为什么在传递非空可迭代对象时枚举为空？

Question

It's my understanding that the enumerate() function will loop over each item in an iterable. However, in the example below is does not loop at all when given a non-empty iterable.

import itertools
x = itertools.chain(
    itertools.combinations([1, 2, 3], 1),
    itertools.combinations([1, 2, 3], 2),
    itertools.combinations([1, 2, 3], 3),
)
print(list(x))
print(list(enumerate(x)))
print(list(enumerate(list(x))))
print(list(enumerate([(1, ), (2, ), (3, ), (1, 2), (1, 3), (2, 3), (1, 2, 3)])))

Output:

[(1,), (2,), (3,), (1, 2), (1, 3), (2, 3), (1, 2, 3)]
[]
[]
[(0, (1,)), (1, (2,)), (2, (3,)), (3, (1, 2)), (4, (1, 3)), (5, (2, 3)), (6, (1, 2, 3))]

Expected output:

[(1,), (2,), (3,), (1, 2), (1, 3), (2, 3), (1, 2, 3)]
[(0, (1,)), (1, (2,)), (2, (3,)), (3, (1, 2)), (4, (1, 3)), (5, (2, 3)), (6, (1, 2, 3))]
[(0, (1,)), (1, (2,)), (2, (3,)), (3, (1, 2)), (4, (1, 3)), (5, (2, 3)), (6, (1, 2, 3))]
[(0, (1,)), (1, (2,)), (2, (3,)), (3, (1, 2)), (4, (1, 3)), (5, (2, 3)), (6, (1, 2, 3))]

I'm running this with Python 3.8.2 on Arch Linux.

Can someone explain why enumerate() does not loop with these inputs?

Answer 1

You exhausted your chained iterator with the first print statement. The second and third did receive an empty iterator. If you want to see your expected output, you need to rebuild the chain for each run-through.

This is not specific to chain , nor to enumerate ; you can get the same effect with

x = iter(range(10))