class LanguageModel(object):
"""ABC for Language Models.
Cannot be directly instantiated itself.
"""
def __init__(self, order, vocabulary=None, counter=None):
"""Creates new LanguageModel.
:param vocabulary: If provided, this vocabulary will be used instead
of creating a new one when training.
:type vocabulary: `nltk.lm.Vocabulary` or None
:param counter: If provided, use this object to count ngrams.
:type vocabulary: `nltk.lm.NgramCounter` or None
:param ngrams_fn: If given, defines how sentences in training text are turned to ngram
sequences.
:type ngrams_fn: function or None
:param pad_fn: If given, defines how senteces in training text are padded.
:type pad_fn: function or None
"""
self.order = order
self.vocab = Vocabulary() if vocabulary is None else vocabulary
self.counts = NgramCounter() if counter is None else counter
def fit(self, text, vocabulary_text=None):
"""Trains the model on a text.
:param text: Training text as a sequence of sentences.
"""
if not self.vocab:
if vocabulary_text is None:
raise ValueError(
"Cannot fit without a vocabulary or text to " "create it from."
)
self.vocab.update(vocabulary_text)
self.counts.update(self.vocab.lookup(sent) for sent in text)
def score(self, word, context=None):
"""Masks out of vocab (OOV) words and computes their model score.
For model-specific logic of calculating scores, see the `unmasked_score`
method.
"""
return self.unmasked_score(
self.vocab.lookup(word), self.vocab.lookup(context) if context else None
)
@abstractmethod
def unmasked_score(self, word, context=None):
"""Score a word given some optional context.
Concrete models are expected to provide an implementation.
Note that this method does not mask its arguments with the OOV label.
Use the `score` method for that.
:param str word: Word for which we want the score
:param tuple(str) context: Context the word is in.
If `None`, compute unigram score.
:param context: tuple(str) or None
:rtype: float
"""
raise NotImplementedError()
def logscore(self, word, context=None):
"""Evaluate the log score of this word in this context.
The arguments are the same as for `score` and `unmasked_score`.
"""
return log_base2(self.score(word, context))
def context_counts(self, context):
"""Helper method for retrieving counts for a given context.
Assumes context has been checked and oov words in it masked.
:type context: tuple(str) or None
"""
return (
self.counts[len(context) + 1][context] if context else self.counts.unigrams
)
def entropy(self, text_ngrams):
"""Calculate cross-entropy of model for given evaluation text.
:param Iterable(tuple(str)) text_ngrams: A sequence of ngram tuples.
:rtype: float
"""
return -1 * _mean(
[self.logscore(ngram[-1], ngram[:-1]) for ngram in text_ngrams]
)
def perplexity(self, text_ngrams):
"""Calculates the perplexity of the given text.
This is simply 2 ** cross-entropy for the text, so the arguments are the same.
"""
return pow(2.0, self.entropy(text_ngrams))
def generate(self, num_words=1, text_seed=None, random_seed=None):
"""Generate words from the model.
:param int num_words: How many words to generate. By default 1.
:param text_seed: Generation can be conditioned on preceding context.
:param random_seed: A random seed or an instance of `random.Random`. If provided,
makes the random sampling part of generation reproducible.
:return: One (str) word or a list of words generated from model.
Examples:
>>> from nltk.lm import MLE
>>> lm = MLE(2)
>>> lm.fit([[("a", "b"), ("b", "c")]], vocabulary_text=['a', 'b', 'c'])
>>> lm.fit([[("a",), ("b",), ("c",)]])
>>> lm.generate(random_seed=3)
'a'
>>> lm.generate(text_seed=['a'])
'b'
"""
text_seed = [] if text_seed is None else list(text_seed)
random_generator = _random_generator(random_seed)
# base recursion case
if num_words == 1:
context = (
text_seed[-self.order + 1 :]
if len(text_seed) >= self.order
else text_seed
)
samples = self.context_counts(self.vocab.lookup(context))
while context and not samples:
context = context[1:] if len(context) > 1 else []
samples = self.context_counts(self.vocab.lookup(context))
# sorting achieves two things:
# - reproducible randomness when sampling
# - turning Mapping into Sequence which _weighted_choice expects
samples = sorted(samples)
return _weighted_choice(
samples, tuple(self.score(w, context) for w in samples), random_generator
)
# build up text one word at a time
generated = []
for _ in range(num_words):
generated.append(
self.generate(
num_words=1,
text_seed=text_seed + generated,
random_seed=random_generator,
)
)
return generated
class LanguageModel(object):
"""ABC for Language Models.
Cannot be directly instantiated itself.
"""
def __init__(self, order, vocabulary=None, counter=None):
"""Creates new LanguageModel.
