How to elegantly loop over multiple lists

Question

Say, I have two lists a and b:

a = [10, 20]
b = [40, 50]

I want to loop over all these values (10, 20, 40, 50) in one go.

Simply doing two loops is not what I want (repetition is ugly).

I also don't want to modify one of the lists:

a.extend(b)
for i in a:
    print(i)

So how do I do this elgantly in Python?

It's a duplicate, unfortunate that I did not find that question in the first place. Must be a wording thing. — florisla, Sep 09 '15 at 13:15
@Hacketo If you're lucky enough to think about 'concatenate' as the verb. Not trivial for non-native English speakers. — florisla, Sep 09 '15 at 13:17
http://stackoverflow.com/questions/1720421/join-two-lists-in-python — heinst, Sep 09 '15 at 13:20

score 8 · Answer 1 · edited Oct 12 '16 at 17:33

8

You can use chain from itertools:

from itertools import chain

a = [10, 20]
b = [40, 50]

for i in chain(a, b):
    print(i)

This will not create a new list (as a + b does) and is therefore more (memory-)efficient should your lists be huge.

This also works for generators and other iterables.

edited Oct 12 '16 at 17:33

pillravi

answered Sep 09 '15 at 13:13

hiro protagonist

score 1 · Answer 2 · edited Sep 09 '15 at 13:16

1

for i in a + b:
    print(i)

Note: I answered this myself. Was wondering about the question, found the answer but not through SO, and felt that it should be added.

edited Sep 09 '15 at 13:16

Delgan

answered Sep 09 '15 at 13:11

florisla

http://stackoverflow.com/questions/1720421/join-two-lists-in-python – heinst Sep 09 '15 at 13:20
@heinst By not thinking of using terms 'join' or 'concatenate' I guess. – florisla Sep 09 '15 at 13:21
1

Searching for add or append also leads to that link – Tim Sep 09 '15 at 13:22

2 Answers2