python迭代器和生成器

1.經典迭代器

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: #隱式調用iter(s),加入s存在__iter__則調用返回迭代器，不然若s存在__getitem__，則默認生成迭代器，調用__getitem__生成元素
        print(word)
if __name__ == '__main__':
    main()

 

2.將Sentence中的__iter__改爲生成器函數

def __iter__(self): 
    for word in self.words:
    yield word #yield爲生成器關鍵字

改爲生成器後用法不變，但更加簡潔。

3.惰性實現

當列表比較大，佔內存較大時，咱們能夠採用惰性實現，每次只讀取一個元素到內存。

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

或者使用更簡潔的生成器表達式

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

4.yield from

itertools模塊含有大量生成器函數可供利用

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

等價於

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