python - NLTK 标记化 - 更快的方式？

Question

我有一个接受字符串参数的方法，并使用 NLTK 将字符串分解为句子，然后分解为单词。然后，它将每个单词转换为小写，最后创建一个包含每个单词出现频率的字典。

import nltk
from collections import Counter

def freq(string):
    f = Counter()
    sentence_list = nltk.tokenize.sent_tokenize(string)
    for sentence in sentence_list:
        words = nltk.word_tokenize(sentence)
        words = [word.lower() for word in words]
        for word in words:
            f[word] += 1
    return f

我应该进一步优化上述代码以缩短预处理时间，但我不确定该怎么做。返回值显然应该与上面的完全相同，所以我希望使用 nltk，尽管没有明确要求这样做。

有什么方法可以加快上面的代码？谢谢。

Answer 1

如果您只想要一个简单的标记列表，请注意 word_tokenize 会隐式调用 sent_tokenize，请参阅 https://github.com/nltk/nltk/blob/develop/nltk/tokenize/init.py#L98

_treebank_word_tokenize = TreebankWordTokenizer().tokenize
def word_tokenize(text, language='english'):
    """
    Return a tokenized copy of *text*,
    using NLTK's recommended word tokenizer
    (currently :class:`.TreebankWordTokenizer`
    along with :class:`.PunktSentenceTokenizer`
    for the specified language).
    :param text: text to split into sentences
    :param language: the model name in the Punkt corpus
    """
    return [token for sent in sent_tokenize(text, language)
            for token in _treebank_word_tokenize(sent)]

以棕色语料库为例，配合Counter(word_tokenize(string_corpus)):

>>> from collections import Counter
>>> from nltk.corpus import brown
>>> from nltk import sent_tokenize, word_tokenize
>>> string_corpus = brown.raw() # Plaintext, str type.
>>> start = time.time(); fdist = Counter(word_tokenize(string_corpus)); end = time.time() - start
>>> end
12.662328958511353
>>> fdist.most_common(5)
[(u',', 116672), (u'/', 89031), (u'the/at', 62288), (u'.', 60646), (u'./', 48812)]
>>> sum(fdist.values())
1423314

~140 万个单词在我的机器上用了 12 秒(没有保存标记化的语料库):

alvas@ubi:~$ cat /proc/cpuinfo
processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 69
model name  : Intel(R) Core(TM) i5-4200U CPU @ 1.60GHz
stepping    : 1
microcode   : 0x17
cpu MHz     : 1600.027
cache size  : 3072 KB
physical id : 0
siblings    : 4
core id     : 0
cpu cores   : 2

$ cat /proc/meminfo
MemTotal:       12004468 kB

首先保存标记化语料库tokenized_corpus = [word_tokenize(sent) for sent in sent_tokenize(string_corpus)]，然后使用Counter(chain*(tokenized_corpus)):

>>> from itertools import chain
>>> start = time.time(); tokenized_corpus = [word_tokenize(sent) for sent in sent_tokenize(string_corpus)]; fdist = Counter(chain(*tokenized_corpus)); end = time.time() - start
>>> end
16.421464920043945

使用 ToktokTokenizer()

>>> from collections import Counter
>>> import time
>>> from itertools import chain
>>> from nltk.corpus import brown
>>> from nltk import sent_tokenize, word_tokenize
>>> from nltk.tokenize import ToktokTokenizer
>>> toktok = ToktokTokenizer()
>>> string_corpus = brown.raw()

>>> start = time.time(); tokenized_corpus = [toktok.tokenize(sent) for sent in sent_tokenize(string_corpus)]; fdist = Counter(chain(*tokenized_corpus)); end = time.time() - start 
>>> end
10.00472116470337

使用 MosesTokenizer():

>>> from nltk.tokenize.moses import MosesTokenizer
>>> moses = MosesTokenizer()
>>> start = time.time(); tokenized_corpus = [moses.tokenize(sent) for sent in sent_tokenize(string_corpus)]; fdist = Counter(chain(*tokenized_corpus)); end = time.time() - start 
>>> end
30.783339023590088
>>> start = time.time(); tokenized_corpus = [moses.tokenize(sent) for sent in sent_tokenize(string_corpus)]; fdist = Counter(chain(*tokenized_corpus)); end = time.time() - start 
>>> end
30.559681177139282

为什么要使用MosesTokenizer

它的实现方式是有一种方法可以将标记反转回字符串，即“detokenize”。

>>> from nltk.tokenize.moses import MosesTokenizer, MosesDetokenizer
>>> t, d = MosesTokenizer(), MosesDetokenizer()
>>> sent = "This ain't funny. It's actually hillarious, yet double Ls. | [] < > [ ] & You're gonna shake it off? Don't?"
>>> expected_tokens = [u'This', u'ain', u'&apos;t', u'funny.', u'It', u'&apos;s', u'actually', u'hillarious', u',', u'yet', u'double', u'Ls.', u'&#124;', u'&#91;', u'&#93;', u'&lt;', u'&gt;', u'&#91;', u'&#93;', u'&amp;', u'You', u'&apos;re', u'gonna', u'shake', u'it', u'off', u'?', u'Don', u'&apos;t', u'?']
>>> expected_detokens = "This ain't funny. It's actually hillarious, yet double Ls. | [] < > [] & You're gonna shake it off? Don't?"
>>> tokens = t.tokenize(sent)
>>> tokens == expected_tokens
True
>>> detokens = d.detokenize(tokens)
>>> " ".join(detokens) == expected_detokens
True

