Update app.py

app.py

@@ -198,10 +198,12 @@ def merge_lines(doc1, doc2, doc3, doc4):
     docs = [doc1, doc2, doc3, doc4]
     merged_lines = []
 
-    with open("merged_lines.txt", "w") as f, *map(open, docs) as handles:
-        for lines in zip(*handles):
+    with open(doc1, "r") as f1, open(doc2, "r") as f2, \
+            open(doc3, "r") as f3, open(doc4, "r") as f4, \
+            open("merged_lines.txt", "w") as f_out:
+        for lines in zip(f1, f2, f3, f4):
             merged_line = "".join(line.strip() for line in lines)
-            f.write(merged_line + "\n")
+            f_out.write(merged_line + "\n")
             merged_lines.append(merged_line)
 
     return merged_lines