迭代器对象(Iterator)和可迭代对象(Itetable)实现

一、可迭代对象实现__iter__方法，返回迭代器对象

二、迭代器对象实现__iter__方法，返回迭代器对象，实现__next__方法，进行迭代操作

三、for循环执行的底层实现：首先调用可迭代对象的iter方法生成迭代器对象，再循环调用迭代器对象的next方法，直到抛出StopIteration错误，循环结束
　　iter(WeatherIterable) -> it -> next(it) -> StopIteration

四、自定义实现迭代器进行for循环实例：

import requests
from collections import Iterable, Iterator

# 实现自定义迭代器对象类
class WeatherIterator(Iterator):
    def __init__(self, cities):
        self.cities = cities
        self.index = 0

    def __next__(self):
        if self.index == len(self.cities):
            raise StopIteration
        self.index += 1
        return self.get_weather()

    def get_weather(self):
        city = self.cities[self.index-1]
        url = 'http://wthrcdn.etouch.cn/weather_mini?city=' + city
        res = requests.get(url=url).json()
        return city, res

# 实现自定义可迭代对象类
class WeatherIterable(Iterable):
    def __init__(self, cities):
        self.cities = cities

    def __iter__(self):
        return WeatherIterator(self.cities)


it = WeatherIterable(['北京', '深圳', '广州'])
for x in it:
    print(x)