Python 2.x和Python 3.x之间的生成器评估差异

Question

Test code could be listed below.

In Python 2.7.8, this code below caused my machine crashed.

for row, current_char in zip(cycle(chain(pattern, pattern[::-1][1:][:-1])), count()):
    print row, current_char
    if current_char >= 14:
        break

While in Python 3.2.5.1, the code below works fine.

for row, current_char in zip(cycle(chain(pattern, pattern[::-1][1:][:-1])), count()):
    print(row, current_char)
    if current_char >= 14:
        break

Result is

0 0
1 1
2 2
1 3
0 4
1 5
2 6
1 7
0 8
1 9
2 10
1 11
0 12
1 13
2 14

Is there anyone knowing the reason? Thanks

Answer 1

zip returns an iterator in Python 3, but a list in Python 2. itertools.count and itertools.cycle are both infinite generators, thus zip ping them together produces an "infinite" list.

As others already mentioned, you should use itertools.izip instead (Python 2 only).

Answer 2

In Python 2, zip like most other "functional" elements in the global namespace, returns a list , meaning it evaluates everything at once. In Python 3 this will be a generator, evaluated on demand and thus needing less memory (although the output you provided doesn't look like memory would be a problem).