# Natural Language Toolkit: Sequential Backoff Taggers
#
# Copyright (C) 2001-2015 NLTK Project
# Author: Edward Loper <edloper@gmail.com>
# Steven Bird <stevenbird1@gmail.com> (minor additions)
# Tiago Tresoldi <tresoldi@users.sf.net> (original affix tagger)
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT
"""
Classes for tagging sentences sequentially, left to right. The
abstract base class SequentialBackoffTagger serves as the base
class for all the taggers in this module. Tagging of individual words
is performed by the method ``choose_tag()``, which is defined by
subclasses of SequentialBackoffTagger. If a tagger is unable to
determine a tag for the specified token, then its backoff tagger is
consulted instead. Any SequentialBackoffTagger may serve as a
backoff tagger for any other SequentialBackoffTagger.
"""
from __future__ import print_function, unicode_literals
import re
from nltk.probability import ConditionalFreqDist
from nltk.classify import NaiveBayesClassifier
from nltk.compat import python_2_unicode_compatible
from nltk.tag.api import TaggerI, FeaturesetTaggerI
from nltk import jsontags
######################################################################
#{ Abstract Base Classes
######################################################################
[docs]class SequentialBackoffTagger(TaggerI):
"""
An abstract base class for taggers that tags words sequentially,
left to right. Tagging of individual words is performed by the
``choose_tag()`` method, which should be defined by subclasses. If
a tagger is unable to determine a tag for the specified token,
then its backoff tagger is consulted.
:ivar _taggers: A list of all the taggers that should be tried to
tag a token (i.e., self and its backoff taggers).
"""
[docs] def __init__(self, backoff=None):
if backoff is None:
self._taggers = [self]
else:
self._taggers = [self] + backoff._taggers
@property
def backoff(self):
"""The backoff tagger for this tagger."""
return self._taggers[1] if len(self._taggers) > 1 else None
[docs] def tag(self, tokens):
# docs inherited from TaggerI
tags = []
for i in range(len(tokens)):
tags.append(self.tag_one(tokens, i, tags))
return list(zip(tokens, tags))
[docs] def tag_one(self, tokens, index, history):
"""
Determine an appropriate tag for the specified token, and
return that tag. If this tagger is unable to determine a tag
for the specified token, then its backoff tagger is consulted.
:rtype: str
:type tokens: list
:param tokens: The list of words that are being tagged.
:type index: int
:param index: The index of the word whose tag should be
returned.
:type history: list(str)
:param history: A list of the tags for all words before *index*.
"""
tag = None
for tagger in self._taggers:
tag = tagger.choose_tag(tokens, index, history)
if tag is not None: break
return tag
[docs] def choose_tag(self, tokens, index, history):
"""
Decide which tag should be used for the specified token, and
return that tag. If this tagger is unable to determine a tag
for the specified token, return None -- do not consult
the backoff tagger. This method should be overridden by
subclasses of SequentialBackoffTagger.
:rtype: str
:type tokens: list
:param tokens: The list of words that are being tagged.
:type index: int
:param index: The index of the word whose tag should be
returned.
:type history: list(str)
:param history: A list of the tags for all words before *index*.
"""
raise NotImplementedError()
@python_2_unicode_compatible
[docs]class ContextTagger(SequentialBackoffTagger):
"""
An abstract base class for sequential backoff taggers that choose
a tag for a token based on the value of its "context". Different
subclasses are used to define different contexts.
A ContextTagger chooses the tag for a token by calculating the
token's context, and looking up the corresponding tag in a table.
This table can be constructed manually; or it can be automatically
constructed based on a training corpus, using the ``_train()``
factory method.
:ivar _context_to_tag: Dictionary mapping contexts to tags.
"""
[docs] def __init__(self, context_to_tag, backoff=None):
"""
:param context_to_tag: A dictionary mapping contexts to tags.
:param backoff: The backoff tagger that should be used for this tagger.
"""
SequentialBackoffTagger.__init__(self, backoff)
self._context_to_tag = (context_to_tag if context_to_tag else {})
[docs] def context(self, tokens, index, history):
"""
:return: the context that should be used to look up the tag
for the specified token; or None if the specified token
should not be handled by this tagger.
