metrics: add BLEU (Lightning-AI#2535)

* metrics: added bleu score and test bleu

* metrics: fixed type hints in bleu

* bleu score moved to metrics/functional/nlp.py

* refactor with torch.Tensor

* Update test_sequence.py

* refactor as Borda requests and nltk==3.2

* locked nltk==3.3

* nltk>=3.3, parametrized smooth argument for test

* fix bleu_score example

* added class BLEUScore metrics and test

* added class BLEUScore metrics and test

* update CHANGELOG

* refactor with torchtext

* torchtext changed to optional import

* fix E501 line too long

* add else: in optional import

* remove pragma: no-cover

* constants changed to CAPITALS

* remove class in tests

* List -> Sequence, conda -> pip, cast with tensor

* add torchtext in test.txt

* remove torchtext from test.txt

* bump torchtext to 0.5.0

* bump torchtext to 0.5.0

* Apply suggestions from code review

* ignore bleu score in doctest, renamed to nlp.py

* back to implementation with torch

* remove --ignore in CI test, proper reference format

* apply justus comment

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

Author: Jeff Yang

CHANGELOG.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+- Added BLEU metrics ([#2535](https://github.com/PyTorchLightning/pytorch-lightning/pull/2535))
 
 ### Changed
 

docs/source/metrics.rst

@@ -28,7 +28,7 @@ Example::
 
 .. warning::
     The metrics package is still in development! If we're missing a metric or you find a mistake, please send a PR!
-    to a few metrics. Please feel free to create an issue/PR if you have a proposed  metric or have found a bug.
+    to a few metrics. Please feel free to create an issue/PR if you have a proposed metric or have found a bug.
 
 ----------------
 
@@ -73,7 +73,7 @@ Here's an example showing how to implement a NumpyMetric
     class RMSE(NumpyMetric):
         def forward(self, x, y):
             return np.sqrt(np.mean(np.power(x-y, 2.0)))
-        
+
 
 .. autoclass:: pytorch_lightning.metrics.metric.NumpyMetric
     :noindex:
@@ -138,6 +138,12 @@ AUROC
 .. autoclass:: pytorch_lightning.metrics.classification.AUROC
     :noindex:
 
+BLEUScore
+^^^^^^^^^
+
+.. autoclass:: pytorch_lightning.metrics.nlp.BLEUScore
+    :noindex:
+
 ConfusionMatrix
 ^^^^^^^^^^^^^^^
 
@@ -283,6 +289,12 @@ average_precision (F)
 .. autofunction:: pytorch_lightning.metrics.functional.average_precision
     :noindex:
 
+bleu_score (F)
+^^^^^^^^^^^^^^
+
+.. autofunction:: pytorch_lightning.metrics.functional.bleu_score
+    :noindex:
+
 confusion_matrix (F)
 ^^^^^^^^^^^^^^^^^^^^
 
@@ -418,22 +430,22 @@ to_onehot (F)
 
 Sklearn interface
 -----------------
-    
-Lightning supports `sklearns metrics module <https://scikit-learn.org/stable/modules/classes.html#module-sklearn.metrics>`_ 
-as a backend for calculating metrics. Sklearns metrics are well tested and robust, 
+
+Lightning supports `sklearns metrics module <https://scikit-learn.org/stable/modules/classes.html#module-sklearn.metrics>`_
+as a backend for calculating metrics. Sklearns metrics are well tested and robust,
 but requires conversion between pytorch and numpy thus may slow down your computations.
 
 To use the sklearn backend of metrics simply import as
 
 .. code-block:: python
-    
+
     import pytorch_lightning.metrics.sklearns import plm
     metric = plm.Accuracy(normalize=True)
     val = metric(pred, target)
-    
-Each converted sklearn metric comes has the same interface as its 
-original counterpart (e.g. accuracy takes the additional `normalize` keyword). 
-Like the native Lightning metrics, these converted sklearn metrics also come 
+
+Each converted sklearn metric comes has the same interface as its
+original counterpart (e.g. accuracy takes the additional `normalize` keyword).
+Like the native Lightning metrics, these converted sklearn metrics also come
 with built-in distributed (ddp) support.
 
