Python基础:一起来面向对象 (二) 之搜索引擎

实例 搜索引擎

　　一个搜索引擎由搜索器、索引器、检索器和用户接口四个部分组成

　　搜索器就是爬虫(scrawler),爬出的内容送给索引器生成索引(Index)存储在内部数据库。用户通过用户接口发出询问(query),询问解析后送达检索器，检索器高效检索后，将结果返回给用户。

　　以下5个文件为爬取的搜索样本。

# # 1.txt
# I have a dream that my four little children will one day live in a nation where they will not be judged by the color of their skin but by the content of their character. I have a dream today.

# # 2.txt
# I have a dream that one day down in Alabama, with its vicious racists, . . . one day right there in Alabama little black boys and black girls will be able to join hands with little white boys and white girls as sisters and brothers. I have a dream today.

# # 3.txt
# I have a dream that one day every valley shall be exalted, every hill and mountain shall be made low, the rough places will be made plain, and the crooked places will be made straight, and the glory of the Lord shall be revealed, and all flesh shall see it together.

# # 4.txt
# This is our hope. . . With this faith we will be able to hew out of the mountain of despair a stone of hope. With this faith we will be able to transform the jangling discords of our nation into a beautiful symphony of brotherhood. With this faith we will be able to work together, to pray together, to struggle together, to go to jail together, to stand up for freedom together, knowing that we will be free one day. . . .

# # 5.txt
# And when this happens, and when we allow freedom ring, when we let it ring from every village and every hamlet, from every state and every city, we will be able to speed up that day when all of God's children, black men and white men, Jews and Gentiles, Protestants and Catholics, will be able to join hands and sing in the words of the old Negro spiritual: "Free at last! Free at last! Thank God Almighty, we are free at last!"

简单搜索引擎

　　SearchEngineBase 基类,corpus(语料)

class SearchEngineBase(object):
    def __init__(self):
        pass
    #将文件作为id,与内容一起送到process_corpus
    def add_corpus(self, file_path):
        with open(file_path, 'r') as fin:
            text = fin.read()
        self.process_corpus(file_path, text)
    #索引器 将文件路径作为id,将处理的内容作为索引存储
    def process_corpus(self, id, text):
        raise Exception('process_corpus not implemented.')
    #检索器
    def search(self, query):
        raise Exception('search not implemented.')

#多态
def main(search_engine):
    for file_path in ['1.txt', '2.txt', '3.txt', '4.txt', '5.txt']:
        search_engine.add_corpus(file_path)

    while True:
        query = input()
        results = search_engine.search(query)
        print('found {} result(s):'.format(len(results)))
        for result in results:
            print(result)

class SimpleEngine(SearchEngineBase):
    def __init__(self):
        super(SimpleEngine, self).__init__()
        self.__id_to_texts = dict()

    def process_corpus(self, id, text):
        self.__id_to_texts[id] = text

    def search(self, query):
        results = []
        for id, text in self.__id_to_texts.items():
            if query in text:
                results.append(id)
        return results

search_engine = SimpleEngine()
main(search_engine)

########## 输出 ##########
# simple
# found 0 result(s):
# whe
# found 2 result(s):
# 1.txt
# 5.txt

　　缺点:每次索引与检索都需要占用大量空间，还有查询只能是一个词或连续的几个词，对分散的在不同位置的多个词无能为力

词袋模型 (bag of words model)

　　运用词袋模型 (bag of words model),NLP领域最简单的模型之一。

　　process_corpus函数中调用parse_text_to_words把文档中的各个单词装进set集合中。

　　search函数中把包含查询关键字也打碎成set,与索引的文档核对，将匹配的id加入结果集。

 

import re
class BOWEngine(SearchEngineBase):
    def __init__(self):
        super(BOWEngine, self).__init__()
        self.__id_to_words = dict()

    def process_corpus(self, id, text):
        self.__id_to_words[id] = self.parse_text_to_words(text)

    def search(self, query):
        query_words = self.parse_text_to_words(query)
        results = []
        for id, words in self.__id_to_words.items():
            if self.query_match(query_words, words):
                results.append(id)
        return results

    @staticmethod
    def query_match(query_words, words):
        for query_word in query_words:
             if query_word not in words:
                 return False
        return True
        #for query_word in query_words:
        #    return False if query_word not in words else True

        #result = filter(lambda x:x not in words,query_words)
        #return False if (len(list(result)) > 0) else True