:param vocabulary: If provided, this vocabulary will be used instead
of creating a new one when training.
:type vocabulary: `nltk.lm.Vocabulary` or None
:param counter: If provided, use this object to count ngrams.
:type vocabulary: `nltk.lm.NgramCounter` or None
:param ngrams_fn: If given, defines how sentences in training text are turned to ngram
sequences.
:type ngrams_fn: function or None
:param pad_fn: If given, defines how senteces in training text are padded.
:type pad_fn: function or None
"""
self.order = order
self.vocab = Vocabulary() if vocabulary is None else vocabulary
self.counts = NgramCounter() if counter is None else counter
def fit(self, text, vocabulary_text=None):
"""Trains the model on a text.
:param text: Training text as a sequence of sentences.
"""
if not self.vocab:
if vocabulary_text is None:
raise ValueError("Cannot fit without a vocabulary or text to "
"create it from.")
self.vocab.update(vocabulary_text)
self.counts.update(self.vocab.lookup(sent) for sent in text)
def score(self, word, context=None):
"""Masks out of vocab (OOV) words and computes their model score.
For model-specific logic of calculating scores, see the `unmasked_score`
method.
"""
return self.unmasked_score(
self.vocab.lookup(word),
self.vocab.lookup(context) if context else None)
@abstractmethod
def unmasked_score(self, word, context=None):
"""Score a word given some optional context.
Concrete models are expected to provide an implementation.
Note that this method does not mask its arguments with the OOV label.
Use the `score` method for that.
:param str word: Word for which we want the score
:param tuple(str) context: Context the word is in.
If `None`, compute unigram score.
:param context: tuple(str) or None
:rtype: float
"""
raise NotImplementedError()
def logscore(self, word, context=None):
"""Evaluate the log score of this word in this context.
The arguments are the same as for `score` and `unmasked_score`.
"""
return log_base2(self.score(word, context))
def context_counts(self, context):
"""Helper method for retrieving counts for a given context.
Assumes context has been checked and oov words in it masked.
:type context: tuple(str) or None
"""
return (self.counts[len(context) +
1][context] if context else self.counts.unigrams)
def entropy(self, text_ngrams):
"""Calculate cross-entropy of model for given evaluation text.
:param Iterable(tuple(str)) text_ngrams: A sequence of ngram tuples.
:rtype: float
"""
return -1 * _mean(
[self.logscore(ngram[-1], ngram[:-1]) for ngram in text_ngrams])
def perplexity(self, text_ngrams):
"""Calculates the perplexity of the given text.
This is simply 2 ** cross-entropy for the text, so the arguments are the same.
"""
return pow(2.0, self.entropy(text_ngrams))
def generate(self, num_words=1, text_seed=None, random_seed=None):
"""Generate words from the model.
:param int num_words: How many words to generate. By default 1.
:param text_seed: Generation can be conditioned on preceding context.
:param random_seed: If provided, makes the random sampling part of
generation reproducible.
:return: One (str) word or a list of words generated from model.
Examples:
>>> from nltk.lm import MLE
>>> lm = MLE(2)
>>> lm.fit([[("a", "b"), ("b", "c")]], vocabulary_text=['a', 'b', 'c'])
>>> lm.fit([[("a",), ("b",), ("c",)]])
>>> lm.generate(random_seed=3)
'a'
>>> lm.generate(text_seed=['a'])
'b'
"""
text_seed = [] if text_seed is None else list(text_seed)
# base recursion case
if num_words == 1:
context = (text_seed[-self.order + 1:]
if len(text_seed) >= self.order else text_seed)
samples = self.context_counts(self.vocab.lookup(context))
while context and not samples:
context = context[1:] if len(context) > 1 else []
samples = self.context_counts(self.vocab.lookup(context))
# sorting achieves two things:
# - reproducible randomness when sampling
# - turning Mapping into Sequence which _weighted_choice expects
samples = sorted(samples)
return _weighted_choice(
samples, tuple(self.score(w, context) for w in samples),
random_seed)
# build up text one word at a time
generated = []
for _ in range(num_words):
generated.append(
self.generate(
num_words=1,
text_seed=text_seed + generated,
random_seed=random_seed,
))
return generated
class LanguageModel(metaclass=ABCMeta):
"""ABC for Language Models.
Cannot be directly instantiated itself.
"""
def __init__(
self,
order,
vocabulary=None,
counter=None,
verbose=True,
):
"""Creates new LanguageModel.
:param vocabulary: If provided, this vocabulary will be used instead
of creating a new one when training.
:type vocabulary: `nltk.lm.Vocabulary` or None
:param counter: If provided, use this object to count ngrams.
:type vocabulary: `nltk.lm.NgramCounter` or None
:param ngrams_fn: If given, defines how sentences in training text are turned to ngram
sequences.
