用Python进行自然语言处理-2. Accessing Text Corpora and Lexical Resources
1. 处理文本语料库
1.1 古腾堡语料库
这是一个电子书语料库，目前提供49,000本免费电子书。
我们可以看看nltk里集成了多少电子书：
>>> import nltk
>>> nltk.corpus.gutenberg.fileids()
['austen-emma.txt', 'austen-persuasion.txt', 'austen-sense.txt', 'bible-kjv.txt',
'blake-poems.txt', 'bryant-stories.txt', 'burgess-busterbrown.txt',
'carroll-alice.txt', 'chesterton-ball.txt', 'chesterton-brown.txt',
'chesterton-thursday.txt', 'edgeworth-parents.txt', 'melville-moby_dick.txt',
'milton-paradise.txt', 'shakespeare-caesar.txt', 'shakespeare-hamlet.txt',
'shakespeare-macbeth.txt', 'whitman-leaves.txt']我们可以用处理text1…text10的方法去处理。包括len、concordance等等。
我们还可以用raw方法获得所有字符，用words方法获得所有单词，用sents方法获得所有句子。进而可以计算平均词长、平均句长、用词多样性。
>>> for fileid in gutenberg.fileids():
... num_chars = len(gutenberg.raw(fileid)) [1]
... num_words = len(gutenberg.words(fileid))
... num_sents = len(gutenberg.sents(fileid))
... num_vocab = len(set(w.lower() for w in gutenberg.words(fileid)))
... print(round(num_chars/num_words), round(num_words/num_sents), round(num_words/num_vocab), fileid)
...
5 25 26 austen-emma.txt
5 26 17 austen-persuasion.txt
5 28 22 austen-sense.txt
4 34 79 bible-kjv.txt
5 19 5 blake-poems.txt
4 19 14 bryant-stories.txt
4 18 12 burgess-busterbrown.txt
4 20 13 carroll-alice.txt
5 20 12 chesterton-ball.txt
5 23 11 chesterton-brown.txt
5 18 11 chesterton-thursday.txt
4 21 25 edgeworth-parents.txt
5 26 15 melville-moby_dick.txt
5 52 11 milton-paradise.txt
4 12 9 shakespeare-caesar.txt
4 12 8 shakespeare-hamlet.txt
4 12 7 shakespeare-macbeth.txt
5 36 12 whitman-leaves.txt1.2 网络聊天语料库
从略
1.3 布朗语料库
这是一个分类语料库：
| ID | File | Genre | Description |
|---|---|---|---|
| A16 | ca16 | news | Chicago Tribune: Society Reportage |
| B02 | cb02 | editorial | Christian Science Monitor: Editorials |
| C17 | cc17 | reviews | Time Magazine: Reviews |
| D12 | cd12 | religion | Underwood: Probing the Ethics of Realtors |
| E36 | ce36 | hobbies | Norling: Renting a Car in Europe |
| F25 | cf25 | lore | Boroff: Jewish Teenage Culture |
| G22 | cg22 | belles_lettres | Reiner: Coping with Runaway Technology |
| H15 | ch15 | government | US Office of Civil and Defence Mobilization: The Family Fallout Shelter |
| J17 | cj19 | learned | Mosteller: Probability with Statistical Applications |
| K04 | ck04 | fiction | W.E.B. Du Bois: Worlds of Color |
| L13 | cl13 | mystery | Hitchens: Footsteps in the Night |
| M01 | cm01 | science_fiction | Heinlein: Stranger in a Strange Land |
| N14 | cn15 | adventure | Field: Rattlesnake Ridge |
| P12 | cp12 | romance | Callaghan: A Passion in Rome |
| R06 | cr06 | humor | Thurber: The Future, If Any, of Comedy |
可以用categories方法获得所有分类：
>>> from nltk.corpus import brown
>>> brown.categories()
['adventure', 'belles_lettres', 'editorial', 'fiction', 'government', 'hobbies',
'humor', 'learned', 'lore', 'mystery', 'news', 'religion', 'reviews', 'romance',
'science_fiction']
>>> brown.words(categories='news')
['The', 'Fulton', 'County', 'Grand', 'Jury', 'said', ...]
>>> brown.words(fileids=['cg22'])
['Does', 'our', 'society', 'have', 'a', 'runaway', ',', ...]
>>> brown.sents(categories=['news', 'editorial', 'reviews'])
[['The', 'Fulton', 'County'...], ['The', 'jury', 'further'...], ...]利用布朗语料库我们可以研究不同文体之间的风格差异。比如，我们可以比较不同文体的情态动词的差异，这里用到了tabulate方法制作表格，也用到了条件概率分布方法ConditionalFreqDist获得不同文体的概率分布：
>>> cfd = nltk.ConditionalFreqDist(
... (genre, word)
... for genre in brown.categories()
... for word in brown.words(categories=genre))
>>> genres = ['news', 'religion', 'hobbies', 'science_fiction', 'romance', 'humor']
>>> modals = ['can', 'could', 'may', 'might', 'must', 'will']
>>> cfd.tabulate(conditions=genres, samples=modals)
can could may might must will
news 93 86 66 38 50 389
religion 82 59 78 12 54 71
hobbies 268 58 131 22 83 264
science_fiction 16 49 4 12 8 16
romance 74 193 11 51 45 43
humor 16 30 8 8 9 131.4 路透社语料库
方法同布朗语料库
1.5 总统就职演讲语料库
>>> from nltk.corpus import inaugural
>>> inaugural.fileids()
['1789-Washington.txt', '1793-Washington.txt', '1797-Adams.txt', ...]
>>> [fileid[:4] for fileid in inaugural.fileids()]
['1789', '1793', '1797', '1801', '1805', '1809', '1813', '1817', '1821', ...]
根据文件名以“年份-总统名”命名的特点，我们可以计算不同时期，某些词的频度分布：
>>> cfd = nltk.ConditionalFreqDist(
... (target, fileid[:4])
... for fileid in inaugural.fileids()
... for w in inaugural.words(fileid)
... for target in ['america', 'citizen']
... if w.lower().startswith(target)) [1]
>>> cfd.plot()