:rtype: (hashable)
"""
raise NotImplementedError()
[docs] def choose_tag(self, tokens, index, history):
context = self.context(tokens, index, history)
return self._context_to_tag.get(context)
[docs] def size(self):
"""
:return: The number of entries in the table used by this
tagger to map from contexts to tags.
"""
return len(self._context_to_tag)
def __repr__(self):
return '<%s: size=%d>' % (self.__class__.__name__, self.size())
def _train(self, tagged_corpus, cutoff=0, verbose=False):
"""
Initialize this ContextTagger's ``_context_to_tag`` table
based on the given training data. In particular, for each
context ``c`` in the training data, set
``_context_to_tag[c]`` to the most frequent tag for that
context. However, exclude any contexts that are already
tagged perfectly by the backoff tagger(s).
The old value of ``self._context_to_tag`` (if any) is discarded.
:param tagged_corpus: A tagged corpus. Each item should be
a list of (word, tag tuples.
:param cutoff: If the most likely tag for a context occurs
fewer than cutoff times, then exclude it from the
context-to-tag table for the new tagger.
"""
token_count = hit_count = 0
# A context is considered 'useful' if it's not already tagged
# perfectly by the backoff tagger.
useful_contexts = set()
# Count how many times each tag occurs in each context.
fd = ConditionalFreqDist()
for sentence in tagged_corpus:
tokens, tags = zip(*sentence)
for index, (token, tag) in enumerate(sentence):
# Record the event.
token_count += 1
context = self.context(tokens, index, tags[:index])
if context is None: continue
fd[context][tag] += 1
# If the backoff got it wrong, this context is useful:
if (self.backoff is None or
tag != self.backoff.tag_one(tokens, index, tags[:index])):
useful_contexts.add(context)
# Build the context_to_tag table -- for each context, figure
# out what the most likely tag is. Only include contexts that
# we've seen at least `cutoff` times.
for context in useful_contexts:
best_tag = fd[context].max()
hits = fd[context][best_tag]
if hits > cutoff:
self._context_to_tag[context] = best_tag
hit_count += hits
# Display some stats, if requested.
if verbose:
size = len(self._context_to_tag)
backoff = 100 - (hit_count * 100.0)/ token_count
pruning = 100 - (size * 100.0) / len(fd.conditions())
print("[Trained Unigram tagger:", end=' ')
print("size=%d, backoff=%.2f%%, pruning=%.2f%%]" % (
size, backoff, pruning))
######################################################################
#{ Tagger Classes
######################################################################
@python_2_unicode_compatible
@jsontags.register_tag
[docs]class DefaultTagger(SequentialBackoffTagger):
"""
A tagger that assigns the same tag to every token.
>>> from nltk.tag import DefaultTagger
>>> default_tagger = DefaultTagger('NN')
>>> list(default_tagger.tag('This is a test'.split()))
[('This', 'NN'), ('is', 'NN'), ('a', 'NN'), ('test', 'NN')]
This tagger is recommended as a backoff tagger, in cases where
a more powerful tagger is unable to assign a tag to the word
(e.g. because the word was not seen during training).
:param tag: The tag to assign to each token
:type tag: str
"""
json_tag = 'nltk.tag.sequential.DefaultTagger'
[docs] def __init__(self, tag):
self._tag = tag
SequentialBackoffTagger.__init__(self, None)
[docs] def encode_json_obj(self):
return self._tag
@classmethod
[docs] def decode_json_obj(cls, obj):
tag = obj
return cls(tag)
[docs] def choose_tag(self, tokens, index, history):
return self._tag # ignore token and history
def __repr__(self):
return '<DefaultTagger: tag=%s>' % self._tag
@jsontags.register_tag
[docs]class NgramTagger(ContextTagger):
"""
A tagger that chooses a token's tag based on its word string and
on the preceding n word's tags. In particular, a tuple
(tags[i-n:i-1], words[i]) is looked up in a table, and the
corresponding tag is returned. N-gram taggers are typically
trained on a tagged corpus.
