gensim实现LDA(Latent Dirichlet Allocation)算法提取主题词(topic)

 Latent Dirichlet Allocation（LDA) 隐含分布作为目前最受欢迎的主题模型算法被广泛使用。LDA能够将文本集合转化为不同概率的主题集合。需要注意的是LDA是利用统计手段对主题词汇进行到的处理，是一种词袋（bag-of-words）方法。如：
 输入：

第一段：“Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure. It is altogether fitting and proper that we should do this.”
第二段：‘Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.’
第三段："We are met on a great battle-field of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that nation might live. "

 输出：

(0, u'0.032*"conceive" + 0.032*"dedicate" + 0.032*"nation" + 0.032*"life"')
(1, u'0.059*"conceive" + 0.059*"score" + 0.059*"seven" + 0.059*"proposition"')
(2, u'0.103*"nation" + 0.071*"dedicate" + 0.071*"great" + 0.071*"field"')
(3, u'0.032*"conceive" + 0.032*"nation" + 0.032*"dedicate" + 0.032*"rest"')
(4, u'0.032*"conceive" + 0.032*"nation" + 0.032*"dedicate" + 0.032*"battle"')

 本文将简单介绍如何使用Python 的nltk、spacy、gensim包，实现包括预处理流程在内的LDA算法。

1. 预处理：

1.1 分词处理

#第一次使用需要首先下载en包:
#python -m spacy download en
import spacy
spacy.load('en_core_web_sm')
from spacy.lang.en import English
parser = English()
#对文章内容进行清洗并将单词统一降为小写
def tokenize(text):
    lda_tokens = []
    tokens = parser(text)
    for token in tokens:
        if token.orth_.isspace():
            continue
        elif token.like_url:
            lda_tokens.append('URL')
        elif token.orth_.startswith('@'):
            lda_tokens.append('SCREEN_NAME')
        else:
            lda_tokens.append(token.lower_)
    return lda_tokens

1.2 lemma处理

 lemma与stem都是NLP中常用的对于单词的处理：
lemma 将变形了的单词还原为元单词 “dictionaries”–>“dictionary”
stem 从单词中抽取词根 “dictionaries”—>“dict”

#引入一个同义词、近义词、反义词包
import nltk
#第一次使用需要下载这个nltk包
# nltk.download('wordnet')

from nltk.corpus import wordnet as wn
def get_lemma(word):
    #dogs->dog
    #aardwolves->aardwolf'
    #sichuan->sichuan
    lemma = wn.morphy(word)
    if lemma is None:
        return word
    else:
        return lemma

1.3 从nltk包中引入英文停顿词停顿词处理

#第一次使用需要下载停顿词
# nltk.download('stopwords')

en_stop = set(nltk.corpus.stopwords.words('english'))

1.4 预处理流程

预处理的过程包括以上所提及的分词、lemma处理及停顿词处理

#定义预处理函数
def prepare_text_for_lda(text):
    #分词处理
    tokens = tokenize(text)
    #取出长度大于4的单词
    tokens = [token for token in tokens if len(token) > 4]
    #取出非停顿词
    tokens = [token for token in tokens if token not in en_stop]
    #对词语进行还原
    tokens = [get_lemma(token) for token in tokens]
    return tokens

2. LDA算法

2.1 预处理文本集合

 通过预处理函数加载文本集合，需要注意的是，gensim:models.ldamodel 处理对象是一个文本集合而不是文本集，因此其输入应该为[[],``````,[]]结构而不是[]

    text_1 = u"Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure. It is altogether fitting and proper that we should do this."
    text_2 = u'Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.'
    text_3 = u"We are met on a great battle-field of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that nation might live. "
    text_data_1 = prepare_text_for_lda(text_1)
    text_data_2 = prepare_text_for_lda(text_2)
    text_data_3 = prepare_text_for_lda(text_3)
    text_data =[]
    text_data.append(text_data_1)
    text_data.append(text_data_2)
    text_data.append(text_data_3)
    print "text_data :",text_data

 通过对于三个string的预处理并组合成为一个list集合，数据如下：

[[u'engage', u'great', u'civil', u'testing', u'whether', u'nation', u'nation', u'conceive', u'dedicate', u'endure', u'altogether', u'fitting', u'proper'], [u'score', u'seven', u'years', u'father', u'bring', u'forth', u'continent', u'nation', u'conceive', u'liberty', u'dedicate', u'proposition', u'create', u'equal'], [u'great', u'battle', u'field', u'dedicate', u'portion', u'field', u'final', u'rest', u'place', u'life', u'nation', u'might']]

2.2 使用LDA算法提取主题词

 需要注意的是，如下实现LDA算法的gensim.models.ldamodel.LdaModel()与生成的corpus、dictionary密切相关。

    #加载gensim 
    #使用gensim.Dictionary从text_data中生成一个词袋（bag-of-words)
    dictionary = corpora.Dictionary(text_data)
    corpus = [dictionary.doc2bow(text) for text in text_data]

    #加载gensim，使用LDA算法求得前五的topic，
    #同时生成的topic在之后也会被使用到来定义文本所属主题
    
    NUM_TOPICS = 5#定义了生成的主题词的个数
    ldamodel = gensim.models.ldamodel.LdaModel(corpus,              
    	                                       num_topics = NUM_TOPICS,
    	                                       id2word=dictionary,
    	                                       passes=15)
    ldamodel.save('model5.gensim')
    topics = ldamodel.print_topics(num_words=4)
    for topic in topics:
        print(topic)

3. 附录遇到的问题及修改

3.1 来自spacy的报错

import spacy
spacy.load('en')

Traceback (most recent call last):
  File "topial_LDA.py", line 13, in 
    spacy.load('en')
  File "C:\Python27\lib\site-packages\spacy\__init__.py", line 15, in load
    return util.load_model(name, **overrides)
  File "C:\Python27\lib\site-packages\spacy\util.py", line 119, in load_model
    raise IOError(Errors.E050.format(name=name))
IOError: [E050] Can't find model 'en'. It doesn't seem to be a shortcut link, a Python package or a valid path to a data directory.

 这条报错是因为没有向spacy指明引入的english类型的语言包具体是那个，在spacy中我们发现了如下多个包：

修改代码代码，实现功能：

import spacy
spacy.load('en_core_web_sm')

3.2 来自dictionary的报错

这个报错参考2.1

C:\Python27\lib\site-packages\gensim\utils.py:1209: UserWarning:
detected Windows; aliasing chunkize to chunkize_serial
warnings.warn(“detected Windows; aliasing chunkize to
chunkize_serial”) Traceback (most recent call last): File
“topial_LDA.py”, line 122, in
dictionary = corpora.Dictionary(text_data_1) File “C:\Python27\lib\site-packages\gensim\corpora\dictionary.py”, line 81,
in init
self.add_documents(documents, prune_at=prune_at) File “C:\Python27\lib\site-packages\gensim\corpora\dictionary.py”, line
198, in add_documents
self.doc2bow(document, allow_update=True) # ignore the result, here we only care about updating token ids File
“C:\Python27\lib\site-packages\gensim\corpora\dictionary.py”, line
236, in doc2bow
raise TypeError(“doc2bow expects an array of unicode tokens on input, not a single string”) TypeError: doc2bow expects an array of
unicode tokens on input, not a single string