April 2023
Intermediate to advanced
769 pages
25h 16m
Chinese
Content preview from 流畅的Python（第2版）
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迭代器、生成器和经典协程
｜
467
除了
__iter__
方法之外，你可能还想在
Sentence
类中实现
__next__
方法，让
Sentence
实例既是可迭代对象，也是自身的迭代器。可是，这种想法非常糟糕。
Alex Martelli
在
Google
有大量
Python
代码审查经验，他指出，这也是常见的反模式。
《设计模式》一书讲解迭代器设计模式时，在“适用性”一节中说：
迭代器模式可用来：
•
访问一个聚合对象的内容而无须暴露它的内部表示；
•
支持对聚合对象的多种遍历；
•
为遍历不同的聚合结构提供一个统一的接口（即支持多态迭代）。
为了“支持多种遍历”，必须能从同一个可迭代对象中获取多个独立的迭代器，而且各个
迭代器要能维护自身的内部状态。因此这一模式正确的实现方式是，每次调用
iter(my_
iterable)
都新建一个独立的迭代器。这就是本例需要
SentenceIterator
类的原因。
至此，我们演示了如何正确实现经典迭代器模式。现在，把本节内容忘掉。
Python
从
Barbara Liskov
发明的
CLU
语言中借鉴了
yield
关键字，因此我们无须自己动手编写实现
迭代器的代码。
17.5.3
节将展示如何使用更地道的方式实现
Sentence
类。
17.5.3
Sentence
类第
3
版
：
生成器函数
实现相同功能，但符合
Python
习惯的方式是，用生成器代替
SentenceIterator
类。先看
示例
17-5
，然后详细说明生成器。
示例
17-5
sentence_gen.py
：使用生成器实现
Sentence ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.