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# Natural Language Toolkit: Texts # # Copyright (C) 2001-2012 NLTK Project # Author: Steven Bird <sb@csse.unimelb.edu.au> # Edward Loper <edloper@gradient.cis.upenn.edu> # URL: <http://www.nltk.org/> # For license information, see LICENSE.TXT
This module brings together a variety of NLTK functionality for text analysis, and provides simple, interactive interfaces. Functionality includes: concordancing, collocation discovery, regular expression search over tokenized strings, and distributional similarity. """
""" A bidirectional index between words and their 'contexts' in a text. The context of a word is usually defined to be the words that occur in a fixed window around the word; but other definitions may also be used by providing a custom context function. """ def _default_context(tokens, i): """One left token and one right token, normalized to lowercase""" if i == 0: left = '*START*' else: left = tokens[i-1].lower() if i == len(tokens) - 1: right = '*END*' else: right = tokens[i+1].lower() return (left, right)
self._key = key self._tokens = tokens if not context_func: self._context_func = self._default_context if filter: tokens = [t for t in tokens if filter(t)] self._word_to_contexts = CFD((self._key(w), self._context_func(tokens, i)) for i, w in enumerate(tokens)) self._context_to_words = CFD((self._context_func(tokens, i), self._key(w)) for i, w in enumerate(tokens))
""" :rtype: list(str) :return: The document that this context index was created from. """ return self._tokens
""" Return a dictionary mapping from words to 'similarity scores,' indicating how often these two words occur in the same context. """ word = self._key(word) word_contexts = set(self._word_to_contexts[word])
scores = {} for w, w_contexts in self._word_to_contexts.items(): scores[w] = f_measure(word_contexts, set(w_contexts))
return scores
scores = defaultdict(int) for c in self._word_to_contexts[self._key(word)]: for w in self._context_to_words[c]: if w != word: print(w, c, self._context_to_words[c][word], self._context_to_words[c][w]) scores[w] += self._context_to_words[c][word] * self._context_to_words[c][w] return sorted(scores, key=scores.get)[:n]
""" Find contexts where the specified words can all appear; and return a frequency distribution mapping each context to the number of times that context was used.
:param words: The words used to seed the similarity search :type words: str :param fail_on_unknown: If true, then raise a value error if any of the given words do not occur at all in the index. """ words = [self._key(w) for w in words] contexts = [set(self._word_to_contexts[w]) for w in words] empty = [words[i] for i in range(len(words)) if not contexts[i]] common = reduce(set.intersection, contexts) if empty and fail_on_unknown: raise ValueError("The following word(s) were not found:", " ".join(words)) elif not common: # nothing in common -- just return an empty freqdist. return FreqDist() else: fd = FreqDist(c for w in words for c in self._word_to_contexts[w] if c in common) return fd
""" An index that can be used to look up the offset locations at which a given word occurs in a document. """ """ Construct a new concordance index.
:param tokens: The document (list of tokens) that this concordance index was created from. This list can be used to access the context of a given word occurrence. :param key: A function that maps each token to a normalized version that will be used as a key in the index. E.g., if you use ``key=lambda s:s.lower()``, then the index will be case-insensitive. """ self._tokens = tokens """The document (list of tokens) that this concordance index was created from."""
self._key = key """Function mapping each token to an index key (or None)."""
self._offsets = defaultdict(list) """Dictionary mapping words (or keys) to lists of offset indices."""
# Initialize the index (self._offsets) for index, word in enumerate(tokens): word = self._key(word) self._offsets[word].append(index)
""" :rtype: list(str) :return: The document that this concordance index was created from. """ return self._tokens
""" :rtype: list(int) :return: A list of the offset positions at which the given word occurs. If a key function was specified for the index, then given word's key will be looked up. """ word = self._key(word) return self._offsets[word]
return '<ConcordanceIndex for %d tokens (%d types)>' % ( len(self._tokens), len(self._offsets))
""" Print a concordance for ``word`` with the specified context window.
:param word: The target word :type word: str :param width: The width of each line, in characters (default=80) :type width: int :param lines: The number of lines to display (default=25) :type lines: int """ half_width = (width - len(word) - 2) / 2 context = width/4 # approx number of words of context
offsets = self.offsets(word) if offsets: lines = min(lines, len(offsets)) print("Displaying %s of %s matches:" % (lines, len(offsets))) for i in offsets: if lines <= 0: break left = (' ' * half_width + ' '.join(self._tokens[i-context:i])) right = ' '.join(self._tokens[i+1:i+context]) left = left[-half_width:] right = right[:half_width] print(left, self._tokens[i], right) lines -= 1 else: print("No matches")
""" A class that makes it easier to use regular expressions to search over tokenized strings. The tokenized string is converted to a string where tokens are marked with angle brackets -- e.g., ``'<the><window><is><still><open>'``. The regular expression passed to the ``findall()`` method is modified to treat angle brackets as nongrouping parentheses, in addition to matching the token boundaries; and to have ``'.'`` not match the angle brackets. """
""" Find instances of the regular expression in the text. The text is a list of tokens, and a regexp pattern to match a single token must be surrounded by angle brackets. E.g.