使用ReppTokenizer():

>>> repp = ReppTokenizer('/home/alvas/repp')
>>> start = time.time(); sentences = sent_tokenize(string_corpus); tokenized_corpus = repp.tokenize_sents(sentences); fdist = Counter(chain(*tokenized_corpus)); end = time.time() - start
>>> end
76.44129395484924

为什么要使用 ReppTokenizer？

它返回原始字符串中标记的偏移量。

>>> sents = ['Tokenization is widely regarded as a solved problem due to the high accuracy that rulebased tokenizers achieve.' ,
... 'But rule-based tokenizers are hard to maintain and their rules language specific.' ,
... 'We evaluated our method on three languages and obtained error rates of 0.27% (English), 0.35% (Dutch) and 0.76% (Italian) for our best models.'
... ]
>>> tokenizer = ReppTokenizer('/home/alvas/repp/') # doctest: +SKIP
>>> for sent in sents:                             # doctest: +SKIP
...     tokenizer.tokenize(sent)                   # doctest: +SKIP
... 
(u'Tokenization', u'is', u'widely', u'regarded', u'as', u'a', u'solved', u'problem', u'due', u'to', u'the', u'high', u'accuracy', u'that', u'rulebased', u'tokenizers', u'achieve', u'.')
(u'But', u'rule-based', u'tokenizers', u'are', u'hard', u'to', u'maintain', u'and', u'their', u'rules', u'language', u'specific', u'.')
(u'We', u'evaluated', u'our', u'method', u'on', u'three', u'languages', u'and', u'obtained', u'error', u'rates', u'of', u'0.27', u'%', u'(', u'English', u')', u',', u'0.35', u'%', u'(', u'Dutch', u')', u'and', u'0.76', u'%', u'(', u'Italian', u')', u'for', u'our', u'best', u'models', u'.')
>>> for sent in tokenizer.tokenize_sents(sents): 
...     print sent                               
... 
(u'Tokenization', u'is', u'widely', u'regarded', u'as', u'a', u'solved', u'problem', u'due', u'to', u'the', u'high', u'accuracy', u'that', u'rulebased', u'tokenizers', u'achieve', u'.')
(u'But', u'rule-based', u'tokenizers', u'are', u'hard', u'to', u'maintain', u'and', u'their', u'rules', u'language', u'specific', u'.')
(u'We', u'evaluated', u'our', u'method', u'on', u'three', u'languages', u'and', u'obtained', u'error', u'rates', u'of', u'0.27', u'%', u'(', u'English', u')', u',', u'0.35', u'%', u'(', u'Dutch', u')', u'and', u'0.76', u'%', u'(', u'Italian', u')', u'for', u'our', u'best', u'models', u'.')
>>> for sent in tokenizer.tokenize_sents(sents, keep_token_positions=True): 
...     print sent
... 
[(u'Tokenization', 0, 12), (u'is', 13, 15), (u'widely', 16, 22), (u'regarded', 23, 31), (u'as', 32, 34), (u'a', 35, 36), (u'solved', 37, 43), (u'problem', 44, 51), (u'due', 52, 55), (u'to', 56, 58), (u'the', 59, 62), (u'high', 63, 67), (u'accuracy', 68, 76), (u'that', 77, 81), (u'rulebased', 82, 91), (u'tokenizers', 92, 102), (u'achieve', 103, 110), (u'.', 110, 111)]
[(u'But', 0, 3), (u'rule-based', 4, 14), (u'tokenizers', 15, 25), (u'are', 26, 29), (u'hard', 30, 34), (u'to', 35, 37), (u'maintain', 38, 46), (u'and', 47, 50), (u'their', 51, 56), (u'rules', 57, 62), (u'language', 63, 71), (u'specific', 72, 80), (u'.', 80, 81)]
[(u'We', 0, 2), (u'evaluated', 3, 12), (u'our', 13, 16), (u'method', 17, 23), (u'on', 24, 26), (u'three', 27, 32), (u'languages', 33, 42), (u'and', 43, 46), (u'obtained', 47, 55), (u'error', 56, 61), (u'rates', 62, 67), (u'of', 68, 70), (u'0.27', 71, 75), (u'%', 75, 76), (u'(', 77, 78), (u'English', 78, 85), (u')', 85, 86), (u',', 86, 87), (u'0.35', 88, 92), (u'%', 92, 93), (u'(', 94, 95), (u'Dutch', 95, 100), (u')', 100, 101), (u'and', 102, 105), (u'0.76', 106, 110), (u'%', 110, 111), (u'(', 112, 113), (u'Italian', 113, 120), (u')', 120, 121), (u'for', 122, 125), (u'our', 126, 129), (u'best', 130, 134), (u'models', 135, 141), (u'.', 141, 142)]

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长话短说

不同分词器的优势

• word_tokenize() 隐式调用 sent_tokenize()
• ToktokTokenizer() 最快
• MosesTokenizer() 能够对文本进行去标记
• ReppTokenizer() 能够提供 token 偏移量

问:在 NLTK 中是否有一个快速分词器可以去分词并提供偏移量并进行句子分词？

A:我不这么认为，试试gensim或spacy。