flatten treats iterable-once objects not carefully enough

[aananko]

>>> (range(2) for _ in range(2)) |> flatten |> list
[]

but

>>> (range(2) for _ in range(2)) |> flatten |> iter |> list
[0, 1, 0, 1]

Looks like list() constructor does notice __len__ and calls it first, and flatten.__len__ consumes the generator.

[evhub]

Good catch! I thought I added a test for that case, but I guess not.

[aananko]

Your response time is incredible, thanks!