    @staticmethod
    def parse_text_to_words(text):
        # 使用正则表达式去除标点符号和换行符
        text = re.sub(r'[^\w ]', ' ', text)
        # 转为小写
        text = text.lower()
        # 生成所有单词的列表
        word_list = text.split(' ')
        # 去除空白单词
        word_list = filter(None, word_list)
        # 返回单词的 set
        return set(word_list)

search_engine = BOWEngine()
main(search_engine)

########## 输出 ##########
# i have a dream
# found 3 result(s):
# 1.txt
# 2.txt
# 3.txt
# freedom children
# found 1 result(s):
# 5.txt

　　缺点:每次search还是需要遍历所有文档

Inverted index 倒序索引

　　Inverted index 倒序索引，现在保留的是 word -> id 的字典

import re
class BOWInvertedIndexEngine(SearchEngineBase):
    def __init__(self):
        super(BOWInvertedIndexEngine, self).__init__()
        self.inverted_index = dict()

    #生成索引 word -> id
    def process_corpus(self, id, text):
        words = self.parse_text_to_words(text)
        for word in words:
            if word not in self.inverted_index:
                self.inverted_index[word] = []
            self.inverted_index[word].append(id) #{'little':['1.txt','2.txt'],...}

    def search(self, query):
        query_words = list(self.parse_text_to_words(query))
        query_words_index = list()
        for query_word in query_words:
            query_words_index.append(0)

        # 如果某一个查询单词的倒序索引为空，我们就立刻返回
        for query_word in query_words:
            if query_word not in self.inverted_index:
                return []

        result = []
        while True:

            # 首先，获得当前状态下所有倒序索引的 index
            current_ids = []

            for idx, query_word in enumerate(query_words):
                current_index = query_words_index[idx]
                current_inverted_list = self.inverted_index[query_word] #['1.txt','2.txt']

                # 已经遍历到了某一个倒序索引的末尾，结束 search
                if current_index >= len(current_inverted_list):
                    return result
                current_ids.append(current_inverted_list[current_index])

            # 然后，如果 current_ids 的所有元素都一样，那么表明这个单词在这个元素对应的文档中都出现了
            if all(x == current_ids[0] for x in current_ids):
                result.append(current_ids[0])
                query_words_index = [x + 1 for x in query_words_index]
                continue

            # 如果不是，我们就把最小的元素加一
            min_val = min(current_ids)
            min_val_pos = current_ids.index(min_val)
            query_words_index[min_val_pos] += 1

    @staticmethod
    def parse_text_to_words(text):
        # 使用正则表达式去除标点符号和换行符
        text = re.sub(r'[^\w ]', ' ', text)
        # 转为小写
        text = text.lower()
        # 生成所有单词的列表
        word_list = text.split(' ')
        # 去除空白单词
        word_list = filter(None, word_list)
        # 返回单词的 set
        return set(word_list)

search_engine = BOWInvertedIndexEngine()
main(search_engine)

########## 输出 ##########

# little
# found 2 result(s):
# 1.txt
# 2.txt
# little vicious
# found 1 result(s):
# 2.txt

 

LRUCache

　　如果90%以上都是重复搜索，为了提高性能，考虑增加缓存，使用Least Recently Used 近期最少使用算法实现

import pylru
class LRUCache(object):
    def __init__(self, size=32):
        self.cache = pylru.lrucache(size)

    def has(self, key):
        return key in self.cache

    def get(self, key):
        return self.cache[key]

    def set(self, key, value):
        self.cache[key] = value

class BOWInvertedIndexEngineWithCache(BOWInvertedIndexEngine, LRUCache):
    def __init__(self):
        super(BOWInvertedIndexEngineWithCache, self).__init__()
        LRUCache.__init__(self)

    def search(self, query):
        if self.has(query):
            print('cache hit!')
            return self.get(query)

        result = super(BOWInvertedIndexEngineWithCache, self).search(query)
        self.set(query, result)

        return result

search_engine = BOWInvertedIndexEngineWithCache()
main(search_engine)

########## 输出 ##########
# little
# found 2 result(s):
# 1.txt
# 2.txt
# little
# cache hit!
# found 2 result(s):
# 1.txt
# 2.txt

　　注意BOWInvertedIndexEngineWithCache继承了两个类。

　　在构造函数里直接使用super(BOWInvertedIndexEngineWithCache, self).__init__()来初始化BOWInvertedIndexEngine父类

　　对于多重继承的父类就要使用LRUCache.__init__(self)来初始化

　　

　　BOWInvertedIndexEngineWithCache里重载了search函数，在函数里面要调用父类BOWInvertedIndexEngine的search函数，使用：

　　result = super(BOWInvertedIndexEngineWithCache, self).search(query)

参考

　　极客时间《Python核心技术与实战》专栏

　　https://time.geekbang.org/column/intro/176