Train a new NgramTagger using the given training data or
the supplied model. In particular, construct a new tagger
whose table maps from each context (tag[i-n:i-1], word[i])
to the most frequent tag for that context. But exclude any
contexts that are already tagged perfectly by the backoff
tagger.
:param train: A tagged corpus consisting of a list of tagged
sentences, where each sentence is a list of (word, tag) tuples.
:param backoff: A backoff tagger, to be used by the new
tagger if it encounters an unknown context.
:param cutoff: If the most likely tag for a context occurs
fewer than *cutoff* times, then exclude it from the
context-to-tag table for the new tagger.
"""
json_tag = 'nltk.tag.sequential.NgramTagger'
[docs] def __init__(self, n, train=None, model=None,
backoff=None, cutoff=0, verbose=False):
self._n = n
self._check_params(train, model)
ContextTagger.__init__(self, model, backoff)
if train:
self._train(train, cutoff, verbose)
[docs] def encode_json_obj(self):
return self._n, self._context_to_tag, self.backoff
@classmethod
[docs] def decode_json_obj(cls, obj):
_n, _context_to_tag, backoff = obj
return cls(_n, model=_context_to_tag, backoff=backoff)
[docs] def context(self, tokens, index, history):
tag_context = tuple(history[max(0,index-self._n+1):index])
return tag_context, tokens[index]
@jsontags.register_tag
[docs]class UnigramTagger(NgramTagger):
"""
Unigram Tagger
The UnigramTagger finds the most likely tag for each word in a training
corpus, and then uses that information to assign tags to new tokens.
>>> from nltk.corpus import brown
>>> from nltk.tag import UnigramTagger
>>> test_sent = brown.sents(categories='news')[0]
>>> unigram_tagger = UnigramTagger(brown.tagged_sents(categories='news')[:500])
>>> for tok, tag in unigram_tagger.tag(test_sent):
... print("(%s, %s), " % (tok, tag))
(The, AT), (Fulton, NP-TL), (County, NN-TL), (Grand, JJ-TL),
(Jury, NN-TL), (said, VBD), (Friday, NR), (an, AT),
(investigation, NN), (of, IN), (Atlanta's, NP$), (recent, JJ),
(primary, NN), (election, NN), (produced, VBD), (``, ``),
(no, AT), (evidence, NN), ('', ''), (that, CS), (any, DTI),
(irregularities, NNS), (took, VBD), (place, NN), (., .),
:param train: The corpus of training data, a list of tagged sentences
:type train: list(list(tuple(str, str)))
:param model: The tagger model
:type model: dict
:param backoff: Another tagger which this tagger will consult when it is
unable to tag a word
:type backoff: TaggerI
:param cutoff: The number of instances of training data the tagger must see
in order not to use the backoff tagger
:type cutoff: int
"""
json_tag = 'nltk.tag.sequential.UnigramTagger'
[docs] def __init__(self, train=None, model=None,
backoff=None, cutoff=0, verbose=False):
NgramTagger.__init__(self, 1, train, model,
backoff, cutoff, verbose)
[docs] def encode_json_obj(self):
return self._context_to_tag, self.backoff
@classmethod
[docs] def decode_json_obj(cls, obj):
_context_to_tag, backoff = obj
return cls(model=_context_to_tag, backoff=backoff)
[docs] def context(self, tokens, index, history):
return tokens[index]
@jsontags.register_tag
[docs]class BigramTagger(NgramTagger):
"""
A tagger that chooses a token's tag based its word string and on
the preceding words' tag. In particular, a tuple consisting
of the previous tag and the word is looked up in a table, and
the corresponding tag is returned.
