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FLEX is a benchmark and framework for unified, rigorous few-shot NLP evaluation. FLEX enables:

  • First-class NLP support
  • Support for meta-training
  • Reproducible fewshot evaluations
  • Extensible benchmark creation (benchmarks defined using HuggingFace Datasets)
  • Advanced sampling functions for creating episodes with class imbalance, etc.

Together with FLEX, we also released a simple yet strong few-shot model called UniFew.

For more context and details, see our NeurIPS 2021 paper: FLEX: Unifying Evaluation for Few-Shot NLP.


These instructions are geared towards users of the first benchmark created with this framework. The benchmark has two leaderboards, for the Pretraining-Only and Meta-Trained protocols described in Section 4.2 of our paper:

  • FLEX (Pretraining-Only): for models that do not use meta-training data related to the test tasks (do not follow the Model Training section below).
  • FLEX-META (Meta-Trained): for models that use only the provided meta-training and meta-validation data (please do see the Model Training section below).


  • Clone the repository: git clone
  • Create a Python 3 environment (3.7 or greater), eg using conda create --name flex python=3.9
  • Activate the environment: conda activate flex
  • Install the package locally with pip install -r requirements.txt && pip install -e .

Data Preparation

Creating the data for the flex challenge for the first time takes about 10 minutes (using a recent Macbook Pro on a broadband connection) and requires 3GB of disk space. You can initiate this process by running

python -c "import fewshot; fewshot.make_challenge('flex');"

You can control the location of the cached data by setting the environment variable HF_DATASETS_CACHE. If you have not set this variable, the location should default to ~/.cache/huggingface/datasets/. See the HuggingFace docs for more details.

Model Evaluation

"Challenges" are datasets of sampled tasks for evaluation. They are defined in fewshot/challenges/

To evaluate a model on challenge flex (our first challenge), you should write a program that produces a predictions.json, for example:

#!/usr/bin/env python3
import random
from typing import Iterable, Dict, Any, Sequence
import fewshot

class YourModel(fewshot.Model):
    def fit_and_predict(
        support_x: Iterable[Dict[str, Any]],
        support_y: Iterable[str],
        target_x: Iterable[Dict[str, Any]],
        metadata: Dict[str, Any]
    ) -> Sequence[str]:
        """Return random label predictions for a fewshot task."""
        train_x = [d['txt'] for d in support_x]
        train_y = support_y
        test_x = [d['txt'] for d in target_x]
        test_y = [random.choice(metadata['labels']) for _ in test_x]
        # >>> print(test_y)
        # ['some', 'list', 'of', 'label', 'predictions']
        return test_y

if __name__ == '__main__':
    evaluator = fewshot.make_challenge("flex")
    model = YourModel()
    evaluator.save_model_predictions(model=model, save_path='/path/to/predictions.json')

Warning: Calling fewshot.make_challenge("flex") above requires some time to prepare all the necessary data (see "Data preparation" section).

Running the above script produces /path/to/predictions.json with contents formatted as:

    "[QUESTION_ID]": {
        "label": "[CLASS_LABEL]",  # Currently an integer converted to a string
        "score": float  # Only used for ranking tasks

Each [QUESTION_ID] is an ID for a test example in a few-shot problem.

[Optional] Parallelizing Evaluation

Two options are available for parallelizing evaluation.

First, one can restrict evaluation to a subset of tasks with indices from [START] to [STOP] (exclusive) via

evaluator.save_model_predictions(model=model, start_task_index=[START], stop_task_index=[STOP])


  • You may use stop_task_index=None (or omit it) to avoid specifying an end.
  • You can find the total number of tasks in the challenge with fewshot.get_challenge_spec([CHALLENGE]).num_tasks.
  • To merge partial evaluation outputs into a complete predictions.json file, use fewshot merge partial1.json partial2.json ... predictions.json.

The second option will call your model's .fit_and_predict() method with batches of [BATCH_SIZE] tasks, via

evaluator.save_model_predictions(model=model, batched=True, batch_size=[BATCH_SIZE])

Result Validation and Scoring

To validate the contents of your predictions, run:

fewshot validate --challenge_name flex --predictions /path/to/predictions.json

This validates all the inputs and takes some time. Substitute flex for another challenge to evaluate on a different challenge.

(There is also a score CLI command which should not be used on the final challenge except when reporting final results.)

Model Training

For the meta-training protocol (e.g., the FLEX-META leaderboard), challenges come with a set of related training and validation data. This data is most easily accessible in one of two formats:

  1. Iterable from sampled episodes. fewshot.get_challenge_spec('flex').get_sampler(split='[SPLIT]') returns an iterable that samples datasets and episodes from meta-training or meta-validation datasets, via [SPLIT]='train' or [SPLIT]='val', respectively. The sampler defaults to the fewshot.samplers.Sample2WayMax8ShotCfg sampler configuration (for the fewshot.samplers.sample.Sampler class), but can be reconfigured.

  2. Raw dataset stores. This option is for directly accessing the raw data. fewshot.get_challenge_spec('flex').get_stores(split='[SPLIT']) returns a mapping from dataset names to instances. Each Store instance has a attribute containing a raw HuggingFace Dataset instance. The Store instance has a Store.label attribute with the Dataset object key for accessing the target label (e.g., via[Store.label]) and the FLEX-formatted text available at the flex.txt key (e.g., via['flex.txt']).

Two examples of these respective approaches are available at:

  1. The UniFew model repository. For more details on Unifew, see also the FLEX NeurIPS 2021 paper.
  2. The baselines/bao/ directory, for training and evaluating the approach described in the following paper:

Yujia Bao*, Menghua Wu*, Shiyu Chang, and Regina Barzilay. Few-shot Text Classification with Distributional Signatures. In International Conference on Learning Representations 2020

Benchmark Construction and Optimization

To add a new benchmark (challenge) named [NEW_CHALLENGE], you must edit fewshot/challenges/ or otherwise add it to the registry. The above usage instructions would change to substitute [NEW_CHALLENGE] in place of flex when calling fewshot.get_challenge_spec('[NEW_CHALLENGE]') and fewshot.make_challenge('[NEW_CHALLENGE]').

For an example of how to optimize the sample size of the challenge, see scripts/


If you make use of our framework, benchmark, or model, please cite our NeurIPS 2021 paper:

      title={FLEX: Unifying Evaluation for Few-Shot NLP},
      author={Jonathan Bragg and Arman Cohan and Kyle Lo and Iz Beltagy},
      booktitle={NeurIPS 2021}


Few-shot NLP benchmark for unified, rigorous eval







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