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408 lines
14 KiB
408 lines
14 KiB
# Natural Language Toolkit: Collocations and Association Measures
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#
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# Copyright (C) 2001-2019 NLTK Project
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# Author: Joel Nothman <jnothman@student.usyd.edu.au>
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# URL: <http://nltk.org>
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# For license information, see LICENSE.TXT
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#
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"""
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Tools to identify collocations --- words that often appear consecutively
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--- within corpora. They may also be used to find other associations between
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word occurrences.
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See Manning and Schutze ch. 5 at http://nlp.stanford.edu/fsnlp/promo/colloc.pdf
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and the Text::NSP Perl package at http://ngram.sourceforge.net
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Finding collocations requires first calculating the frequencies of words and
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their appearance in the context of other words. Often the collection of words
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will then requiring filtering to only retain useful content terms. Each ngram
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of words may then be scored according to some association measure, in order
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to determine the relative likelihood of each ngram being a collocation.
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The ``BigramCollocationFinder`` and ``TrigramCollocationFinder`` classes provide
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these functionalities, dependent on being provided a function which scores a
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ngram given appropriate frequency counts. A number of standard association
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measures are provided in bigram_measures and trigram_measures.
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"""
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from __future__ import print_function
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# Possible TODOs:
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# - consider the distinction between f(x,_) and f(x) and whether our
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# approximation is good enough for fragmented data, and mention it
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# - add a n-gram collocation finder with measures which only utilise n-gram
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# and unigram counts (raw_freq, pmi, student_t)
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import itertools as _itertools
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from six import iteritems
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from nltk.probability import FreqDist
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from nltk.util import ngrams
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# these two unused imports are referenced in collocations.doctest
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from nltk.metrics import ContingencyMeasures, BigramAssocMeasures, TrigramAssocMeasures
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from nltk.metrics.spearman import ranks_from_scores, spearman_correlation
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class AbstractCollocationFinder(object):
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"""
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An abstract base class for collocation finders whose purpose is to
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collect collocation candidate frequencies, filter and rank them.
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As a minimum, collocation finders require the frequencies of each
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word in a corpus, and the joint frequency of word tuples. This data
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should be provided through nltk.probability.FreqDist objects or an
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identical interface.
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"""
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def __init__(self, word_fd, ngram_fd):
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self.word_fd = word_fd
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self.N = word_fd.N()
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self.ngram_fd = ngram_fd
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@classmethod
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def _build_new_documents(
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cls, documents, window_size, pad_left=False, pad_right=False, pad_symbol=None
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):
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'''
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Pad the document with the place holder according to the window_size
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'''
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padding = (pad_symbol,) * (window_size - 1)
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if pad_right:
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return _itertools.chain.from_iterable(
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_itertools.chain(doc, padding) for doc in documents
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)
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if pad_left:
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return _itertools.chain.from_iterable(
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_itertools.chain(padding, doc) for doc in documents
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)
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@classmethod
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def from_documents(cls, documents):
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"""Constructs a collocation finder given a collection of documents,
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each of which is a list (or iterable) of tokens.
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"""
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# return cls.from_words(_itertools.chain(*documents))
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return cls.from_words(
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cls._build_new_documents(documents, cls.default_ws, pad_right=True)
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)
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@staticmethod
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def _ngram_freqdist(words, n):
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return FreqDist(tuple(words[i : i + n]) for i in range(len(words) - 1))
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def _apply_filter(self, fn=lambda ngram, freq: False):
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"""Generic filter removes ngrams from the frequency distribution
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if the function returns True when passed an ngram tuple.
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"""
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tmp_ngram = FreqDist()
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for ngram, freq in iteritems(self.ngram_fd):
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if not fn(ngram, freq):
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tmp_ngram[ngram] = freq
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self.ngram_fd = tmp_ngram
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def apply_freq_filter(self, min_freq):
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"""Removes candidate ngrams which have frequency less than min_freq."""
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self._apply_filter(lambda ng, freq: freq < min_freq)
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def apply_ngram_filter(self, fn):
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"""Removes candidate ngrams (w1, w2, ...) where fn(w1, w2, ...)
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evaluates to True.
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"""
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self._apply_filter(lambda ng, f: fn(*ng))
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def apply_word_filter(self, fn):
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"""Removes candidate ngrams (w1, w2, ...) where any of (fn(w1), fn(w2),
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...) evaluates to True.
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"""
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self._apply_filter(lambda ng, f: any(fn(w) for w in ng))
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def _score_ngrams(self, score_fn):
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"""Generates of (ngram, score) pairs as determined by the scoring
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function provided.
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"""
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for tup in self.ngram_fd:
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score = self.score_ngram(score_fn, *tup)
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if score is not None:
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yield tup, score
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def score_ngrams(self, score_fn):
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"""Returns a sequence of (ngram, score) pairs ordered from highest to
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lowest score, as determined by the scoring function provided.