:param train: The corpus of training data, a list of tagged sentences
:type train: list(list(tuple(str, str)))
:param model: The tagger model
:type model: dict
:param backoff: Another tagger which this tagger will consult when it is
unable to tag a word
:type backoff: TaggerI
:param cutoff: The number of instances of training data the tagger must see
in order not to use the backoff tagger
:type cutoff: int
"""
json_tag = 'nltk.tag.sequential.BigramTagger'
[docs] def __init__(self, train=None, model=None,
backoff=None, cutoff=0, verbose=False):
NgramTagger.__init__(self, 2, train, model,
backoff, cutoff, verbose)
[docs] def encode_json_obj(self):
return self._context_to_tag, self.backoff
@classmethod
[docs] def decode_json_obj(cls, obj):
_context_to_tag, backoff = obj
return cls(model=_context_to_tag, backoff=backoff)
@jsontags.register_tag
[docs]class TrigramTagger(NgramTagger):
"""
A tagger that chooses a token's tag based its word string and on
the preceding two words' tags. In particular, a tuple consisting
of the previous two tags and the word is looked up in a table, and
the corresponding tag is returned.
:param train: The corpus of training data, a list of tagged sentences
:type train: list(list(tuple(str, str)))
:param model: The tagger model
:type model: dict
:param backoff: Another tagger which this tagger will consult when it is
unable to tag a word
:type backoff: TaggerI
:param cutoff: The number of instances of training data the tagger must see
in order not to use the backoff tagger
:type cutoff: int
"""
json_tag = 'nltk.tag.sequential.TrigramTagger'
[docs] def __init__(self, train=None, model=None,
backoff=None, cutoff=0, verbose=False):
NgramTagger.__init__(self, 3, train, model,
backoff, cutoff, verbose)
[docs] def encode_json_obj(self):
return self._context_to_tag, self.backoff
@classmethod
[docs] def decode_json_obj(cls, obj):
_context_to_tag, backoff = obj
return cls(model=_context_to_tag, backoff=backoff)
@jsontags.register_tag
[docs]class AffixTagger(ContextTagger):
"""
A tagger that chooses a token's tag based on a leading or trailing
substring of its word string. (It is important to note that these
substrings are not necessarily "true" morphological affixes). In
particular, a fixed-length substring of the word is looked up in a
table, and the corresponding tag is returned. Affix taggers are
typically constructed by training them on a tagged corpus.
Construct a new affix tagger.
:param affix_length: The length of the affixes that should be
considered during training and tagging. Use negative
numbers for suffixes.
:param min_stem_length: Any words whose length is less than
min_stem_length+abs(affix_length) will be assigned a
tag of None by this tagger.
"""
json_tag = 'nltk.tag.sequential.AffixTagger'
[docs] def __init__(self, train=None, model=None, affix_length=-3,
min_stem_length=2, backoff=None, cutoff=0, verbose=False):
self._check_params(train, model)
ContextTagger.__init__(self, model, backoff)
self._affix_length = affix_length
self._min_word_length = min_stem_length + abs(affix_length)
if train:
self._train(train, cutoff, verbose)
[docs] def encode_json_obj(self):
return self._affix_length, self._min_word_length, self._context_to_tag, self.backoff
@classmethod
[docs] def decode_json_obj(cls, obj):
_affix_length, _min_word_length, _context_to_tag, backoff = obj
return cls(
affix_length=_affix_length,
min_stem_length=_min_word_length - abs(_affix_length),
model=_context_to_tag,
backoff=backoff
)
[docs] def context(self, tokens, index, history):
token = tokens[index]
if len(token) < self._min_word_length:
return None
elif self._affix_length > 0:
return token[:self._affix_length]
else:
return token[self._affix_length:]
@python_2_unicode_compatible
@jsontags.register_tag
[docs]class RegexpTagger(SequentialBackoffTagger):
"""
Regular Expression Tagger
The RegexpTagger assigns tags to tokens by comparing their
