120 lines
4.1 KiB
Python
120 lines
4.1 KiB
Python
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#!/usr/bin/env python
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# -*- coding: utf-8 -*-
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#
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# Licensed under the GNU LGPL v2.1 - http://www.gnu.org/licenses/lgpl.html
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from gensim.summarization.textcleaner import tokenize_by_word as _tokenize_by_word
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from gensim.utils import to_unicode
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import numpy
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import scipy
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def mz_keywords(text, blocksize=1024, scores=False, split=False, weighted=True, threshold=0.0):
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"""Extract keywords from text using the Montemurro and Zanette entropy algorithm. [1]_
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Parameters
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----------
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text: str
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Document for summarization.
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blocksize: int, optional
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Size of blocks to use in analysis.
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scores: bool, optional
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Whether to return score with keywords.
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split: bool, optional
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Whether to return results as list.
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weighted: bool, optional
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Whether to weight scores by word frequency.
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False can useful for shorter texts, and allows automatic thresholding.
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threshold: float or 'auto', optional
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Minimum score for returned keywords, 'auto' calculates the threshold as n_blocks / (n_blocks + 1.0) + 1e-8,
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use 'auto' with `weighted=False`.
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Returns
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-------
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results: str
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newline separated keywords if `split` == False **OR**
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results: list(str)
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list of keywords if `scores` == False **OR**
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results: list(tuple(str, float))
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list of (keyword, score) tuples if `scores` == True
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Results are returned in descending order of score regardless of the format.
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Note
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----
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This algorithm looks for keywords that contribute to the structure of the
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text on scales of `blocksize` words of larger. It is suitable for extracting
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keywords representing the major themes of long texts.
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References
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----------
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.. [1] Marcello A Montemurro, Damian Zanette, "Towards the quantification of the semantic information encoded in
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written language". Advances in Complex Systems, Volume 13, Issue 2 (2010), pp. 135-153,
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DOI: 10.1142/S0219525910002530, https://arxiv.org/abs/0907.1558
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"""
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text = to_unicode(text)
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words = [word for word in _tokenize_by_word(text)]
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vocab = sorted(set(words))
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word_counts = numpy.array(
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[
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[words[i:i + blocksize].count(word) for word in vocab]
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for i in range(0, len(words), blocksize)
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]
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).astype('d')
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n_blocks = word_counts.shape[0]
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totals = word_counts.sum(axis=0)
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n_words = totals.sum()
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p = word_counts / totals
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log_p = numpy.log2(p)
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h = numpy.nan_to_num(p * log_p).sum(axis=0)
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analytic = __analytic_entropy(blocksize, n_blocks, n_words)
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h += analytic(totals).astype('d')
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if weighted:
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h *= totals / n_words
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if threshold == 'auto':
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threshold = n_blocks / (n_blocks + 1.0) + 1.0e-8
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weights = [(word, score) for (word, score) in zip(vocab, h) if score > threshold]
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weights.sort(key=lambda x: -x[1])
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result = weights if scores else [word for (word, score) in weights]
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if not (scores or split):
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result = '\n'.join(result)
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return result
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def __log_combinations_inner(n, m):
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"""Calculates the logarithm of n!/m!(n-m)!"""
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return -(numpy.log(n + 1) + scipy.special.betaln(n - m + 1, m + 1))
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__log_combinations = numpy.frompyfunc(__log_combinations_inner, 2, 1)
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def __marginal_prob(blocksize, n_words):
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def marginal_prob(n, m):
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"""Marginal probability of a word that occurs n times in the document
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occurring m times in a given block"""
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return numpy.exp(
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__log_combinations(n, m) +
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__log_combinations(n_words - n, blocksize - m) -
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__log_combinations(n_words, blocksize)
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)
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return numpy.frompyfunc(marginal_prob, 2, 1)
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def __analytic_entropy(blocksize, n_blocks, n_words):
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marginal = __marginal_prob(blocksize, n_words)
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def analytic_entropy(n):
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"""Predicted entropy for a word that occurs n times in the document"""
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m = numpy.arange(1, min(blocksize, n) + 1).astype('d')
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p = m / n
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elements = numpy.nan_to_num(p * numpy.log2(p)) * marginal(n, m)
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return -n_blocks * elements.sum()
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return numpy.frompyfunc(analytic_entropy, 1, 1)
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