Python中的生成器和for循环

Question

所以我有一个生成器函数,看起来像这样.

def generator():
    while True:
        for x in range(3):
            for j in range(5):
                yield x

在我加载此函数并多次调用"next"之后,我希望它能够产生值

0 0 0 0 0 1 1 1 1 1 2 2 2 2 2 0 0 0 0 0 ...

但它只是一直产生0.这是为什么？

>>> execfile("test.py")
>>> generator
<function generator at 0x10b6121b8>
>>> generator().next()
0
>>> generator().next()
0
>>> generator().next()
0
>>> generator().next()
0
>>> generator().next()
0
>>> generator().next()
0
>>> generator().next()
0

Answer 1

generator() 初始化新的生成器对象:

In [4]: generator() is generator() # Creating 2 separate objects
Out[4]: False

然后generator().next()从新创建的生成器对象中获取第一个值(在您的情况下为0).

你应该打generator一次电话:

In [5]: gen = generator() # Storing new generator object, will reuse it

In [6]: [gen.next() for _ in range(6)] # Get first 6 values for demonstration purposes
Out[6]: [0, 0, 0, 0, 0, 1]

注意:generator.next已从Python 3中删除(PEP 3114) - 请改用此next函数:

In [7]: next(gen)
Out[7]: 1

Answer 2

每次调用generator都会创建一个新的生成器对象:

generator().next() # 1st item in 1st generator 
generator().next() # 1st item in 2nd generator

创建一个生成器,然后调用nextfor后续项:

g = generator()

g.next() # 1st item in 1st generator
g.next() # 2nd item in 1st generator