word strings to a series of regular expressions. The following tagger
uses word suffixes to make guesses about the correct Brown Corpus part
of speech tag:
>>> from nltk.corpus import brown
>>> from nltk.tag import RegexpTagger
>>> test_sent = brown.sents(categories='news')[0]
>>> regexp_tagger = RegexpTagger(
... [(r'^-?[0-9]+(.[0-9]+)?$', 'CD'), # cardinal numbers
... (r'(The|the|A|a|An|an)$', 'AT'), # articles
... (r'.*able$', 'JJ'), # adjectives
... (r'.*ness$', 'NN'), # nouns formed from adjectives
... (r'.*ly$', 'RB'), # adverbs
... (r'.*s$', 'NNS'), # plural nouns
... (r'.*ing$', 'VBG'), # gerunds
... (r'.*ed$', 'VBD'), # past tense verbs
... (r'.*', 'NN') # nouns (default)
... ])
>>> regexp_tagger
<Regexp Tagger: size=9>
>>> regexp_tagger.tag(test_sent)
[('The', 'AT'), ('Fulton', 'NN'), ('County', 'NN'), ('Grand', 'NN'), ('Jury', 'NN'),
('said', 'NN'), ('Friday', 'NN'), ('an', 'AT'), ('investigation', 'NN'), ('of', 'NN'),
("Atlanta's", 'NNS'), ('recent', 'NN'), ('primary', 'NN'), ('election', 'NN'),
('produced', 'VBD'), ('``', 'NN'), ('no', 'NN'), ('evidence', 'NN'), ("''", 'NN'),
('that', 'NN'), ('any', 'NN'), ('irregularities', 'NNS'), ('took', 'NN'),
('place', 'NN'), ('.', 'NN')]
:type regexps: list(tuple(str, str))
:param regexps: A list of ``(regexp, tag)`` pairs, each of
which indicates that a word matching ``regexp`` should
be tagged with ``tag``. The pairs will be evalutated in
order. If none of the regexps match a word, then the
optional backoff tagger is invoked, else it is
assigned the tag None.
"""
json_tag = 'nltk.tag.sequential.RegexpTagger'
[docs] def __init__(self, regexps, backoff=None):
"""
"""
SequentialBackoffTagger.__init__(self, backoff)
self._regexs = [(re.compile(regexp), tag,) for regexp, tag in regexps]
[docs] def encode_json_obj(self):
return [(regexp.patten, tag,) for regexp, tag in self._regexs], self.backoff
@classmethod
[docs] def decode_json_obj(cls, obj):
regexps, backoff = obj
self = cls(())
self._regexs = [(re.compile(regexp), tag,) for regexp, tag in regexps]
SequentialBackoffTagger.__init__(self, backoff)
return self
[docs] def choose_tag(self, tokens, index, history):
for regexp, tag in self._regexs:
if re.match(regexp, tokens[index]):
return tag
return None
def __repr__(self):
return '<Regexp Tagger: size=%d>' % len(self._regexs)
@python_2_unicode_compatible
[docs]class ClassifierBasedTagger(SequentialBackoffTagger, FeaturesetTaggerI):
"""
A sequential tagger that uses a classifier to choose the tag for
each token in a sentence. The featureset input for the classifier
is generated by a feature detector function::
feature_detector(tokens, index, history) -> featureset
Where tokens is the list of unlabeled tokens in the sentence;
index is the index of the token for which feature detection
should be performed; and history is list of the tags for all
tokens before index.
Construct a new classifier-based sequential tagger.
:param feature_detector: A function used to generate the
featureset input for the classifier::
feature_detector(tokens, index, history) -> featureset
:param train: A tagged corpus consisting of a list of tagged
sentences, where each sentence is a list of (word, tag) tuples.
:param backoff: A backoff tagger, to be used by the new tagger
if it encounters an unknown context.
:param classifier_builder: A function used to train a new
classifier based on the data in *train*. It should take
one argument, a list of labeled featuresets (i.e.,
(featureset, label) tuples).
:param classifier: The classifier that should be used by the
tagger. This is only useful if you want to manually
construct the classifier; normally, you would use *train*
instead.
:param backoff: A backoff tagger, used if this tagger is
unable to determine a tag for a given token.
:param cutoff_prob: If specified, then this tagger will fall
back on its backoff tagger if the probability of the most
likely tag is less than *cutoff_prob*.