>>> from nltk.text import TokenSearcher >>> from nltk.book import text1, text5, text9 >>> text5.findall("<.*><.*><bro>") you rule bro; telling you bro; u twizted bro >>> text1.findall("<a>(<.*>)<man>") monied; nervous; dangerous; white; white; white; pious; queer; good; mature; white; Cape; great; wise; wise; butterless; white; fiendish; pale; furious; better; certain; complete; dismasted; younger; brave; brave; brave; brave >>> text9.findall("<th.*>{3,}") thread through those; the thought that; that the thing; the thing that; that that thing; through these than through; them that the; through the thick; them that they; thought that the
:param regexp: A regular expression :type regexp: str """ # preprocess the regular expression
# perform the search
# Sanity check raise ValueError('Bad regexp for TokenSearcher.findall')
# postprocess the output
""" A wrapper around a sequence of simple (string) tokens, which is intended to support initial exploration of texts (via the interactive console). Its methods perform a variety of analyses on the text's contexts (e.g., counting, concordancing, collocation discovery), and display the results. If you wish to write a program which makes use of these analyses, then you should bypass the ``Text`` class, and use the appropriate analysis function or class directly instead.
A ``Text`` is typically initialized from a given document or corpus. E.g.:
>>> import nltk.corpus >>> from nltk.text import Text >>> moby = Text(nltk.corpus.gutenberg.words('melville-moby_dick.txt'))
""" # This defeats lazy loading, but makes things faster. This # *shouldn't* be necessary because the corpus view *should* be # doing intelligent caching, but without this it's running slow. # Look into whether the caching is working correctly.
""" Create a Text object.
:param tokens: The source text. :type tokens: sequence of str """
else: self.name = " ".join(map(str, tokens[:8])) + "..."
#//////////////////////////////////////////////////////////// # Support item & slice access #////////////////////////////////////////////////////////////
else:
#//////////////////////////////////////////////////////////// # Interactive console methods #////////////////////////////////////////////////////////////
""" Print a concordance for ``word`` with the specified context window. Word matching is not case-sensitive. :seealso: ``ConcordanceIndex`` """ if '_concordance_index' not in self.__dict__: print("Building index...") self._concordance_index = ConcordanceIndex(self.tokens, key=lambda s:s.lower())
self._concordance_index.print_concordance(word, width, lines)
""" Print collocations derived from the text, ignoring stopwords.
:seealso: find_collocations :param num: The maximum number of collocations to print. :type num: int :param window_size: The number of tokens spanned by a collocation (default=2) :type window_size: int """ if not ('_collocations' in self.__dict__ and self._num == num and self._window_size == window_size): self._num = num self._window_size = window_size
print("Building collocations list") from nltk.corpus import stopwords ignored_words = stopwords.words('english') finder = BigramCollocationFinder.from_words(self.tokens, window_size) finder.apply_freq_filter(2) finder.apply_word_filter(lambda w: len(w) < 3 or w.lower() in ignored_words) bigram_measures = BigramAssocMeasures() self._collocations = finder.nbest(bigram_measures.likelihood_ratio, num) colloc_strings = [w1+' '+w2 for w1, w2 in self._collocations] print(tokenwrap(colloc_strings, separator="; "))
""" Count the number of times this word appears in the text. """ return self.tokens.count(word)
""" Find the index of the first occurrence of the word in the text. """ return self.tokens.index(word)
# code from nltk_contrib.readability raise NotImplementedError
""" Print random text, generated using a trigram language model.
:param length: The length of text to generate (default=100) :type length: int :seealso: NgramModel """ if '_trigram_model' not in self.__dict__: print("Building ngram index...") estimator = lambda fdist, bins: LidstoneProbDist(fdist, 0.2) self._trigram_model = NgramModel(3, self, estimator) text = self._trigram_model.generate(length) print(tokenwrap(text))
""" Distributional similarity: find other words which appear in the same contexts as the specified word; list most similar words first.
:param word: The word used to seed the similarity search :type word: str :param num: The number of words to generate (default=20) :type num: int :seealso: ContextIndex.similar_words() """ if '_word_context_index' not in self.__dict__: print('Building word-context index...') self._word_context_index = ContextIndex(self.tokens, filter=lambda x:x.isalpha(), key=lambda s:s.lower())
# words = self._word_context_index.similar_words(word, num)
word = word.lower() wci = self._word_context_index._word_to_contexts if word in wci.conditions(): contexts = set(wci[word]) fd = FreqDist(w for w in wci.conditions() for c in wci[w] if c in contexts and not w == word) words = fd.keys()[:num] print(tokenwrap(words)) else: print("No matches")
""" Find contexts where the specified words appear; list most frequent common contexts first.
