Question

I have a small snippet that does not work in an inexplicable way.
The purpose is to generate all combinations of two or more sequences.
It works when called with lists, but it doesn’t when called with generators.

def comb(seqs):
    if seqs:
        for item in seqs[0]:
            for rest in comb(seqs[1:]):
                yield [item] + rest
    else:
        yield []

if __name__=="__main__":
    x=[1,2]
    y=[3,4]
    print list(comb([x,y])) # prints [[1, 3], [1, 4], [2, 3], [2, 4]]

    def gen1(): yield 1; yield 2
    def gen2(): yield 3; yield 4
    x=gen1()
    y=gen2()
    print list(comb([x,y])) # prints [[1, 3], [1, 4]  WHY ????

Answer 1

The reason is that you can only iterate over a generator once. In the line

for item in seqs[0]

When you get to the second element of the gen1, you make a recursive call to iterate over the elements of gen2. The problem is that you have already iterated over gen2 in the previous recursive call, so it will not yield any items.