 SklearnMetric (sk)
@@ -460,7 +472,7 @@ AveragePrecision (sk)
 .. autofunction:: pytorch_lightning.metrics.sklearns.AveragePrecision
     :noindex:
 
-    
+
 ConfusionMatrix (sk)
 ^^^^^^^^^^^^^^^^^^^^
 

environment.yml

@@ -30,6 +30,7 @@ dependencies:
     - twine==1.13.0
     - pillow<7.0.0
     - scikit-image
+    - nltk>=3.3
 
     # Optional
     - scipy>=0.13.3

pytorch_lightning/metrics/__init__.py

@@ -5,7 +5,7 @@
     MSE,
     PSNR,
     RMSE,
-    RMSLE
+    RMSLE,
 )
 from pytorch_lightning.metrics.classification import (
     Accuracy,
@@ -28,30 +28,32 @@
     PrecisionRecallCurve,
     SklearnMetric,
 )
+from pytorch_lightning.metrics.nlp import BLEUScore
 
 __classification_metrics = [
-    'AUC',
-    'AUROC',
-    'Accuracy',
-    'AveragePrecision',
-    'ConfusionMatrix',
-    'DiceCoefficient',
-    'F1',
-    'FBeta',
-    'MulticlassPrecisionRecall',
-    'MulticlassROC',
-    'Precision',
-    'PrecisionRecall',
-    'PrecisionRecallCurve',
-    'ROC',
-    'Recall',
-    'IoU',
+    "AUC",
+    "AUROC",
+    "Accuracy",
+    "AveragePrecision",
+    "ConfusionMatrix",
+    "DiceCoefficient",
+    "F1",
+    "FBeta",
+    "MulticlassPrecisionRecall",
+    "MulticlassROC",
+    "Precision",
+    "PrecisionRecall",
+    "PrecisionRecallCurve",
+    "ROC",
+    "Recall",
+    "IoU",
 ]
 __regression_metrics = [
-    'MAE',
-    'MSE',
-    'PSNR',
-    'RMSE',
-    'RMSLE'
+    "MAE",
+    "MSE",
+    "PSNR",
+    "RMSE",
+    "RMSLE",
 ]
-__all__ = __regression_metrics + __classification_metrics + ['SklearnMetric']
+__sequence_metrics = ["BLEUScore"]
+__all__ = __regression_metrics + __classification_metrics + ["SklearnMetric"] + __sequence_metrics

pytorch_lightning/metrics/functional/__init__.py

@@ -25,5 +25,6 @@
     mse,
     psnr,
     rmse,
-    rmsle
+    rmsle,
 )
+from pytorch_lightning.metrics.functional.nlp import bleu_score

