Writing a metric loading script

If you want to use your own metric, or if you would like to share a new metric with the community, for instance in the HuggingFace Hub, then you can define a new metric loading script.

This chapter will explain how metrics are loaded and how you can write from scratch or adapt a metric loading script.


You can start from the template for a metric loading script when writing a new metric loading script. You can find this template in the templates folder on the github repository.

To create a new metric loading script one mostly needs to specify three methods in a datasets.Metric class:

  • datasets.Metric._info() which is in charge of specifying the metric metadata as a datasets.MetricInfo dataclass and in particular the datasets.Features which defined the types of the predictions and the references,

  • datasets.Metric._compute() which is in charge of computing the actual score(s), given some predictions and references.


Note on naming: the metric class should be camel case, while the metric name is its snake case equivalent (ex: class Rouge(datasets.Metric) for the metric rouge).

Adding metric metadata

The datasets.Metric._info() method is in charge of specifying the metric metadata as a datasets.MetricInfo dataclass and in particular the datasets.Features which defined the types of the predictions and the references. datasets.MetricInfo has a predefined set of attributes and cannot be extended. The full list of attributes can be found in the package reference.

The most important attributes to specify are:

  • datasets.MetricInfo.features: a datasets.Features instance defining the name and the type the predictions and references,

  • datasets.MetricInfo.description: a str describing the metric,

  • datasets.MetricInfo.citation: a str containing the citation for the metric in a BibTex format for inclusion in communications citing the metric,

  • datasets.MetricInfo.homepage: a str containing an URL to an original homepage of the metric.

  • datasets.MetricInfo.format: an optional str to tell what is the format of the predictions and the references passed to _compute. It can be set to “numpy”, “torch”, “tensorflow” or “pandas”.

Here is for instance the datasets.Metric._info() for the Sacrebleu metric for instance, which is taken from the sacrebleu metric loading script

def _info(self):
    return datasets.MetricInfo(
            'predictions': datasets.Value('string'),
            'references': datasets.Sequence(datasets.Value('string')),

The datasets.Features define the type of the predictions and the references and can define arbitrary nested objects with fields of various types. More details on the available features can be found in the guide on features Dataset features and in the package reference on datasets.Features. Many examples of features can also be found in the various metric scripts provided on the GitHub repository and even in dataset scripts provided on the GitHub repository or directly inspected on the datasets viewer.

Here are the features of the SQuAD metric for instance, which is taken from the squad metric loading script:

    'predictions': datasets.Value('string'),
    'references': datasets.Sequence(datasets.Value('string')),

We can see that each prediction is a string, and each reference is a sequence of strings. Indeed we can use the metric the following way:

>>> import datasets

>>> metric = datasets.load_metric('./metrics/sacrebleu')
>>> reference_batch = [['The dog bit the man.', 'The dog had bit the man.'],
...                    ['It was not unexpected.', 'No one was surprised.'],
...                    ['The man bit him first.', 'The man had bitten the dog.']]
>>> sys_batch = ['The dog bit the man.', "It wasn't surprising.", 'The man had just bitten him.']
>>> score = metric.add_batch(predictions=sys_batch, references=reference_batch)
>>> print(metric)

Downloading data files

The datasets.Metric._download_and_prepare() method is in charge of downloading (or retrieving locally the data files) if needed.

This method takes as input a datasets.DownloadManager which is a utility which can be used to download files (or to retrieve them from the local filesystem if they are local files or are already in the cache).

Let’s have a look at a simple example of a datasets.Metric._download_and_prepare() method. We’ll take the example of the bleurt metric loading script:

def _download_and_prepare(self, dl_manager):

    # check that config name specifies a valid BLEURT model
    if self.config_name not in CHECKPOINT_URLS.keys():
        raise KeyError(f"{self.config_name} model not found. You should supply the name of a model checkpoint for bleurt in {CHECKPOINT_URLS.keys()}")

    # download the model checkpoint specified by self.config_name and set up the scorer
    model_path = dl_manager.download_and_extract(CHECKPOINT_URLS[self.config_name])
    self.scorer = score.BleurtScorer(os.path.join(model_path, self.config_name))

As you can see this method downloads a model checkpoint depending of the configuration name of the metric. The checkpoint url is then provided to the datasets.DownloadManager.download_and_extract() method which will take care of downloading or retrieving the file from the local file system and returning a object of the same type and organization (here a just one path, but it could be a list or a dict of paths) with the path to the local version of the requested files. datasets.DownloadManager.download_and_extract() can take as input a single URL/path or a list or dictionary of URLs/paths and will return an object of the same structure (single URL/path, list or dictionary of URLs/paths) with the path to the local files. This method also takes care of extracting compressed tar, gzip and zip archives.

datasets.DownloadManager.download_and_extract() can download files from a large set of origins but if your data files are hosted on a special access server, it’s also possible to provide a callable which will take care of the downloading process to the DownloadManager using datasets.DownloadManager.download_custom().


In addition to datasets.DownloadManager.download_and_extract() and datasets.DownloadManager.download_custom(), the datasets.DownloadManager class also provide more fine-grained control on the download and extraction process through several methods including: datasets.DownloadManager.download(), datasets.DownloadManager.extract() and datasets.DownloadManager.iter_archive(). Please refer to the package reference on datasets.DownloadManager for details on these methods.

Computing the scores

The datasets.DatasetBuilder._compute() is in charge of computing the metric scores given predictions and references that are in the format specified in the features set in datasets.DatasetBuilder._info().

Here again, let’s take the simple example of the xnli metric loading script:

def simple_accuracy(preds, labels):
    return (preds == labels).mean()

class Xnli(datasets.Metric):
    def _info(self):
        return datasets.MetricInfo(
                'predictions': datasets.Value('int64' if self.config_name != 'sts-b' else 'float32'),
                'references': datasets.Value('int64' if self.config_name != 'sts-b' else 'float32'),

    def _compute(self, predictions, references):
        return {"accuracy": simple_accuracy(predictions, references)}

Here to compute the accuracy it uses the simple_accuracy function, that uses numpy to compute the accuracy using .mean()

The predictions and references objects passes to _compute are sequences of integers or floats, and the sequences are formated as numpy arrays since the format specified in the datasets.MetricInfo object is set to “numpy”.

Specifying several metric configurations

Sometimes you want to provide several ways of computing the scores.

It is possible to gave different configurations for a metric. The configuration name is stored in datasets.Metric.config_name attribute. The configuration name can be specified by the user when instantiating a metric:

>>> from datasets import load_metric
>>> metric = load_metric('bleurt', name='bleurt-base-128')
>>> metric = load_metric('bleurt', name='bleurt-base-512')

Here depending on the configuration name, a different checkpoint will be downloaded and used to compute the BLEURT score.

You can access datasets.Metric.config_name from inside datasets.Metric._info(), datasets.Metric._download_and_prepare() and datasets.Metric._compute()

Testing the metric loading script

Once you’re finished with creating or adapting a metric loading script, you can try it locally by giving the path to the metric loading script:

>>> from datasets import load_metric
>>> metric = load_metric('PATH/TO/MY/SCRIPT.py')

If your metric has several configurations you can use the arguments of datasets.load_metric() accordingly:

>>> from datasets import load_metric
>>> metric = load_metric('PATH/TO/MY/SCRIPT.py', 'my_configuration')