python迭代器和生成器

1.经典迭代器

import re

RE_WORD = re.compile('w+')


class Sentence:

    def __init__(self, text):
        self.text = text
        self.words = RE_WORD.findall(text)


    def __iter__(self):  # <1>
        return SentenceIterator(self.words)  # <2>


class SentenceIterator:

    def __init__(self, words):
        self.words = words  # <3>
        self.index = 0  # <4>

    def __next__(self):
        try:
            word = self.words[self.index]  # <5>
        except IndexError:
            raise StopIteration()  # <6>
        self.index += 1  # <7>
        return word  # <8>

    def __iter__(self):  # <9>
        return self

def main():
   
    s = Sentence('hello all')
    for word in s: #隐式调用iter(s),加入s存在__iter__则调用返回迭代器，否则若s存在__getitem__，则默认生成迭代器，调用__getitem__生成元素
        print(word)
if __name__ == '__main__':
    main()

 

2.将Sentence中的__iter__改成生成器函数

def __iter__(self): 
    for word in self.words:
    yield word #yield为生成器关键字

改成生成器后用法不变，但更加简洁。

3.惰性实现

当列表比较大，占内存较大时，我们可以采用惰性实现，每次只读取一个元素到内存。

def __iter__(self):
   for match in RE_WORD.finditer(self.text): 
      yield match.group() 
　　 

或者使用更简洁的生成器表达式

 def __iter__(self):
    return (match.group() for match in RE_WORD.finditer(self.text))

4.yield from

itertools模块含有大量生成器函数可供利用

def chain(*iterables):
    for it in iterables:
        for i in it:
            yield i

等价于

def chain(*iterables):
    for it in iterables:
        yield from it