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| # Natural Language Toolkit: Utility functions | |
| # | |
| # Copyright (C) 2001-2020 NLTK Project | |
| # Author: Steven Bird <[email protected]> | |
| # URL: <http://nltk.org/> | |
| # For license information, see LICENSE.TXT | |
| from itertools import chain | |
| def pad_sequence( | |
| sequence, | |
| n, | |
| pad_left=False, | |
| pad_right=False, | |
| left_pad_symbol=None, | |
| right_pad_symbol=None, | |
| ): | |
| """ | |
| Returns a padded sequence of items before ngram extraction. | |
| >>> list(pad_sequence([1,2,3,4,5], 2, pad_left=True, pad_right=True, left_pad_symbol='<s>', right_pad_symbol='</s>')) | |
| ['<s>', 1, 2, 3, 4, 5, '</s>'] | |
| >>> list(pad_sequence([1,2,3,4,5], 2, pad_left=True, left_pad_symbol='<s>')) | |
| ['<s>', 1, 2, 3, 4, 5] | |
| >>> list(pad_sequence([1,2,3,4,5], 2, pad_right=True, right_pad_symbol='</s>')) | |
| [1, 2, 3, 4, 5, '</s>'] | |
| :param sequence: the source data to be padded | |
| :type sequence: sequence or iter | |
| :param n: the degree of the ngrams | |
| :type n: int | |
| :param pad_left: whether the ngrams should be left-padded | |
| :type pad_left: bool | |
| :param pad_right: whether the ngrams should be right-padded | |
| :type pad_right: bool | |
| :param left_pad_symbol: the symbol to use for left padding (default is None) | |
| :type left_pad_symbol: any | |
| :param right_pad_symbol: the symbol to use for right padding (default is None) | |
| :type right_pad_symbol: any | |
| :rtype: sequence or iter | |
| """ | |
| sequence = iter(sequence) | |
| if pad_left: | |
| sequence = chain((left_pad_symbol,) * (n - 1), sequence) | |
| if pad_right: | |
| sequence = chain(sequence, (right_pad_symbol,) * (n - 1)) | |
| return sequence | |
| # add a flag to pad the sequence so we get peripheral ngrams? | |
| def ngrams( | |
| sequence, | |
| n, | |
| pad_left=False, | |
| pad_right=False, | |
| left_pad_symbol=None, | |
| right_pad_symbol=None, | |
| ): | |
| """ | |
| Return the ngrams generated from a sequence of items, as an iterator. | |
| For example: | |
| >>> from nltk.util import ngrams | |
| >>> list(ngrams([1,2,3,4,5], 3)) | |
| [(1, 2, 3), (2, 3, 4), (3, 4, 5)] | |
| Wrap with list for a list version of this function. Set pad_left | |
| or pad_right to true in order to get additional ngrams: | |
| >>> list(ngrams([1,2,3,4,5], 2, pad_right=True)) | |
| [(1, 2), (2, 3), (3, 4), (4, 5), (5, None)] | |
| >>> list(ngrams([1,2,3,4,5], 2, pad_right=True, right_pad_symbol='</s>')) | |
| [(1, 2), (2, 3), (3, 4), (4, 5), (5, '</s>')] | |
| >>> list(ngrams([1,2,3,4,5], 2, pad_left=True, left_pad_symbol='<s>')) | |
| [('<s>', 1), (1, 2), (2, 3), (3, 4), (4, 5)] | |
| >>> list(ngrams([1,2,3,4,5], 2, pad_left=True, pad_right=True, left_pad_symbol='<s>', right_pad_symbol='</s>')) | |
| [('<s>', 1), (1, 2), (2, 3), (3, 4), (4, 5), (5, '</s>')] | |
| :param sequence: the source data to be converted into ngrams | |
| :type sequence: sequence or iter | |
| :param n: the degree of the ngrams | |
| :type n: int | |
| :param pad_left: whether the ngrams should be left-padded | |
| :type pad_left: bool | |
| :param pad_right: whether the ngrams should be right-padded | |
| :type pad_right: bool | |
| :param left_pad_symbol: the symbol to use for left padding (default is None) | |
| :type left_pad_symbol: any | |
| :param right_pad_symbol: the symbol to use for right padding (default is None) | |
| :type right_pad_symbol: any | |
| :rtype: sequence or iter | |
| """ | |
| sequence = pad_sequence( | |
| sequence, n, pad_left, pad_right, left_pad_symbol, right_pad_symbol | |
| ) | |
| history = [] | |
| while n > 1: | |
| # PEP 479, prevent RuntimeError from being raised when StopIteration bubbles out of generator | |
| try: | |
| next_item = next(sequence) | |
| except StopIteration: | |
| # no more data, terminate the generator | |
| return | |
| history.append(next_item) | |
| n -= 1 | |
| for item in sequence: | |
| history.append(item) | |
| yield tuple(history) | |
| del history[0] |