Python实现基于最大概率法的汉语分词算法

import re
import math

MAX_SPLITLEN = 4#最大切分长度
corpus_lib = ''#corpus:语料


def init_corpus_lib(path): # 初始化语料库
 global corpus_lib
 with open(path, 'r', encoding='utf-8', errors='ignore') as file:
  corpus_lib = str(file.readlines())


def get_candidate_words(sen):
 global MAX_SPLITLEN
 global corpus_lib
 candidate_words = []
 for sp in range(len(sen)):
  w = sen[sp]
  candidate_words.append([w, sp, sp]) # 有些字可能不在语料库中，把它作为单个字加进去
  for mp in range(1, MAX_SPLITLEN): # 判断1 ~ MAX_SPLITLEN-1这3种词中是否有候选词.
   if sp + mp < len(sen):
    w += sen[sp + mp]
    if w in corpus_lib:
     candidate_words.append([w, sp, sp + mp]) # 存储词，初始位置，结束位置
 print('候选词有：%s' % candidate_words)
 return candidate_words


def segment_sentence(sen): # sen:sentence即要切分的句子
 global MAX_SPLITLEN
 global corpus_lib

 candidate_words = get_candidate_words(sen)
 count = 0
 for word in candidate_words:
  if count > 1000: # 为防止对长句子解析时间过长，放弃一部分精度追求效率
   break
  if word[1] == 0 and word[2] != len(sen) - 1: # 如果句子中开头的部分，还没有拼凑成整个词序列的话
   no_whitespace_sen = ''.join(word[0].split())
   for word in candidate_words: # word比如：['今天', 1, 2]，1是今在句子中的位置，2是天的位置
    if word[1] == 0 and word[2] != len(sen) - 1:
     end = word[2]
     for later_word in candidate_words:
      if later_word[1] == end + 1: # 如果later_word是当前词的后续词，那么拼接到当前词上
       word_seq = [word[0] + ' ' + later_word[0], word[1], later_word[2]] # 合并
       candidate_words.append(word_seq)
       # print('拼出了新词：%s' % word_seq)
       count += 1
     candidate_words.remove(word) # 遍历完后，这个开头部分短语要移除掉，不然下次遍历还会对它做无用功
 print('所有结果词序列有：%s' % candidate_words)

 word_segment_res_list = [] # 存储分词结果序列
 for seque in candidate_words:
  if seque[1] == 0 and seque[2] == len(sen) - 1:
   word_segment_res_list.append(seque[0])
 print('获得的所有分词结果是：')
 print(word_segment_res_list)
 return word_segment_res_list


# P(w1,w2,...,wn) = P(w1/start)P(w2/w1)P(w3/w2).....P(Wn/Wn-1)
# 下标从0开始： = P(w0/start)P(w1/w0)...P(Wn-1/Wn-2)
def calculate_word_sequence_probability(sequence):
 global corpus_lib
 word_list = sequence.split(' ')
 total_word_num = len(corpus_lib)
 prob_total = 0.0
 word_start = word_list[0]
 # 计算第一个词出现的概率P(w1/start)=Count(w1)/total
 count = len(re.findall(r'\s' + word_start + r'\s', corpus_lib)) + 1 # 加1平滑
 prob_total += math.log(count / total_word_num)
 # 计算P(w2/w1)P(w3/w2).....P(Wn/Wn-1)
 for i in range(len(word_list) - 1): # 0~ n-2
  prev_w = word_list[i]
  later_w = word_list[i + 1]
  count = len(re.findall(r'\s' + prev_w + r'\s' + later_w + r'\s', corpus_lib))
  count += 1 # 做一次加1平滑
  prob_total += math.log(count / total_word_num)
 print('%s的概率是：' % sequence)
 print(prob_total)
 return prob_total


def calculate_biggest_prob(word_segm_res):
 best_w_s = ''
 max_prob = 0.0
 for w_s in word_segm_res: # 改进：先只计算词的数目<=0.6 句子字数的，如果不行再计算全部的概率
  no_whitespace_sen = ''.join(w_s.split())
  zi_shu = len(no_whitespace_sen)
  if len(w_s.split(' ')) <= zi_shu * 0.6:
   prob = calculate_word_sequence_probability(w_s)
   if max_prob == 0 or max_prob < prob:
    best_w_s = w_s
    max_prob = prob
  if best_w_s == '': # 如果上面的0.6不行的话，再计算全部的概率
   prob = calculate_word_sequence_probability(w_s)
   if max_prob == 0 or max_prob < prob:
    best_w_s = w_s
    max_prob = prob
 print('最好的分词结果（概率为%s）是 ：%s' % (math.pow(math.e, max_prob), best_w_s))
 return best_w_s


def split_middle(sen_to_segment): # 从中间切分一下，返回中间切分的位置
 length = len(sen_to_segment)
 start = int(length / 2) - 2
 end = start + 5
 # 对中间的5个字进行切分，然后找第一个空格，按此把整个句子一分为二
 middle_part = sen_to_segment[start:end]
 best_segm_res = calculate_biggest_prob(segment_sentence(middle_part))
 return start + best_segm_res.index(' ') - 1


def split_mark_and_too_long_sent(sentences): # 按任意标点符号划分句子，对每个短句进行分词
 sen_list = sentences.splitlines()
 print(sen_list)

 out_text = ''
 for line in sen_list:
  sen_to_segment = '' #
  for single_char in line:
   if single_char.isalpha(): # isalpha()表示是否是单词，如果是单词的为True，标点符号等为False
    sen_to_segment += single_char
   elif not single_char.isalpha() and sen_to_segment == '': # 如果single_char是标点符号、数字,且前面没有待分词的句子
    out_text += single_char + ' '
    print(single_char)

   else: # 如果single_char是标点符号、数字,
    # 如果句子太长，先从中间切分一下
    if len(sen_to_segment) >= 20:
     middle = split_middle(sen_to_segment)
     left_half = sen_to_segment[0:middle + 1] # 左半部分
     best_segm_res = calculate_biggest_prob(segment_sentence(left_half))
     out_text += best_segm_res + ' '
     sen_to_segment = sen_to_segment[middle + 1:len(sen_to_segment)] # 右半部分交给后面几行处理

    best_segm_res = calculate_biggest_prob(segment_sentence(sen_to_segment))
    print(single_char)
    sen_to_segment = ''
    out_text += best_segm_res + ' ' + single_char + ' ' # 标点两侧也用空格隔起来

  # 如果这行句子最后还有一些文字没有切分的话
  if sen_to_segment != '':
   best_segm_res = calculate_biggest_prob(segment_sentence(sen_to_segment))
   out_text += best_segm_res + ' '
  out_text += '\n'

 with open('D:/1佩王的文件/计算语言学基础/生成结果.txt','w') as file:
  file.write(out_text)
 print(out_text)


if __name__ == '__main__':
 path = 'D:/1佩王的文件/计算语言学基础/北大(人民日报)语料库199801.txt'
 init_corpus_lib(path)#初始化语料库

 sentences = ''
 path = 'E:/study/1.研一的课/计算语言学基础课件/testset.txt'#读取要切分的文章
 with open(path, 'r', encoding='gbk', errors='ignore') as file:
  for line in file.readlines():
   sentences += line

 # 改进：先对句子按标点符号划分成多个短句，然后对每个短句进行切分、计算概率
 split_mark_and_too_long_sent(sentences)