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"""
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return sorted(self._score_ngrams(score_fn), key=lambda t: (-t[1], t[0]))
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def nbest(self, score_fn, n):
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"""Returns the top n ngrams when scored by the given function."""
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return [p for p, s in self.score_ngrams(score_fn)[:n]]
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def above_score(self, score_fn, min_score):
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"""Returns a sequence of ngrams, ordered by decreasing score, whose
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scores each exceed the given minimum score.
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"""
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for ngram, score in self.score_ngrams(score_fn):
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if score > min_score:
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yield ngram
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else:
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break
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class BigramCollocationFinder(AbstractCollocationFinder):
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"""A tool for the finding and ranking of bigram collocations or other
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association measures. It is often useful to use from_words() rather than
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constructing an instance directly.
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"""
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default_ws = 2
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def __init__(self, word_fd, bigram_fd, window_size=2):
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"""Construct a BigramCollocationFinder, given FreqDists for
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appearances of words and (possibly non-contiguous) bigrams.
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"""
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AbstractCollocationFinder.__init__(self, word_fd, bigram_fd)
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self.window_size = window_size
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@classmethod
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def from_words(cls, words, window_size=2):
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"""Construct a BigramCollocationFinder for all bigrams in the given
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sequence. When window_size > 2, count non-contiguous bigrams, in the
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style of Church and Hanks's (1990) association ratio.
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"""
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wfd = FreqDist()
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bfd = FreqDist()
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if window_size < 2:
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raise ValueError("Specify window_size at least 2")
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for window in ngrams(words, window_size, pad_right=True):
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w1 = window[0]
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if w1 is None:
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continue
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wfd[w1] += 1
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for w2 in window[1:]:
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if w2 is not None:
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bfd[(w1, w2)] += 1
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return cls(wfd, bfd, window_size=window_size)
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def score_ngram(self, score_fn, w1, w2):
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"""Returns the score for a given bigram using the given scoring
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function. Following Church and Hanks (1990), counts are scaled by
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a factor of 1/(window_size - 1).
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"""
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n_all = self.N
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n_ii = self.ngram_fd[(w1, w2)] / (self.window_size - 1.0)
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if not n_ii:
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return
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n_ix = self.word_fd[w1]
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n_xi = self.word_fd[w2]
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return score_fn(n_ii, (n_ix, n_xi), n_all)
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class TrigramCollocationFinder(AbstractCollocationFinder):
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"""A tool for the finding and ranking of trigram collocations or other
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association measures. It is often useful to use from_words() rather than
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constructing an instance directly.
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"""
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default_ws = 3
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def __init__(self, word_fd, bigram_fd, wildcard_fd, trigram_fd):
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"""Construct a TrigramCollocationFinder, given FreqDists for
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appearances of words, bigrams, two words with any word between them,
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and trigrams.
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"""
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AbstractCollocationFinder.__init__(self, word_fd, trigram_fd)
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self.wildcard_fd = wildcard_fd
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self.bigram_fd = bigram_fd
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@classmethod
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def from_words(cls, words, window_size=3):
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"""Construct a TrigramCollocationFinder for all trigrams in the given
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sequence.
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"""
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if window_size < 3:
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raise ValueError("Specify window_size at least 3")
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wfd = FreqDist()
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wildfd = FreqDist()
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bfd = FreqDist()
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tfd = FreqDist()
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for window in ngrams(words, window_size, pad_right=True):
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w1 = window[0]
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if w1 is None:
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continue
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for w2, w3 in _itertools.combinations(window[1:], 2):
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wfd[w1] += 1
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if w2 is None:
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continue
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bfd[(w1, w2)] += 1
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if w3 is None:
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continue
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wildfd[(w1, w3)] += 1
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tfd[(w1, w2, w3)] += 1
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return cls(wfd, bfd, wildfd, tfd)
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def bigram_finder(self):
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"""Constructs a bigram collocation finder with the bigram and unigram
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data from this finder. Note that this does not include any filtering
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applied to this finder.
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"""
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return BigramCollocationFinder(self.word_fd, self.bigram_fd)
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def score_ngram(self, score_fn, w1, w2, w3):
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"""Returns the score for a given trigram using the given scoring
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function.
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"""
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n_all = self.N
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n_iii = self.ngram_fd[(w1, w2, w3)]
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if not n_iii:
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return
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n_iix = self.bigram_fd[(w1, w2)]
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n_ixi = self.wildcard_fd[(w1, w3)]
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n_xii = self.bigram_fd[(w2, w3)]
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n_ixx = self.word_fd[w1]
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n_xix = self.word_fd[w2]
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n_xxi = self.word_fd[w3]
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return score_fn(n_iii, (n_iix, n_ixi, n_xii), (n_ixx, n_xix, n_xxi), n_all)
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class QuadgramCollocationFinder(AbstractCollocationFinder):
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"""A tool for the finding and ranking of quadgram collocations or other association measures.