pytorch_lightning/metrics/functional/nlp.py

@@ -0,0 +1,92 @@
+# referenced from
+# Library Name: torchtext
+# Authors: torchtext authors and @sluks
+# Date: 2020-07-18
+# Link: https://pytorch.org/text/_modules/torchtext/data/metrics.html#bleu_score
+from typing import Sequence, List
+from collections import Counter
+
+import torch
+
+
+def _count_ngram(ngram_input_list: List[str], n_gram: int) -> Counter:
+    """Counting how many times each word appears in a given text with ngram
+
+    Args:
+        ngram_input_list: A list of translated text or reference texts
+        n_gram: gram value ranged 1 to 4
+
+    Return:
+        ngram_counter: a collections.Counter object of ngram
+    """
+
+    ngram_counter = Counter()
+
+    for i in range(1, n_gram + 1):
+        for j in range(len(ngram_input_list) - i + 1):
+            ngram_key = tuple(ngram_input_list[j : i + j])
+            ngram_counter[ngram_key] += 1
+
+    return ngram_counter
+
+
+def bleu_score(
+    translate_corpus: Sequence[str], reference_corpus: Sequence[str], n_gram: int = 4, smooth: bool = False
+) -> torch.Tensor:
+    """Calculate BLEU score of machine translated text with one or more references.
+
+    Args:
+        translate_corpus: An iterable of machine translated corpus
+        reference_corpus: An iterable of iterables of reference corpus
+        n_gram: Gram value ranged from 1 to 4 (Default 4)
+        smooth: Whether or not to apply smoothing – Lin et al. 2004
+
+    Return:
+        A Tensor with BLEU Score
+
+    Example:
+
+        >>> translate_corpus = ['the cat is on the mat'.split()]
+        >>> reference_corpus = [['there is a cat on the mat'.split(), 'a cat is on the mat'.split()]]
+        >>> bleu_score(translate_corpus, reference_corpus)
+        tensor(0.7598)
+    """
+
+    assert len(translate_corpus) == len(reference_corpus)
+    numerator = torch.zeros(n_gram)
+    denominator = torch.zeros(n_gram)
+    precision_scores = torch.zeros(n_gram)
+    c = 0.0
+    r = 0.0
+    for (translation, references) in zip(translate_corpus, reference_corpus):
+        c += len(translation)
+        ref_len_list = [len(ref) for ref in references]
+        ref_len_diff = [abs(len(translation) - x) for x in ref_len_list]
+        r += ref_len_list[ref_len_diff.index(min(ref_len_diff))]
+        translation_counter = _count_ngram(translation, n_gram)
+        reference_counter = Counter()
+        for ref in references:
+            reference_counter |= _count_ngram(ref, n_gram)
+
+        ngram_counter_clip = translation_counter & reference_counter
+        for counter_clip in ngram_counter_clip:
+            numerator[len(counter_clip) - 1] += ngram_counter_clip[counter_clip]
+
+        for counter in translation_counter:
+            denominator[len(counter) - 1] += translation_counter[counter]
+
+    trans_len = torch.tensor(c)
+    ref_len = torch.tensor(r)
+    if min(numerator) == 0.0:
+        return torch.tensor(0.0)
+
+    if smooth:
+        precision_scores = torch.add(numerator, torch.ones(n_gram)) / torch.add(denominator, torch.ones(n_gram))
+    else:
+        precision_scores = numerator / denominator
+    log_precision_scores = torch.tensor([1.0 / n_gram] * n_gram) * torch.log(precision_scores)
+    geometric_mean = torch.exp(torch.sum(log_precision_scores))
+    brevity_penalty = torch.tensor(1.0) if c > r else torch.exp(1 - (ref_len / trans_len))
+    bleu = brevity_penalty * geometric_mean
+
+    return bleu

pytorch_lightning/metrics/nlp.py

@@ -0,0 +1,46 @@
+import torch
+
+from pytorch_lightning.metrics.functional.nlp import bleu_score
+from pytorch_lightning.metrics.metric import Metric
+
+
+class BLEUScore(Metric):
+    """
+    Calculate BLEU score of machine translated text with one or more references.
+
+    Example:
+
+        >>> translate_corpus = ['the cat is on the mat'.split()]
+        >>> reference_corpus = [['there is a cat on the mat'.split(), 'a cat is on the mat'.split()]]
+        >>> metric = BLEUScore()
+        >>> metric(translate_corpus, reference_corpus)
+        tensor(0.7598)
+    """
+
+    def __init__(self, n_gram: int = 4, smooth: bool = False):
+        """
+        Args:
+            n_gram: Gram value ranged from 1 to 4 (Default 4)
+            smooth: Whether or not to apply smoothing – Lin et al. 2004
+        """
+        super().__init__(name="bleu")
+        self.n_gram = n_gram
+        self.smooth = smooth
+
+    def forward(self, translate_corpus: list, reference_corpus: list) -> torch.Tensor:
+        """
+        Actual metric computation
+
+        Args:
+            translate_corpus: An iterable of machine translated corpus
+            reference_corpus: An iterable of iterables of reference corpus
+
+        Return:
+            torch.Tensor: BLEU Score
+        """
+        return bleu_score(
+            translate_corpus=translate_corpus,
+            reference_corpus=reference_corpus,
+            n_gram=self.n_gram,
+            smooth=self.smooth,
+        ).to(self.device, self.dtype)

