Question

对于以下代码，生成器在python2和python3中的行为有所不同。

def g1():
  for i in range(3):
    print("in g1: {}".format(i))
    yield i


def g2():
  for i in range(3):
    print("in g2: {}".format(i))
    yield 2*i


def g3(f1,f2):
  for (i,(x,y)) in enumerate(zip(f1,f2)):
    print("in g3: {} ---- {},{}".format(i,x,y))
    yield (x,y)

h1 = g1()
h2 = g2()

h3=g3(h1,h2)

print(list(h3))

python2的输出

in g1: 0
in g2: 0
in g1: 1
in g2: 1
in g1: 2
in g2: 2
in g3: 0 ---- 0,0
in g3: 1 ---- 1,2
in g3: 2 ---- 2,4
[(0, 0), (1, 2), (2, 4)]

python3的输出

in g1: 0
in g2: 0
in g3: 0 ---- 0,0
in g1: 1
in g2: 1
in g3: 1 ---- 1,2
in g1: 2
in g2: 2
in g3: 2 ---- 2,4
[(0, 0), (1, 2), (2, 4)]

为什么会这样？需要python3行为。可以在python2中实现吗？

Answer 1

在Python 2中，zip不是惰性的，它返回一个列表，并完全消耗其参数。但是，您可以使用izip from itertools模拟Python 3的行为：

from itertools import izip

...
for (i,(x,y)) in enumerate(izip(f1,f2)):
...

在2.7.15上，将zip更改为izip，代码输出：

in g1: 0
in g2: 0
in g3: 0 ---- 0,0
in g1: 1
in g2: 1
in g3: 1 ---- 1,2
in g1: 2
in g2: 2
in g3: 2 ---- 2,4
[(0, 0), (1, 2), (2, 4)]

生成器在python-2.7.12和python-3.5.2中的不同行为

1 个答案: