00Z

How to check set of files conform to a naming scheme

<p>I have a bunch of files (TV episodes, although that is fairly arbitrary) that I want to check match a specific naming/organisation scheme..</p> <p>Currently: I have three arrays of regex, one for valid filenames, one for files missing an episode name, and one for valid paths.</p> <p>Then, I loop though each valid-filename regex, if it matches, append it to a "valid" dict, if not, do the same with the missing-ep-name regexs, if it matches this I append it to an "invalid" dict with an error code (2:'missing epsiode name'), if it matches neither, it gets added to invalid with the 'malformed name' error code.</p> <p>The current code can be found <a href="http://github.com/dbr/checktveps/tree/8a6dc68ad61e684c8d8f0ca1dc37a22d1c51aa82/2checkTvEps.py" rel="nofollow">here</a></p> <p>I want to add a rule that checks for the presence of a folder.jpg file in each directory, but to add this would make the code substantially more messy in it's current state.. </p> <p>How could I write this system in a more expandable way?</p> <p>The rules it needs to check would be..</p> <ul> <li>File is in the format <code>Show Name - [01x23] - Episode Name.avi</code> or <code>Show Name - [01xSpecial02] - Special Name.avi</code> or <code>Show Name - [01xExtra01] - Extra Name.avi</code></li> <li>If filename is in the format <code>Show Name - [01x23].avi</code> display it a 'missing episode name' section of the output</li> <li>The path should be in the format <code>Show Name/season 2/the_file.avi</code> (where season 2 should be the correct season number in the filename)</li> <li>each <code>Show Name/season 1/</code> folder should contain "folder.jpg"</li> </ul> <p>.any ideas? While I'm trying to check TV episodes, this concept/code should be able to apply to many things..</p> <p>The only thought I had was a list of dicts in the format:</p> <pre><code>checker = [ { 'name':'valid files', 'type':'file', 'function':check_valid(), # runs check_valid() on all files 'status':0 # if it returns True, this is the status the file gets } </code></pre>

2008-08-20T22:50:55Z

<blockquote> <p>I want to add a rule that checks for the presence of a folder.jpg file in each directory, but to add this would make the code substantially more messy in it's current state..</p> </blockquote> <p>This doesn't look bad. In fact your current code does it very nicely, and Sven mentioned a good way to do it as well:</p> <ol> <li>Get a list of all the files</li> <li>Check for "required" files</li> </ol> <p>You would just have have add to your dictionary a list of required files:</p> <pre><code>checker = { ... 'required': ['file', 'list', 'for_required'] } </code></pre> <p>As far as there being a better/extensible way to do this? I am not exactly sure. I could only really think of a way to possibly drop the "multiple" regular expressions and build off of Sven's idea for using a delimiter. So my strategy would be defining a dictionary as follows (and I'm sorry I don't know Python syntax and I'm a tad to lazy to look it up but it should make sense. The /regex/ is shorthand for a regex):</p> <pre><code>check_dict = { 'delim' : /\-/, 'parts' : [ 'Show Name', 'Episode Name', 'Episode Number' ], 'patterns' : [/valid name/, /valid episode name/, /valid number/ ], 'required' : ['list', 'of', 'files'], 'ignored' : ['.*', 'hidden.txt'], 'start_dir': '/path/to/dir/to/test/' } </code></pre> <ol> <li>Split the filename based on the delimiter.</li> <li>Check each of the parts.</li> </ol> <p>Because its an ordered list you can determine what parts are missing and if a section doesn't match any pattern it is malformed. Here the <code>parts</code> and <code>patterns</code> have a 1 to 1 ratio. Two arrays instead of a dictionary enforces the order.</p> <p>Ignored and required files can be listed. The <code>.</code> and <code>..</code> files should probably be ignored automatically. The user should be allowed to input "globs" which can be shell expanded. I'm thinking here of <code>svn:ignore</code> properties, but globbing is natural for listing files.</p> <p>Here <code>start_dir</code> would be default to the current directory but if you wanted a single file to run automated testing of a bunch of directories this would be useful.</p> <p>The real loose end here is the path template and along the same lines what path is required for "valid files". I really couldn't come up with a solid idea without writing one large regular expression and taking groups from it... to build a template. It felt a lot like writing a TextMate language grammar. But that starts to stray on the ease of use. The real problem was that the path template was not composed of <code>parts</code>, which makes sense but adds complexity.</p> <p>Is this strategy in tune with what you were thinking of?</p>

2008-08-21T22:02:01Z

Build a Basic Python Iterator

<p>How would one create an iterative function (or iterator object) in python?</p>

2008-08-21T00:36:11Z

<p>First of all the <a href="https://docs.python.org/3/library/itertools.html">itertools module</a> is incredibly useful for all sorts of cases in which an iterator would be useful, but here is all you need to create an iterator in python:</p> <blockquote> <p>yield</p> </blockquote> <p>Isn't that cool? Yield can be used to replace a normal <strong>return</strong> in a function. It returns the object just the same, but instead of destroying state and exiting, it saves state for when you want to execute the next iteration. Here is an example of it in action pulled directly from the <a href="http://docs.python.org/lib/itertools-functions.html">itertools function list</a>:</p> <pre><code> def count(n=0): while True: yield n n += 1 </code></pre> <p>As stated in the functions description (it's the <strong>count()</strong> function from the itertools module...) , it produces an iterator that returns consecutive integers starting with n.</p> <p><a href="https://docs.python.org/2/reference/expressions.html#generator-expressions">Generator expressions</a> are a whole other can of worms (awesome worms!). They may be used in place of a <a href="http://www.secnetix.de/olli/Python/list_comprehensions.hawk">List Comprehension</a> to save memory (list comprehensions create a list in memory that is destroyed after use if not assigned to a variable, but generator expressions can create a Generator Object... which is a fancy way of saying Iterator). Here is an example of a generator expression definition:</p> <pre><code>gen = (n for n in xrange(0,11)) </code></pre> <p>This is very similar to our iterator definition above except the full range is predetermined to be between 0 and 10.</p> <p>I just found <strong>xrange()</strong> (suprised I hadn't seen it before...) and added it to the above example. <strong>xrange()</strong> is an iterable version of <strong>range()</strong> which has the advantage of not prebuilding the list. It would be very useful if you had a giant corpus of data to iterate over and only had so much memory to do it in.</p>

2008-08-21T00:36:33Z

Build a Basic Python Iterator

<p>How would one create an iterative function (or iterator object) in python?</p>

2008-08-21T00:36:11Z

<p>Iterator objects in python conform to the iterator protocol, which basically means they provide two methods: <code>__iter__()</code> and <code>next()</code>. The <code>__iter__</code> returns the iterator object and is implicitly called at the start of loops. The <code>next()</code> method returns the next value and is implicitly called at each loop increment. <code>next()</code> raises a StopIteration exception when there are no more value to return, which is implicitly captured by looping constructs to stop iterating.</p> <p>Here's a simple example of a counter:</p> <pre><code>class Counter: def __init__(self, low, high): self.current = low self.high = high def __iter__(self): return self def next(self): # Python 3: def __next__(self) if self.current &gt; self.high: raise StopIteration else: self.current += 1 return self.current - 1 for c in Counter(3, 8): print c </code></pre> <p>This will print:</p> <pre><code>3 4 5 6 7 8 </code></pre> <p>This is easier to write using a generator, as covered in a previous answer:</p> <pre><code>def counter(low, high): current = low while current &lt;= high: yield current current += 1 for c in counter(3, 8): print c </code></pre> <p>The printed output will be the same. Under the hood, the generator object supports the iterator protocol and does something roughly similar to the class Counter.</p> <p>David Mertz's article, <a href="https://www.ibm.com/developerworks/library/l-pycon/">Iterators and Simple Generators</a>, is a pretty good introduction. </p>

2008-08-23T16:57:28Z

Build a Basic Python Iterator

<p>How would one create an iterative function (or iterator object) in python?</p>

2008-08-21T00:36:11Z

<p>There are four ways to build an iterative function:</p> <ul> <li>create a generator (uses the <a href="http://docs.python.org/py3k/reference/expressions.html#yield-expressions">yield keyword</a>)</li> <li>use a generator expression (<a href="http://docs.python.org/py3k/reference/expressions.html#generator-expressions">genexp</a>)</li> <li>create an iterator (defines <a href="http://docs.python.org/py3k/library/stdtypes.html?highlight=__iter__#iterator-types"><code>__iter__</code> and <code>__next__</code></a> (or <code>next</code> in Python 2.x))</li> <li>create a function that Python can iterate over on its own (<a href="http://docs.python.org/py3k/reference/datamodel.html?highlight=__getitem__#object.__getitem__">defines <code>__getitem__</code></a>)</li> </ul> <p>Examples:</p> <pre><code># generator def uc_gen(text): for char in text: yield char.upper() # generator expression def uc_genexp(text): return (char.upper() for char in text) # iterator protocol class uc_iter(): def __init__(self, text): self.text = text self.index = 0 def __iter__(self): return self def __next__(self): try: result = self.text[self.index].upper() except IndexError: raise StopIteration self.index += 1 return result # getitem method class uc_getitem(): def __init__(self, text): self.text = text def __getitem__(self, index): result = self.text[index].upper() return result </code></pre> <p>To see all four methods in action:</p> <pre><code>for iterator in uc_gen, uc_genexp, uc_iter, uc_getitem: for ch in iterator('abcde'): print ch, print </code></pre> <p>Which results in:</p> <pre><code>A B C D E A B C D E A B C D E A B C D E </code></pre> <p><strong>Note</strong>:</p> <p>The two generator types (<code>uc_gen</code> and <code>uc_genexp</code>) cannot be <code>reversed()</code>; the plain iterator (<code>uc_iter</code>) would need the <code>__reversed__</code> magic method (which must return a new iterator that goes backwards); and the getitem iteratable (<code>uc_getitem</code>) must have the <code>__len__</code> magic method:</p> <pre><code> # for uc_iter def __reversed__(self): return reversed(self.text) # for uc_getitem def __len__(self) return len(self.text) </code></pre> <hr> <p>To answer Colonel Panic's secondary question about an infinite lazily evaluated iterator, here are those examples, using each of the four methods above:</p> <pre><code># generator def even_gen(): result = 0 while True: yield result result += 2 # generator expression def even_genexp(): return (num for num in even_gen()) # or even_iter or even_getitem # not much value under these circumstances # iterator protocol class even_iter(): def __init__(self): self.value = 0 def __iter__(self): return self def __next__(self): next_value = self.value self.value += 2 return next_value # getitem method class even_getitem(): def __getitem__(self, index): return index * 2 import random for iterator in even_gen, even_genexp, even_iter, even_getitem: limit = random.randint(15, 30) count = 0 for even in iterator(): print even, count += 1 if count &gt;= limit: break print </code></pre> <p>Which results in (at least for my sample run):</p> <pre><code>0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 </code></pre>

2011-09-24T22:13:44Z

Build a Basic Python Iterator

<p>How would one create an iterative function (or iterator object) in python?</p>

2008-08-21T00:36:11Z

<p>I see some of you doing <code>return self</code> in <code>__iter__</code>. I just wanted to note that <code>__iter__</code> itself can be a generator (thus removing the need for <code>__next__</code> and raising <code>StopIteration</code> exceptions)</p> <pre><code>class range: def __init__(self,a,b): self.a = a self.b = b def __iter__(self): i = self.a while i &lt; self.b: yield i i+=1 </code></pre> <p>Of course here one might as well directly make a generator, but for more complex classes it can be useful.</p>

2012-07-27T15:05:12Z

Build a Basic Python Iterator

<p>How would one create an iterative function (or iterator object) in python?</p>

2008-08-21T00:36:11Z

<p>This is an iterable function without <code>yield</code>. It make use of the <code>iter</code> function and a closure which keeps it's state in a mutable (<code>list</code>) in the enclosing scope for python 2. </p> <pre><code>def count(low, high): counter = [0] def tmp(): val = low + counter[0] if val &lt; high: counter[0] += 1 return val return None return iter(tmp, None) </code></pre> <p>For Python 3, closure state is kept in an immutable in the enclosing scope and <code>nonlocal</code> is used in local scope to update the state variable. </p> <pre><code>def count(low, high): counter = 0 def tmp(): nonlocal counter val = low + counter if val &lt; high: counter += 1 return val return None return iter(tmp, None) </code></pre> <p>Test;</p> <pre><code>for i in count(1,10): print(i) 1 2 3 4 5 6 7 8 9 </code></pre>

2016-03-03T17:55:26Z

Build a Basic Python Iterator

<p>How would one create an iterative function (or iterator object) in python?</p>

2008-08-21T00:36:11Z

<p>This question is about iterable objects, not about iterators. In Python, sequences are iterable too so one way to make an iterable class is to make it behave like a sequence, i.e. give it <code>__getitem__</code> and <code>__len__</code> methods. I have tested this on Python 2 and 3.</p> <pre><code>class CustomRange: def __init__(self, low, high): self.low = low self.high = high def __getitem__(self, item): if item &gt;= len(self): raise IndexError("CustomRange index out of range") return self.low + item def __len__(self): return self.high - self.low cr = CustomRange(0, 10) for i in cr: print(i) </code></pre>

2016-03-21T17:39:14Z

A Transpose/Unzip Function in Python (inverse of zip)

<p>I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.</p> <p><strong>For example:</strong></p> <pre><code>original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)] # and I want to become... result = (['a', 'b', 'c', 'd'], [1, 2, 3, 4]) </code></pre> <p>Is there a builtin function that does that?</p>

2008-08-21T04:29:07Z

<p><a href="http://docs.python.org/library/functions.html#zip"><code>zip</code></a> is its own inverse! Provided you use the special * operator.</p> <pre><code>&gt;&gt;&gt; zip(*[('a', 1), ('b', 2), ('c', 3), ('d', 4)]) [('a', 'b', 'c', 'd'), (1, 2, 3, 4)] </code></pre> <p>The way this works is by calling <code>zip</code> with the arguments:</p> <pre><code>zip(('a', 1), ('b', 2), ('c', 3), ('d', 4)) </code></pre> <p>… except the arguments are passed to <code>zip</code> directly (after being converted to a tuple), so there's no need to worry about the number of arguments getting too big.</p>

2008-08-21T04:36:29Z

A Transpose/Unzip Function in Python (inverse of zip)

<p>I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.</p> <p><strong>For example:</strong></p> <pre><code>original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)] # and I want to become... result = (['a', 'b', 'c', 'd'], [1, 2, 3, 4]) </code></pre> <p>Is there a builtin function that does that?</p>

2008-08-21T04:29:07Z

<p>You could also do</p> <pre><code>result = ([ a for a,b in original ], [ b for a,b in original ]) </code></pre> <p>It <em>should</em> scale better. Especially if Python makes good on not expanding the list comprehensions unless needed.</p> <p>(Incidentally, it makes a 2-tuple (pair) of lists, rather than a list of tuples, like <code>zip</code> does.)</p> <p>If generators instead of actual lists are ok, this would do that:</p> <pre><code>result = (( a for a,b in original ), ( b for a,b in original )) </code></pre> <p>The generators don't munch through the list until you ask for each element, but on the other hand, they do keep references to the original list.</p>

2008-08-24T17:07:57Z

A Transpose/Unzip Function in Python (inverse of zip)

<p>I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.</p> <p><strong>For example:</strong></p> <pre><code>original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)] # and I want to become... result = (['a', 'b', 'c', 'd'], [1, 2, 3, 4]) </code></pre> <p>Is there a builtin function that does that?</p>

2008-08-21T04:29:07Z

<p>If you have lists that are not the same length, you may not want to use zip as per Patricks answer. This works:</p> <pre><code>&gt;&gt;&gt; zip(*[('a', 1), ('b', 2), ('c', 3), ('d', 4)]) [('a', 'b', 'c', 'd'), (1, 2, 3, 4)] </code></pre> <p>But with different length lists, zip truncates each item to the length of the shortest list:</p> <pre><code>&gt;&gt;&gt; zip(*[('a', 1), ('b', 2), ('c', 3), ('d', 4), ('e', )]) [('a', 'b', 'c', 'd', 'e')] </code></pre> <p>You can use map with no function to fill empty results with None:</p> <pre><code>&gt;&gt;&gt; map(None, *[('a', 1), ('b', 2), ('c', 3), ('d', 4), ('e', )]) [('a', 'b', 'c', 'd', 'e'), (1, 2, 3, 4, None)] </code></pre> <p>zip() is marginally faster though.</p>

2011-01-02T12:14:17Z

A Transpose/Unzip Function in Python (inverse of zip)

<p>I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.</p> <p><strong>For example:</strong></p> <pre><code>original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)] # and I want to become... result = (['a', 'b', 'c', 'd'], [1, 2, 3, 4]) </code></pre> <p>Is there a builtin function that does that?</p>

2008-08-21T04:29:07Z

<p>I like to use <code>zip(*iterable)</code> (which is the piece of code you're looking for) in my programs as so:</p> <pre><code>def unzip(iterable): return zip(*iterable) </code></pre> <p>I find <code>unzip</code> more readable.</p>

2014-03-01T15:00:15Z

A Transpose/Unzip Function in Python (inverse of zip)

<p>I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.</p> <p><strong>For example:</strong></p> <pre><code>original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)] # and I want to become... result = (['a', 'b', 'c', 'd'], [1, 2, 3, 4]) </code></pre> <p>Is there a builtin function that does that?</p>

2008-08-21T04:29:07Z

<p>It's only another way to do it but it helped me a lot so I write it here:</p> <p>Having this data structure:</p> <pre><code>X=[1,2,3,4] Y=['a','b','c','d'] XY=zip(X,Y) </code></pre> <p>Resulting in:</p> <pre><code>In: XY Out: [(1, 'a'), (2, 'b'), (3, 'c'), (4, 'd')] </code></pre> <p>The more pythonic way to unzip it and go back to the original is this one in my opinion:</p> <pre><code>x,y=zip(*XY) </code></pre> <p>But this return a tuple so if you need an array you can use:</p> <pre><code>xy=(list(x),list(y)) </code></pre>

2016-01-26T10:45:23Z

A Transpose/Unzip Function in Python (inverse of zip)

<p>I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.</p> <p><strong>For example:</strong></p> <pre><code>original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)] # and I want to become... result = (['a', 'b', 'c', 'd'], [1, 2, 3, 4]) </code></pre> <p>Is there a builtin function that does that?</p>

2008-08-21T04:29:07Z

<pre><code>&gt;&gt;&gt; original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)] &gt;&gt;&gt; tuple([list(tup) for tup in zip(*original)]) (['a', 'b', 'c', 'd'], [1, 2, 3, 4]) </code></pre> <p>Gives a tuple of lists as in the question.</p> <pre><code>list1, list2 = [list(tup) for tup in zip(*original)] </code></pre> <p>Unpacks the two lists.</p>

2016-03-05T11:08:28Z

Introducing Python

<p>The company I used to work with has two developers working fulltime, and a handful of freelancers. They're in the process of hiring a new lead developer to try to bring order and management to the development.</p> <p>But, currently, one of the developers has seen the light of Django (the company has only developed in PHP to date) while the other developer is concerned that introducing a new language (Python) is a bad idea right now.</p> <p>How should they approach introducing this new technology? Obviously with only one of the developers actually knowing Python, there will be no redundancy when that dev is away or leaves the company.</p> <p>Should they bother to introduce Python, or should they look for PHP-only solutions until such a time when the team actually have more than one Pythonion? Without a team leader, the decisions are having to fall to them.</p>

2008-08-21T11:48:03Z

<p>Well, python is a high level language.. its not hard to learn and if the guys already have programming knowledge it should be much easier to learn.. i like django.. i think it should be a nice try to use django .. </p>

2008-08-21T11:53:38Z

Introducing Python

<p>The company I used to work with has two developers working fulltime, and a handful of freelancers. They're in the process of hiring a new lead developer to try to bring order and management to the development.</p> <p>But, currently, one of the developers has seen the light of Django (the company has only developed in PHP to date) while the other developer is concerned that introducing a new language (Python) is a bad idea right now.</p> <p>How should they approach introducing this new technology? Obviously with only one of the developers actually knowing Python, there will be no redundancy when that dev is away or leaves the company.</p> <p>Should they bother to introduce Python, or should they look for PHP-only solutions until such a time when the team actually have more than one Pythonion? Without a team leader, the decisions are having to fall to them.</p>

2008-08-21T11:48:03Z

<p>If the mandate of the new lead is to put the house in order, the current situation should likely be simplified as much as possible prior. If I had to bring things to order, I wouldn't want to have to manage an ongoing language conversion project on top of everything else, or at least I'd like some choice when initiating the project. When making your recommendation, did you think about the additional managerial complexity that coming into the middle of a conversion would entail?</p>

2008-08-21T11:56:00Z

Introducing Python

<p>The company I used to work with has two developers working fulltime, and a handful of freelancers. They're in the process of hiring a new lead developer to try to bring order and management to the development.</p> <p>But, currently, one of the developers has seen the light of Django (the company has only developed in PHP to date) while the other developer is concerned that introducing a new language (Python) is a bad idea right now.</p> <p>How should they approach introducing this new technology? Obviously with only one of the developers actually knowing Python, there will be no redundancy when that dev is away or leaves the company.</p> <p>Should they bother to introduce Python, or should they look for PHP-only solutions until such a time when the team actually have more than one Pythonion? Without a team leader, the decisions are having to fall to them.</p>

2008-08-21T11:48:03Z

<p>I don't think it's a matter of a programming language as such. </p> <p>What is the proficiency level of PHP in the team you're talking about? Are they doing spaghetti code or using some structured framework like Zend? If this is the first case then I absolutely understand the guy's interest in Python and Django. It this is the latter, it's just a hype.</p>

2008-08-21T12:03:43Z

Introducing Python

<p>The company I used to work with has two developers working fulltime, and a handful of freelancers. They're in the process of hiring a new lead developer to try to bring order and management to the development.</p> <p>But, currently, one of the developers has seen the light of Django (the company has only developed in PHP to date) while the other developer is concerned that introducing a new language (Python) is a bad idea right now.</p> <p>How should they approach introducing this new technology? Obviously with only one of the developers actually knowing Python, there will be no redundancy when that dev is away or leaves the company.</p> <p>Should they bother to introduce Python, or should they look for PHP-only solutions until such a time when the team actually have more than one Pythonion? Without a team leader, the decisions are having to fall to them.</p>

2008-08-21T11:48:03Z

<p>@darkdog:</p> <p>Using a new language in production code is about more than easy syntax and high-level capability. You want to be familiar with core APIs and feel like you can fix something through logic instead of having to comb through the documentation.</p> <p>I'm not saying transitioning to Python would be a bad idea for this company, but I'm with John--keep things simple during the transition. The new lead will appreciate having a say in such decisions.</p> <p>If you'd really, really, really like to introduce Python, consider writing some extensions or utilities in straight-up Python or in the framework. You won't be upsetting your core initiatives, so it will be a low/no-risk opportunity to prove the merits of a switch.</p>

2008-08-21T12:09:46Z

Introducing Python

<p>The company I used to work with has two developers working fulltime, and a handful of freelancers. They're in the process of hiring a new lead developer to try to bring order and management to the development.</p> <p>But, currently, one of the developers has seen the light of Django (the company has only developed in PHP to date) while the other developer is concerned that introducing a new language (Python) is a bad idea right now.</p> <p>How should they approach introducing this new technology? Obviously with only one of the developers actually knowing Python, there will be no redundancy when that dev is away or leaves the company.</p> <p>Should they bother to introduce Python, or should they look for PHP-only solutions until such a time when the team actually have more than one Pythonion? Without a team leader, the decisions are having to fall to them.</p>

2008-08-21T11:48:03Z

<p>I think the language itself is not an issue here, as python is really nice high level language with good and easy to find, thorough documentation.</p> <p>From what I've seen, the Django framework is also a great tooklit for web development, giving much the same developer performance boost Rails is touted to give.</p> <p>The real issue is at the maintenance and management level.</p> <p>How will this move fragment the maintenance between PHP and Python code. Is there a need to migrate existing code from one platform to another? What problems will adopting Python and Django solve that you have in your current development workflow and frameworks, etc.</p>

2008-08-21T12:13:51Z