生成器实现(yield)

一、生成器既是迭代器对象也是可迭代对象

二、函数中含有yield关键字，即是生成器函数

三、自定义实现生成器进行for循环实例：

class WeatherIterable(Iterable):
    def __init__(self, cities):
        self.cities = cities
        self.index = 0

    def __iter__(self):
        for _ in range(len(self.cities)):
            yield self.get_weather()
            self.index += 1

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


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