Itertools equivalent of nested loop “for x in xs: for y in ys…”

问题

I have a nested loop to create all combinations in a set of conjugated verbs. The aim to to get all possible combinations of verb, person and tense, e.g. [['to be', 'first person singular', 'future'],['to be', 'second person singular', 'future'], ...].

for v in verbs:
    for p in persons:
        for t in tenses:
            return [v, p, t]

Is there a way of reducing the nesting, perhaps using itertools?

回答1:

for v, p, t in itertools.product(verbs, persons, tenses):
    ...

回答2:

You can use itertools.product for this task:

Cartesian product of input iterables. Equivalent to nested for-loops in a generator expression. For example, product(A, B) returns the same as ((x,y) for x in A for y in B).

a = [1,2,3]
b = [4,5,6]
c = [7,8,9]
import itertools
for p in itertools.product(a,b,c):
    print(p)

The alternative would be a list comprehension expression:

for p in [(x,y,z) for x in a for y in b for z in c]:
    print(p)

来源：https://stackoverflow.com/questions/22473053/itertools-equivalent-of-nested-loop-for-x-in-xs-for-y-in-ys