:type ngrams_fn: function or None
:param pad_fn: If given, defines how senteces in training text are padded.
:type pad_fn: function or None
"""
self.order = order
self.vocab = Vocabulary() if vocabulary is None else vocabulary
self.counts = NgramCounter() if counter is None else counter
def_dict_callable = partial(defaultdict, float)
self._cache = defaultdict(def_dict_callable)
self.verbose = verbose
def _update_cache(self, word):
i, word = word
ret_list = []
for order in range(2, self.order + 1):
for context in self.counts[order].keys():
if self.counts[order][context].N() > self.cache_limit:
ret_list.append((context, word, self.score(word, context)))
return ret_list
def _check_cache_size(self):
return getsizeof(self._cache) / 1e6
def fit(self, text, vocabulary_text=None, verbose=True):
"""Trains the model on a text.
:param text: Training text as a sequence of sentences.
"""
if not self.vocab:
if vocabulary_text is None:
raise ValueError(
"Cannot fit without a vocabulary or text to create it from."
)
self.vocab.update(vocabulary_text)
_iter = (self.vocab.lookup(sent) for sent in text)
self.counts.update(
progress(_iter, desc="Fitting the model") if self.
verbose else _iter)
def score(self, word, context=None):
"""Masks out of vocab (OOV) words and computes their model score.
For model-specific logic of calculating scores, see the `unmasked_score`
method.
"""
return self.unmasked_score(
self.vocab.lookup(word),
self.vocab.lookup(context) if context else None)
@abstractmethod
def unmasked_score(self, word, context=None):
"""Score a word given some optional context.
Concrete models are expected to provide an implementation.
Note that this method does not mask its arguments with the OOV label.
Use the `score` method for that.
:param str word: Word for which we want the score
:param tuple(str) context: Context the word is in.
If `None`, compute unigram score.
:param context: tuple(str) or None
:rtype: float
"""
raise NotImplementedError()
def logscore(self, word, context=None):
"""Evaluate the log score of this word in this context.
The arguments are the same as for `score` and `unmasked_score`.
"""
return log_base2(self.score(word, context))
def context_counts(self, context):
"""Helper method for retrieving counts for a given context.
Assumes context has been checked and oov words in it masked.
:type context: tuple(str) or None
"""
return (self.counts[len(context) +
1][context] if context else self.counts.unigrams)
def entropy(self, text_ngrams):
"""Calculate cross-entropy of model for given evaluation text.
:param Iterable(tuple(str)) text_ngrams: A sequence of ngram tuples.
:rtype: float
"""
return -1 * _mean(
[self.logscore(ngram[-1], ngram[:-1]) for ngram in text_ngrams])
def perplexity(self, text_ngrams):
"""Calculates the perplexity of the given text.
This is simply 2 ** cross-entropy for the text, so the arguments are the same.
"""
return pow(
2.0,
self.entropy(
progress(text_ngrams, desc="Calculating Perplexity") if self.
verbose else text_ngrams))
def context_probabilities(self, context):
"""Helper method for retrieving probabilities for a given context,
including all the words in the vocabulary
Assumes context has been checked and oov words in it masked.
:type context: tuple(str) or None
"""
if context not in self._cache.keys():
self._cache[context] = {
word: self.score(word, context)
for word in self.vocab.counts.keys()
}
return self._cache[context]
def _generate_single_word(self, sampler_func, text_seed, random_generator,
sampler_kwargs):
context = tuple(
text_seed[-self.order +
1:] if len(text_seed) >= self.order else text_seed)
distribution = self.context_probabilities(context)
# Sorting distribution achieves two things:
# - reproducible randomness when sampling
# - turns Dictionary into Sequence which `sampler` expects
distribution = sorted(distribution.items(),
key=lambda x: x[1],
reverse=True)
return sampler_func(distribution,
random_generator=random_generator,
**sampler_kwargs)
def generate(
self,
sampler_func=greedy_decoding,
num_words=1,
text_seed=None,
random_seed=None,
sampler_kwargs={},
EOS=None,
):
text_seed = (random.sample(self.vocab.counts.keys(), 1)
if text_seed is None else list(text_seed))
random_generator = _random_generator(random_seed)
if EOS:
sampler_kwargs["EOS"] = EOS
# We build up text one word at a time using the preceding context.
generated = []
_iter = range(num_words)
for _ in (progress(_iter, desc="Generating words")
if self.verbose else _iter):
token = self._generate_single_word(
sampler_func=sampler_func,
text_seed=text_seed + generated,
random_generator=random_generator,
sampler_kwargs=sampler_kwargs,
)
generated.append(token)
if token == EOS:
break
return generated