"""
[docs] def __init__(self, feature_detector=None, train=None,
classifier_builder=NaiveBayesClassifier.train,
classifier=None, backoff=None,
cutoff_prob=None, verbose=False):
self._check_params(train, classifier)
SequentialBackoffTagger.__init__(self, backoff)
if (train and classifier) or (not train and not classifier):
raise ValueError('Must specify either training data or '
'trained classifier.')
if feature_detector is not None:
self._feature_detector = feature_detector
# The feature detector function, used to generate a featureset
# or each token: feature_detector(tokens, index, history) -> featureset
self._cutoff_prob = cutoff_prob
"""Cutoff probability for tagging -- if the probability of the
most likely tag is less than this, then use backoff."""
self._classifier = classifier
"""The classifier used to choose a tag for each token."""
if train:
self._train(train, classifier_builder, verbose)
[docs] def choose_tag(self, tokens, index, history):
# Use our feature detector to get the featureset.
featureset = self.feature_detector(tokens, index, history)
# Use the classifier to pick a tag. If a cutoff probability
# was specified, then check that the tag's probability is
# higher than that cutoff first; otherwise, return None.
if self._cutoff_prob is None:
return self._classifier.classify(featureset)
pdist = self._classifier.prob_classify(featureset)
tag = pdist.max()
return tag if pdist.prob(tag) >= self._cutoff_prob else None
def _train(self, tagged_corpus, classifier_builder, verbose):
"""
Build a new classifier, based on the given training data
*tagged_corpus*.
"""
classifier_corpus = []
if verbose:
print('Constructing training corpus for classifier.')
for sentence in tagged_corpus:
history = []
untagged_sentence, tags = zip(*sentence)
for index in range(len(sentence)):
featureset = self.feature_detector(untagged_sentence,
index, history)
classifier_corpus.append( (featureset, tags[index]) )
history.append(tags[index])
if verbose:
print('Training classifier (%d instances)' % len(classifier_corpus))
self._classifier = classifier_builder(classifier_corpus)
def __repr__(self):
return '<ClassifierBasedTagger: %r>' % self._classifier
[docs] def feature_detector(self, tokens, index, history):
"""
Return the feature detector that this tagger uses to generate
featuresets for its classifier. The feature detector is a
function with the signature::
feature_detector(tokens, index, history) -> featureset
See ``classifier()``
"""
return self._feature_detector(tokens, index, history)
[docs] def classifier(self):
"""
Return the classifier that this tagger uses to choose a tag
for each word in a sentence. The input for this classifier is
generated using this tagger's feature detector.
See ``feature_detector()``
"""
return self._classifier
[docs]class ClassifierBasedPOSTagger(ClassifierBasedTagger):
"""
A classifier based part of speech tagger.
"""
[docs] def feature_detector(self, tokens, index, history):
word = tokens[index]
if index == 0:
prevword = prevprevword = None
prevtag = prevprevtag = None
elif index == 1:
prevword = tokens[index-1].lower()
prevprevword = None
prevtag = history[index-1]
prevprevtag = None
else:
prevword = tokens[index-1].lower()
prevprevword = tokens[index-2].lower()
prevtag = history[index-1]
prevprevtag = history[index-2]
if re.match('[0-9]+(\.[0-9]*)?|[0-9]*\.[0-9]+$', word):
shape = 'number'
elif re.match('\W+$', word):
shape = 'punct'
elif re.match('[A-Z][a-z]+$', word):
shape = 'upcase'
elif re.match('[a-z]+$', word):
shape = 'downcase'
elif re.match('\w+$', word):
shape = 'mixedcase'
else:
shape = 'other'
features = {
'prevtag': prevtag,
'prevprevtag': prevprevtag,
'word': word,
'word.lower': word.lower(),
'suffix3': word.lower()[-3:],
'suffix2': word.lower()[-2:],
'suffix1': word.lower()[-1:],
'prevprevword': prevprevword,
'prevword': prevword,
'prevtag+word': '%s+%s' % (prevtag, word.lower()),
'prevprevtag+word': '%s+%s' % (prevprevtag, word.lower()),
'prevword+word': '%s+%s' % (prevword, word.lower()),
'shape': shape,
}
return features