Source code for nltk.classify.textcat
# -*- coding: utf-8 -*-
# Natural Language Toolkit: Language ID module using TextCat algorithm
#
# Copyright (C) 2001-2019 NLTK Project
# Author: Avital Pekker <[email protected]>
#
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT
"""
A module for language identification using the TextCat algorithm.
An implementation of the text categorization algorithm
presented in Cavnar, W. B. and J. M. Trenkle,
"N-Gram-Based Text Categorization".
The algorithm takes advantage of Zipf's law and uses
n-gram frequencies to profile languages and text-yet to
be identified-then compares using a distance measure.
Language n-grams are provided by the "An Crubadan"
project. A corpus reader was created seperately to read
those files.
For details regarding the algorithm, see:
http://www.let.rug.nl/~vannoord/TextCat/textcat.pdf
For details about An Crubadan, see:
http://borel.slu.edu/crubadan/index.html
"""
# Ensure that literal strings default to unicode rather than str.
from __future__ import print_function, unicode_literals
from nltk.compat import PY3
from nltk.util import trigrams
if PY3:
from sys import maxsize
else:
from sys import maxint
# Note: this is NOT "re" you're likely used to. The regex module
# is an alternative to the standard re module that supports
# Unicode codepoint properties with the \p{} syntax.
# You may have to "pip install regx"
try:
import regex as re
except ImportError:
re = None
######################################################################
## Language identification using TextCat
######################################################################
[docs]class TextCat(object):
_corpus = None
fingerprints = {}
_START_CHAR = "<"
_END_CHAR = ">"
last_distances = {}
def __init__(self):
if not re:
raise EnvironmentError(
"classify.textcat requires the regex module that "
"supports unicode. Try '$ pip install regex' and "
"see https://pypi.python.org/pypi/regex for "
"further details."
)
from nltk.corpus import crubadan
self._corpus = crubadan
# Load all language ngrams into cache
for lang in self._corpus.langs():
self._corpus.lang_freq(lang)
[docs] def remove_punctuation(self, text):
''' Get rid of punctuation except apostrophes '''
return re.sub(r"[^\P{P}\']+", "", text)
[docs] def profile(self, text):
''' Create FreqDist of trigrams within text '''
from nltk import word_tokenize, FreqDist
clean_text = self.remove_punctuation(text)
tokens = word_tokenize(clean_text)
fingerprint = FreqDist()
for t in tokens:
token_trigram_tuples = trigrams(self._START_CHAR + t + self._END_CHAR)
token_trigrams = [''.join(tri) for tri in token_trigram_tuples]
for cur_trigram in token_trigrams:
if cur_trigram in fingerprint:
fingerprint[cur_trigram] += 1
else:
fingerprint[cur_trigram] = 1
return fingerprint
[docs] def calc_dist(self, lang, trigram, text_profile):
''' Calculate the "out-of-place" measure between the
text and language profile for a single trigram '''
lang_fd = self._corpus.lang_freq(lang)
dist = 0
if trigram in lang_fd:
idx_lang_profile = list(lang_fd.keys()).index(trigram)
idx_text = list(text_profile.keys()).index(trigram)
# print(idx_lang_profile, ", ", idx_text)
dist = abs(idx_lang_profile - idx_text)
else:
# Arbitrary but should be larger than
# any possible trigram file length
# in terms of total lines
if PY3:
dist = maxsize
else:
dist = maxint
return dist
[docs] def lang_dists(self, text):
''' Calculate the "out-of-place" measure between
the text and all languages '''
distances = {}
profile = self.profile(text)
# For all the languages
for lang in self._corpus._all_lang_freq.keys():
# Calculate distance metric for every trigram in
# input text to be identified
lang_dist = 0
for trigram in profile:
lang_dist += self.calc_dist(lang, trigram, profile)
distances[lang] = lang_dist
return distances
[docs] def guess_language(self, text):
''' Find the language with the min distance
to the text and return its ISO 639-3 code '''
self.last_distances = self.lang_dists(text)
return min(self.last_distances, key=self.last_distances.get)
#################################################')
[docs]def demo():
from nltk.corpus import udhr
langs = [
'Kurdish-UTF8',
'Abkhaz-UTF8',
'Farsi_Persian-UTF8',
'Hindi-UTF8',
'Hawaiian-UTF8',
'Russian-UTF8',
'Vietnamese-UTF8',
'Serbian_Srpski-UTF8',
'Esperanto-UTF8',
]
friendly = {
'kmr': 'Northern Kurdish',
'abk': 'Abkhazian',
'pes': 'Iranian Persian',
'hin': 'Hindi',
'haw': 'Hawaiian',
'rus': 'Russian',
'vie': 'Vietnamese',
'srp': 'Serbian',
'epo': 'Esperanto',
}
tc = TextCat()
for cur_lang in langs:
# Get raw data from UDHR corpus
raw_sentences = udhr.sents(cur_lang)
rows = len(raw_sentences) - 1
cols = list(map(len, raw_sentences))
sample = ''
# Generate a sample text of the language
for i in range(0, rows):
cur_sent = ''
for j in range(0, cols[i]):
cur_sent += ' ' + raw_sentences[i][j]
sample += cur_sent
# Try to detect what it is
print('Language snippet: ' + sample[0:140] + '...')
guess = tc.guess_language(sample)
print('Language detection: %s (%s)' % (guess, friendly[guess]))
print('#' * 140)
if __name__ == '__main__':
demo()