Python combine two for loops

Question

Currently I would do:

for x in [1,2,3]:
    for y in [1,2,3]:
        print x,y

Is there way of doing something like below,

for x,y in ([1,2,3],[1,2,3]):
    print x,y

Would like to shorten this kind of loop and this throws the "too many to unpack" exception.

eumiro · Accepted Answer · 2012-02-22T13:08:51.787

32

import itertools
for x, y in itertools.product([1,2,3], [1,2,3]):
    print x, y

prints all nine pairs:

UPDATE: If the two variables x and y are to be chosen from one list, you can use the repeat keyword (as proposed by agf):

import itertools
for x, y in itertools.product([1,2,3], repeat=2):
    print x, y

edited Feb 22 '12 at 13:08

answered Feb 22 '12 at 12:34

eumiro

8

or `product([1, 2, 3], repeat=2)`. – agf Feb 22 '12 at 13:00
@agf: thanks! I always used `product(**[1,2,3]*2)`. I might still use my method because it's more explicit, but `repeat=...` might be more readable. – ninjagecko Feb 22 '12 at 13:27
@ninjagecko I assume you mean `product(*[[1,2,3]]*2)`, but I don't see that as more explicit. As you've shown, it's also easier to get wrong. Shameless self promotion: See my highly upvoted answer to [What is the best way to generate all possible three letter strings?](http://stackoverflow.com/a/7074066/500584) – agf Feb 22 '12 at 16:59
@agf: Ah yes, I meant `product(*[ [1,2,3] ]*2)`. I feel it's more explicit because `repeat=` requires the user to know about what that keyword does, but `repeat=` is pretty elegant. =) Nice answer; exactly how I'd do it. – ninjagecko Feb 23 '12 at 17:09
if the length is difference in that case – user765443 Oct 08 '13 at 08:00

score 16 · Answer 2 · answered Feb 22 '12 at 12:37

16

You could use a generator expression in the for loop:

for x, y in ((a,b) for a in [1,2,3] for b in [5,6,7]):
  print x, y

answered Feb 22 '12 at 12:37

Amandasaurus

FWIW - please include a reference to "generator expression". Your answer was the one that helped me today. About 8 yeas later. :) Thank you! – Rikki Feb 26 '21 at 04:26

2 Answers2