:param word: The word used to seed the similarity search :type word: str :param num: The number of words to generate (default=20) :type num: int :seealso: ContextIndex.common_contexts() """ if '_word_context_index' not in self.__dict__: print('Building word-context index...') self._word_context_index = ContextIndex(self.tokens, key=lambda s:s.lower())
try: fd = self._word_context_index.common_contexts(words, True) if not fd: print("No common contexts were found") else: ranked_contexts = fd.keys()[:num] print(tokenwrap(w1+"_"+w2 for w1,w2 in ranked_contexts))
except ValueError as e: print(e)
""" Produce a plot showing the distribution of the words through the text. Requires pylab to be installed.
:param words: The words to be plotted :type word: str :seealso: nltk.draw.dispersion_plot() """ from nltk.draw import dispersion_plot dispersion_plot(self, words)
""" See documentation for FreqDist.plot() :seealso: nltk.prob.FreqDist.plot() """ self.vocab().plot(*args)
""" :seealso: nltk.prob.FreqDist """ if "_vocab" not in self.__dict__: print("Building vocabulary index...") self._vocab = FreqDist(self) return self._vocab
""" Find instances of the regular expression in the text. The text is a list of tokens, and a regexp pattern to match a single token must be surrounded by angle brackets. E.g.
>>> from nltk.book import text1, text5, text9 >>> text5.findall("<.*><.*><bro>") you rule bro; telling you bro; u twizted bro >>> text1.findall("<a>(<.*>)<man>") monied; nervous; dangerous; white; white; white; pious; queer; good; mature; white; Cape; great; wise; wise; butterless; white; fiendish; pale; furious; better; certain; complete; dismasted; younger; brave; brave; brave; brave >>> text9.findall("<th.*>{3,}") thread through those; the thought that; that the thing; the thing that; that that thing; through these than through; them that the; through the thick; them that they; thought that the
:param regexp: A regular expression :type regexp: str """
#//////////////////////////////////////////////////////////// # Helper Methods #////////////////////////////////////////////////////////////
""" One left & one right token, both case-normalized. Skip over non-sentence-final punctuation. Used by the ``ContextIndex`` that is created for ``similar()`` and ``common_contexts()``. """ # Left context j = i-1 while j>=0 and not self._CONTEXT_RE.match(tokens[j]): j = j-1 if j == 0: left = '*START*' else: left = tokens[j] # Right context j = i+1 while j<len(tokens) and not self._CONTEXT_RE.match(tokens[j]): j = j+1 if j == len(tokens): right = '*END*' else: right = tokens[j] return (left, right)
#//////////////////////////////////////////////////////////// # String Display #////////////////////////////////////////////////////////////
""" :return: A string representation of this FreqDist. :rtype: string """ return '<Text: %s>' % self.name
# Prototype only; this approach will be slow to load """A collection of texts, which can be loaded with list of texts, or with a corpus consisting of one or more texts, and which supports counting, concordancing, collocation discovery, etc. Initialize a TextCollection as follows:
>>> import nltk.corpus >>> from nltk.text import TextCollection >>> from nltk.book import text1, text2, text3 >>> gutenberg = TextCollection(nltk.corpus.gutenberg) >>> mytexts = TextCollection([text1, text2, text3])
Iterating over a TextCollection produces all the tokens of all the texts in order. """
""" The frequency of the term in text. """ return float(text.count(term)) / len(text)
""" The number of texts in the corpus divided by the number of texts that the term appears in. If a term does not appear in the corpus, 0.0 is returned. """ # idf values are cached for performance. idf = self._idf_cache.get(term) if idf is None: matches = len(list(True for text in self._texts if term in text)) if not matches: # FIXME Should this raise some kind of error instead? idf = 0.0 else: idf = log(float(len(self._texts)) / matches) self._idf_cache[term] = idf return idf
return self.tf(term, text) * self.idf(term)
from nltk.corpus import brown text = Text(brown.words(categories='news')) print(text) print() print("Concordance:") text.concordance('news') print() print("Distributionally similar words:") text.similar('news') print() print("Collocations:") text.collocations() print() print("Automatically generated text:") text.generate() print() print("Dispersion plot:") text.dispersion_plot(['news', 'report', 'said', 'announced']) print() print("Vocabulary plot:") text.plot(50) print() print("Indexing:") print("text[3]:", text[3]) print("text[3:5]:", text[3:5]) print("text.vocab()['news']:", text.vocab()['news'])
demo()
"ConcordanceIndex", "TokenSearcher", "Text", "TextCollection"] |