requirements/extra.txt

@@ -11,4 +11,4 @@ horovod>=0.19.1
 omegaconf>=2.0.0
 # scipy>=0.13.3
 scikit-learn>=0.20.0
-torchtext>=0.3.1
+torchtext>=0.3.1

requirements/test.txt

@@ -12,3 +12,4 @@ black==19.10b0
 pre-commit>=1.0
 
 cloudpickle>=1.2
+nltk>=3.3

tests/metrics/functional/test_nlp.py

@@ -0,0 +1,66 @@
+import pytest
+import torch
+from nltk.translate.bleu_score import SmoothingFunction, corpus_bleu, sentence_bleu
+
+from pytorch_lightning.metrics.functional.nlp import bleu_score
+
+# example taken from
+# https://www.nltk.org/api/nltk.translate.html?highlight=bleu%20score#nltk.translate.bleu_score.sentence_bleu
+HYPOTHESIS1 = tuple(
+    "It is a guide to action which ensures that the military always obeys the commands of the party".split()
+)
+REFERENCE1 = tuple("It is a guide to action that ensures that the military will forever heed Party commands".split())
+REFERENCE2 = tuple(
+    "It is a guiding principle which makes the military forces always being under the command of the Party".split()
+)
+REFERENCE3 = tuple("It is the practical guide for the army always to heed the directions of the party".split())
+
+
+# example taken from
+# https://www.nltk.org/api/nltk.translate.html?highlight=bleu%20score#nltk.translate.bleu_score.corpus_bleu
+HYP1 = "It is a guide to action which ensures that the military always obeys the commands of the party".split()
+HYP2 = "he read the book because he was interested in world history".split()
+
+REF1A = "It is a guide to action that ensures that the military will forever heed Party commands".split()
+REF1B = "It is a guiding principle which makes the military force always being under the command of the Party".split()
+REF1C = "It is the practical guide for the army always to heed the directions of the party".split()
+REF2A = "he was interested in world history because he read the book".split()
+
+LIST_OF_REFERENCES = [[REF1A, REF1B, REF1C], [REF2A]]
+HYPOTHESES = [HYP1, HYP2]
+
+# https://www.nltk.org/api/nltk.translate.html?highlight=bleu%20score#nltk.translate.bleu_score.SmoothingFunction
+smooth_func = SmoothingFunction().method2
+
+
+@pytest.mark.parametrize(
+    ["weights", "n_gram", "smooth_func", "smooth"],
+    [
+        pytest.param([1], 1, None, False),
+        pytest.param([0.5, 0.5], 2, smooth_func, True),
+        pytest.param([0.333333, 0.333333, 0.333333], 3, None, False),
+        pytest.param([0.25, 0.25, 0.25, 0.25], 4, smooth_func, True),
+    ],
+)
+def test_bleu_score(weights, n_gram, smooth_func, smooth):
+    nltk_output = sentence_bleu(
+        [REFERENCE1, REFERENCE2, REFERENCE3], HYPOTHESIS1, weights=weights, smoothing_function=smooth_func
+    )
+    pl_output = bleu_score([HYPOTHESIS1], [[REFERENCE1, REFERENCE2, REFERENCE3]], n_gram=n_gram, smooth=smooth)
+    assert torch.allclose(pl_output, torch.tensor(nltk_output))
+
+    nltk_output = corpus_bleu(LIST_OF_REFERENCES, HYPOTHESES, weights=weights, smoothing_function=smooth_func)
+    pl_output = bleu_score(HYPOTHESES, LIST_OF_REFERENCES, n_gram=n_gram, smooth=smooth)
+    assert torch.allclose(pl_output, torch.tensor(nltk_output))
+
+
+def test_bleu_empty():
+    hyp = [[]]
+    ref = [[[]]]
+    assert bleu_score(hyp, ref) == torch.tensor(0.0)
+
+
+def test_no_4_gram():
+    hyps = [["My", "full", "pytorch-lightning"]]
+    refs = [[["My", "full", "pytorch-lightning", "test"], ["Completely", "Different"]]]
+    assert bleu_score(hyps, refs) == torch.tensor(0.0)