Iterating over several lists at once

[Peter Otten]

Gal Diskin wrote:

> I am writing a code that needs to iterate over 3 lists at the same
> time, i.e something like this:

> for x1 in l1:
>     for x2 in l2:
>         for x3 in l3:
>             print "do something with", x1, x2, x3

> I was wondering if one could write this more easily in some manner
> using only 1 for loop.

def nested_loops(*args):
    assert args
    if len(args) == 1:
        for item in args[0]:
            yield (item,)
    else:
        gap = len(args) // 2
        for left in nested_loops(*args[:gap]):
            for right in nested_loops(*args[gap:]):
                yield left + right

if __name__ == "__main__":
    for i, k in nested_loops("abc", "12"):
        print i, k
    for i, j, k in nested_loops("ab", "123", "xyz"):
        print i, j, k

Be prepared for a significant performance hit.

Peter

PS: Did anybody say macro? No? I must be hallucinating...