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It is often useful to use from_words() rather than constructing an instance directly.
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"""
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default_ws = 4
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def __init__(self, word_fd, quadgram_fd, ii, iii, ixi, ixxi, iixi, ixii):
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"""Construct a QuadgramCollocationFinder, given FreqDists for appearances of words,
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bigrams, trigrams, two words with one word and two words between them, three words
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with a word between them in both variations.
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"""
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AbstractCollocationFinder.__init__(self, word_fd, quadgram_fd)
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self.iii = iii
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self.ii = ii
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self.ixi = ixi
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self.ixxi = ixxi
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self.iixi = iixi
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self.ixii = ixii
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@classmethod
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def from_words(cls, words, window_size=4):
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if window_size < 4:
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raise ValueError("Specify window_size at least 4")
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ixxx = FreqDist()
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iiii = FreqDist()
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ii = FreqDist()
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iii = FreqDist()
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ixi = FreqDist()
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ixxi = FreqDist()
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iixi = FreqDist()
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ixii = FreqDist()
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for window in ngrams(words, window_size, pad_right=True):
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w1 = window[0]
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if w1 is None:
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continue
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for w2, w3, w4 in _itertools.combinations(window[1:], 3):
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ixxx[w1] += 1
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if w2 is None:
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continue
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ii[(w1, w2)] += 1
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if w3 is None:
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continue
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iii[(w1, w2, w3)] += 1
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ixi[(w1, w3)] += 1
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if w4 is None:
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continue
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iiii[(w1, w2, w3, w4)] += 1
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ixxi[(w1, w4)] += 1
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ixii[(w1, w3, w4)] += 1
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iixi[(w1, w2, w4)] += 1
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return cls(ixxx, iiii, ii, iii, ixi, ixxi, iixi, ixii)
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def score_ngram(self, score_fn, w1, w2, w3, w4):
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n_all = self.N
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n_iiii = self.ngram_fd[(w1, w2, w3, w4)]
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if not n_iiii:
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return
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n_iiix = self.iii[(w1, w2, w3)]
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n_xiii = self.iii[(w2, w3, w4)]
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n_iixi = self.iixi[(w1, w2, w4)]
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n_ixii = self.ixii[(w1, w3, w4)]
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n_iixx = self.ii[(w1, w2)]
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n_xxii = self.ii[(w3, w4)]
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n_xiix = self.ii[(w2, w3)]
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n_ixix = self.ixi[(w1, w3)]
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n_ixxi = self.ixxi[(w1, w4)]
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n_xixi = self.ixi[(w2, w4)]
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n_ixxx = self.word_fd[w1]
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n_xixx = self.word_fd[w2]
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n_xxix = self.word_fd[w3]
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n_xxxi = self.word_fd[w4]
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return score_fn(
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n_iiii,
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(n_iiix, n_iixi, n_ixii, n_xiii),
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(n_iixx, n_ixix, n_ixxi, n_xixi, n_xxii, n_xiix),
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(n_ixxx, n_xixx, n_xxix, n_xxxi),
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n_all,
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)
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def demo(scorer=None, compare_scorer=None):
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"""Finds bigram collocations in the files of the WebText corpus."""
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from nltk.metrics import (
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BigramAssocMeasures,
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spearman_correlation,
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ranks_from_scores,
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)
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if scorer is None:
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scorer = BigramAssocMeasures.likelihood_ratio
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if compare_scorer is None:
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compare_scorer = BigramAssocMeasures.raw_freq
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from nltk.corpus import stopwords, webtext
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ignored_words = stopwords.words('english')
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word_filter = lambda w: len(w) < 3 or w.lower() in ignored_words
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for file in webtext.fileids():
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words = [word.lower() for word in webtext.words(file)]
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cf = BigramCollocationFinder.from_words(words)
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cf.apply_freq_filter(3)
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cf.apply_word_filter(word_filter)
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corr = spearman_correlation(
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ranks_from_scores(cf.score_ngrams(scorer)),
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ranks_from_scores(cf.score_ngrams(compare_scorer)),
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)
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print(file)
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print('\t', [' '.join(tup) for tup in cf.nbest(scorer, 15)])
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print('\t Correlation to %s: %0.4f' % (compare_scorer.__name__, corr))
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# Slows down loading too much
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# bigram_measures = BigramAssocMeasures()
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# trigram_measures = TrigramAssocMeasures()
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if __name__ == '__main__':
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import sys
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from nltk.metrics import BigramAssocMeasures
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try:
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scorer = eval('BigramAssocMeasures.' + sys.argv[1])
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except IndexError:
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scorer = None
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try:
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compare_scorer = eval('BigramAssocMeasures.' + sys.argv[2])
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except IndexError:
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compare_scorer = None
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demo(scorer, compare_scorer)
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__all__ = [
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'BigramCollocationFinder',
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'TrigramCollocationFinder',
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'QuadgramCollocationFinder',
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]
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