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233 lines
7.8 KiB
Python
233 lines
7.8 KiB
Python
# Natural Language Toolkit: Language Models
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#
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# Copyright (C) 2001-2020 NLTK Project
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# Authors: Ilia Kurenkov <ilia.kurenkov@gmail.com>
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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"""Language Model Interface."""
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import random
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from abc import ABCMeta, abstractmethod
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from bisect import bisect
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from nltk.lm.counter import NgramCounter
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from nltk.lm.util import log_base2
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from nltk.lm.vocabulary import Vocabulary
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from itertools import accumulate
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class Smoothing(metaclass=ABCMeta):
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"""Ngram Smoothing Interface
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Implements Chen & Goodman 1995's idea that all smoothing algorithms have
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certain features in common. This should ideally allow smoothing algorithms to
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work both with Backoff and Interpolation.
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"""
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def __init__(self, vocabulary, counter):
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"""
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:param vocabulary: The Ngram vocabulary object.
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:type vocabulary: nltk.lm.vocab.Vocabulary
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:param counter: The counts of the vocabulary items.
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:type counter: nltk.lm.counter.NgramCounter
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"""
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self.vocab = vocabulary
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self.counts = counter
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@abstractmethod
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def unigram_score(self, word):
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raise NotImplementedError()
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@abstractmethod
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def alpha_gamma(self, word, context):
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raise NotImplementedError()
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def _mean(items):
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"""Return average (aka mean) for sequence of items."""
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return sum(items) / len(items)
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def _random_generator(seed_or_generator):
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if isinstance(seed_or_generator, random.Random):
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return seed_or_generator
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return random.Random(seed_or_generator)
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def _weighted_choice(population, weights, random_generator=None):
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"""Like random.choice, but with weights.
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Heavily inspired by python 3.6 `random.choices`.
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"""
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if not population:
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raise ValueError("Can't choose from empty population")
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if len(population) != len(weights):
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raise ValueError("The number of weights does not match the population")
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cum_weights = list(accumulate(weights))
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total = cum_weights[-1]
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threshold = random_generator.random()
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return population[bisect(cum_weights, total * threshold)]
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class LanguageModel(metaclass=ABCMeta):
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"""ABC for Language Models.
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Cannot be directly instantiated itself.
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"""
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def __init__(self, order, vocabulary=None, counter=None):
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"""Creates new LanguageModel.
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:param vocabulary: If provided, this vocabulary will be used instead
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of creating a new one when training.
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:type vocabulary: `nltk.lm.Vocabulary` or None
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:param counter: If provided, use this object to count ngrams.
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:type vocabulary: `nltk.lm.NgramCounter` or None
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:param ngrams_fn: If given, defines how sentences in training text are turned to ngram
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sequences.
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:type ngrams_fn: function or None
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:param pad_fn: If given, defines how senteces in training text are padded.
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:type pad_fn: function or None
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"""
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self.order = order
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self.vocab = Vocabulary() if vocabulary is None else vocabulary
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self.counts = NgramCounter() if counter is None else counter
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def fit(self, text, vocabulary_text=None):
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"""Trains the model on a text.
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:param text: Training text as a sequence of sentences.
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"""
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if not self.vocab:
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if vocabulary_text is None:
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raise ValueError(
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"Cannot fit without a vocabulary or text to create it from."
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)
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self.vocab.update(vocabulary_text)
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self.counts.update(self.vocab.lookup(sent) for sent in text)
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def score(self, word, context=None):
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"""Masks out of vocab (OOV) words and computes their model score.
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For model-specific logic of calculating scores, see the `unmasked_score`
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method.
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"""
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return self.unmasked_score(
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self.vocab.lookup(word), self.vocab.lookup(context) if context else None
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)
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@abstractmethod
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def unmasked_score(self, word, context=None):
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"""Score a word given some optional context.
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Concrete models are expected to provide an implementation.
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Note that this method does not mask its arguments with the OOV label.
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Use the `score` method for that.
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:param str word: Word for which we want the score
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:param tuple(str) context: Context the word is in.
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If `None`, compute unigram score.
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:param context: tuple(str) or None
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:rtype: float
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"""
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raise NotImplementedError()
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def logscore(self, word, context=None):
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"""Evaluate the log score of this word in this context.
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The arguments are the same as for `score` and `unmasked_score`.
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"""
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return log_base2(self.score(word, context))
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def context_counts(self, context):
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"""Helper method for retrieving counts for a given context.
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Assumes context has been checked and oov words in it masked.
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:type context: tuple(str) or None
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"""
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return (
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self.counts[len(context) + 1][context] if context else self.counts.unigrams
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)
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def entropy(self, text_ngrams):
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"""Calculate cross-entropy of model for given evaluation text.
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:param Iterable(tuple(str)) text_ngrams: A sequence of ngram tuples.
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:rtype: float
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"""
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return -1 * _mean(
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[self.logscore(ngram[-1], ngram[:-1]) for ngram in text_ngrams]
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)
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def perplexity(self, text_ngrams):
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"""Calculates the perplexity of the given text.
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This is simply 2 ** cross-entropy for the text, so the arguments are the same.
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"""
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return pow(2.0, self.entropy(text_ngrams))
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def generate(self, num_words=1, text_seed=None, random_seed=None):
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"""Generate words from the model.
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:param int num_words: How many words to generate. By default 1.
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:param text_seed: Generation can be conditioned on preceding context.
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:param random_seed: A random seed or an instance of `random.Random`. If provided,
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makes the random sampling part of generation reproducible.
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:return: One (str) word or a list of words generated from model.
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Examples:
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>>> from nltk.lm import MLE
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>>> lm = MLE(2)
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>>> lm.fit([[("a", "b"), ("b", "c")]], vocabulary_text=['a', 'b', 'c'])
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>>> lm.fit([[("a",), ("b",), ("c",)]])
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>>> lm.generate(random_seed=3)
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'a'
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>>> lm.generate(text_seed=['a'])
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'b'
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"""
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text_seed = [] if text_seed is None else list(text_seed)
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random_generator = _random_generator(random_seed)
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# This is the base recursion case.
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if num_words == 1:
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context = (
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text_seed[-self.order + 1 :]
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if len(text_seed) >= self.order
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else text_seed
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)
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samples = self.context_counts(self.vocab.lookup(context))
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while context and not samples:
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context = context[1:] if len(context) > 1 else []
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samples = self.context_counts(self.vocab.lookup(context))
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# Sorting samples achieves two things:
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# - reproducible randomness when sampling
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# - turns Mapping into Sequence which `_weighted_choice` expects
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samples = sorted(samples)
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return _weighted_choice(
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samples,
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tuple(self.score(w, context) for w in samples),
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random_generator,
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)
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# We build up text one word at a time using the preceding context.
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generated = []
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for _ in range(num_words):
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generated.append(
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self.generate(
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num_words=1,
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text_seed=text_seed + generated,
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random_seed=random_generator,
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)
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)
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return generated
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