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# Natural Language Toolkit: Sequential Backoff Taggers # # Copyright (C) 2001-2012 NLTK Project # Author: Edward Loper <edloper@gradient.cis.upenn.edu> # Steven Bird <sb@csse.unimelb.edu.au> (minor additions) # Tiago Tresoldi <tresoldi@users.sf.net> (original affix tagger) # URL: <http://www.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. """
###################################################################### #{ Abstract Base Classes ###################################################################### """ 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). """ else: self._taggers = [self] + backoff._taggers
def backoff(self): """The backoff tagger for this tagger.""" else: return self._taggers[1]
# docs inherited from TaggerI
""" 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*. """
""" 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()
""" 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. """ """ :param context_to_tag: A dictionary mapping contexts to tags. :param backoff: The backoff tagger that should be used for this tagger. """ self._context_to_tag = context_to_tag else:
""" :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()
""" :return: The number of entries in the table used by this tagger to map from contexts to tags. """ return len(self._context_to_tag)
return '<%s: size=%d>' % (self.__class__.__name__, self.size())
""" 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. """
# A context is considered 'useful' if it's not already tagged # perfectly by the backoff tagger.
# Count how many times each tag occurs in each context. # Record the event. # If the backoff got it wrong, this context is useful: tag != self.backoff.tag_one(tokens, index, tags[:index])):
# 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.
# Display some stats, if requested. 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 ######################################################################
""" A tagger that assigns the same tag to every token.
>>> from nltk.tag.sequential import DefaultTagger >>> default_tagger = DefaultTagger('NN') >>> 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 """
return '<DefaultTagger: tag=%s>' % self._tag
""" 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. """
backoff=None, cutoff=0, verbose=False):
tag_context = tuple(history[max(0,index-self._n+1):index]) return (tag_context, tokens[index])
""" 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.sequential import UnigramTagger >>> test_sent = brown.sents(categories='news')[0] >>> unigram_tagger = UnigramTagger(brown.tagged_sents(categories='news')[:500]) >>> unigram_tagger.tag(test_sent) [('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 """
backoff=None, cutoff=0, verbose=False): backoff, cutoff, verbose)
""" 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 """
backoff=None, cutoff=0, verbose=False): NgramTagger.__init__(self, 2, train, model, backoff, cutoff, verbose)
""" 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 """
backoff=None, cutoff=0, verbose=False): NgramTagger.__init__(self, 3, train, model, backoff, cutoff, verbose)
""" 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. """
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)
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:]
""" 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.sequential 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.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. """
""" """
return None
return '<Regexp Tagger: size=%d>' % len(self._regexps)
""" 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*. """ 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)
# Use our feature detector to get the featureset.
# 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. else: pdist = self._classifier.prob_classify(featureset) tag = pdist.max() if pdist.prob(tag) >= self._cutoff_prob: return tag else: return None
""" 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)
return '<ClassifierBasedTagger: %r>' % self._classifier
""" 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)
""" 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
""" A classifier based part of speech tagger. """ else:
shape = 'number' shape = 'mixedcase' else:
'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, }
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