python生成器——廖雪峰教程笔记

部分转自https://www.liaoxuefeng.com/wiki/0014316089557264a6b348958f449949df42a6d3a2e542c000/0014317799226173f45ce40636141b6abc8424e12b5fb27000：

一，生成器：

1、 #g 相当于__iter__返回的对象，可调用next(g) 拿到下一个值

>>> g = (x * x for x in range(10))>>> for n in g:...     print(n)

2、g = fib(6)生成一个generator对象<generator object fib at 0x104feaaa0>，然后可调用next(g)，遇到yield返回，下次从上次返回的yield处开始执行

>>> for n in fib(6):...     print(n)

def fib(max):    n, a, b = 0, 0, 1    while n < max:        yield b        a, b = b, a + b        n = n + 1    return 'done'

>>> g = fib(6)>>> while True:...     try:...         x = next(g)...         print('g:', x)...     except StopIteration as e:...         print('Generator return value:', e.value)...         break...g: 1g: 1g: 2g: 3g: 5g: 8Generator return value: done

>>> g = (x*x for x in range(10))>>> g<generator object <genexpr> at 0x02A37D50>>>> def fib(max):a, b = 1, 1n = 0while n < max:yield aa, b = b, a+bn = n+1>>> fib<function fib at 0x02A3F6A8>>>> g = fib(6)>>> g<generator object fib at 0x02A37D80>>>> for n in g:print(n)112358>>> g = fib(6)>>> while True:try:n = next(g)print(n)except StopIteration as e:print('Generator return value:', e.value)break112358Generator return value: None

转自：https://www.liaoxuefeng.com/wiki/0014316089557264a6b348958f449949df42a6d3a2e542c000/0014319098638265527beb24f7840aa97de564ccc7f20f6000

二，

__iter__  类

如果一个类想被用于for ... in循环，类似list或tuple那样，就必须实现一个__iter__()方法，该方法返回一个迭代对象，然后，Python的for循环就会不断调用该迭代对象的__next__()方法拿到循环的下一个值，直到遇到StopIteration错误时退出循环。

我们以斐波那契数列为例，写一个Fib类，可以作用于for循环：

class Fib(object):    def __init__(self):        self.a, self.b = 0, 1 # 初始化两个计数器a，b    def __iter__(self):        return self # 实例本身就是迭代对象，故返回自己    def __next__(self):        self.a, self.b = self.b, self.a + self.b # 计算下一个值        if self.a > 100000: # 退出循环的条件            raise StopIteration()        return self.a # 返回下一个值

现在，试试把Fib实例作用于for循环：

>>> for n in Fib():...     print(n)...11235...4636875025

>>> class Fib(object):def __init__(self):self.a = 0self.b = 1def __iter__(self):return selfdef __next__(self):self.a, self.b = self.b, self.a+self.bif self.a > 100:raise StopIterationreturn self.a>>> Fib<class '__main__.Fib'>>>> g = Fib()>>> g<__main__.Fib object at 0x02A38E70>>>> next(g)1>>> for n in g:print(n)123581321345589