Python builtin “all” with generators

Question

I have the following problem with python\'s \"all\" and generators:

G = (a for a in [0,1])
all(list(G))   # returns False - as I expected

B

Answer 1

>>> G = (a for a in [0,1])
>>> all(list(G))
False
>>> G = (a for a in [0,1])
>>> all(G)
False

No True. However:

>>> G = (a for a in [0,1])
>>> all(list(G))
False
>>> all(G)
True
>>> all([])
True

If you call all a second time on the generator, you'll get True, as there are no False items left in the generator. As you can see, any empty sequence will work the same.

For this particular example, all short-circuits, so you have 1 left to be generated after it returns False because of the leading 0 (if you don't use list) -- so it will return True the second time despite not being empty.