Adding a Custom Task

To add a new task, first either open an issue, to determine whether it will be integrated in the core evaluations of lighteval, in the extended tasks, or the community tasks, and add its dataset on the hub.

• Core evaluations are evaluations that only require standard logic in their metrics and processing, and that we will add to our test suite to ensure non regression through time. They already see high usage in the community.
• Extended evaluations are evaluations that require custom logic in their metrics (complex normalisation, an LLM as a judge, …), that we added to facilitate the life of users. They already see high usage in the community.
• Community evaluations are submissions by the community of new tasks.

A popular community evaluation can move to become an extended or core evaluation over time.

Step by step creation of a custom task

First, create a python file under the community_tasks directory.

You need to define a prompt function that will convert a line from your dataset to a document to be used for evaluation.

# Define as many as you need for your different tasks
def prompt_fn(line, task_name: str = None):
    """Defines how to go from a dataset line to a doc object.
    Follow examples in src/lighteval/tasks/default_prompts.py, or get more info
    about what this function should do in the README.
    """
    return Doc(
        task_name=task_name,
        query=line["question"],
        choices=[f" {c}" for c in line["choices"]],
        gold_index=line["gold"],
        instruction="",
    )

Then, you need to choose a metric: you can either use an existing one (defined in lighteval.metrics.metrics.Metrics) or create a custom one).

custom_metric = SampleLevelMetric(
    metric_name="my_custom_metric_name",
    higher_is_better=True,
    category=MetricCategory.IGNORED,
    use_case=MetricUseCase.NONE,
    sample_level_fn=lambda x: x,  # how to compute score for one sample
    corpus_level_fn=np.mean,  # How to aggreagte the samples metrics
)

Then, you need to define your task using LightevalTaskConfig. You can define a task with or without subsets. To define a task with no subsets:

# This is how you create a simple task (like hellaswag) which has one single subset
# attached to it, and one evaluation possible.
task = LightevalTaskConfig(
    name="myothertask",
    prompt_function=prompt_fn,  # must be defined in the file or imported from src/lighteval/tasks/tasks_prompt_formatting.py
    suite=["community"],
    hf_repo="",
    hf_subset="default",
    hf_avail_splits=[],
    evaluation_splits=[],
    few_shots_split=None,
    few_shots_select=None,
    metric=[],  # select your metric in Metrics
)

If you want to create a task with multiple subset, add them to the SAMPLE_SUBSETS list and create a task for each subset.

SAMPLE_SUBSETS = []  # list of all the subsets to use for this eval


class CustomSubsetTask(LightevalTaskConfig):
    def __init__(
        self,
        name,
        hf_subset,
    ):
        super().__init__(
            name=name,
            hf_subset=hf_subset,
            prompt_function=prompt_fn,  # must be defined in the file or imported from src/lighteval/tasks/tasks_prompt_formatting.py
            hf_repo="",
            metric=[custom_metric],  # select your metric in Metrics or use your custom_metric
            hf_avail_splits=[],
            evaluation_splits=[],
            few_shots_split=None,
            few_shots_select=None,
            suite=["community"],
            generation_size=-1,
            stop_sequence=None,
        )
SUBSET_TASKS = [CustomSubsetTask(name=f"mytask:{subset}", hf_subset=subset) for subset in SAMPLE_SUBSETS]

Here is a list of the parameters and their meaning:

• name (str), your evaluation name
• suite (list), the suite(s) to which your evaluation should belong. This field allows us to compare different task implementations and is used as a task selection to differentiate the versions to launch. At the moment, you’ll find the keywords [“helm”, “bigbench”, “original”, “lighteval”, “community”, “custom”]; for core evals, please choose lighteval.
• prompt_function (Callable), the prompt function you defined in the step above
• hf_repo (str), the path to your evaluation dataset on the hub
• hf_subset (str), the specific subset you want to use for your evaluation (note: when the dataset has no subset, fill this field with "default", not with None or "")
• hf_avail_splits (list), all the splits available for your dataset (train, valid or validation, test, other…)
• evaluation_splits (list), the splits you want to use for evaluation
• few_shots_split (str, can be null), the specific split from which you want to select samples for your few-shot examples. It should be different from the sets included in evaluation_splits
• few_shots_select (str, can be null), the method that you will use to select items for your few-shot examples. Can be null, or one of:
  • balanced select examples from the few_shots_split with balanced labels, to avoid skewing the few shot examples (hence the model generations) toward one specific label
  • random selects examples at random from the few_shots_split
  • random_sampling selects new examples at random from the few_shots_split for every new item, but if a sampled item is equal to the current one, it is removed from the available samples
  • random_sampling_from_train selects new examples at random from the few_shots_split for every new item, but if a sampled item is equal to the current one, it is kept! Only use this if you know what you are doing.
  • sequential selects the first n examples of the few_shots_split
• generation_size (int), the maximum number of tokens allowed for a generative evaluation. If your evaluation is a log likelihood evaluation (multi-choice), this value should be -1
• stop_sequence (list), a list of strings acting as end of sentence tokens for your generation
• metric (list), the metrics you want to use for your evaluation (see next section for a detailed explanation)
• trust_dataset (bool), set to True if you trust the dataset.

Then you need to add your task to the TASKS_TABLE list.

# STORE YOUR EVALS

# tasks with subset:
TASKS_TABLE = SUBSET_TASKS

# tasks without subset:
# TASKS_TABLE = [task]

Once your file is created you can then run the evaluation with the following command:

lighteval accelerate \
    "pretrained=HuggingFaceH4/zephyr-7b-beta" \
    "community|{custom_task}|{fewshots}|{truncate_few_shot}" \
    --custom-tasks {path_to_your_custom_task_file}