CODEWITHSUNDEEP
pythonlistlist-comprehension
cara_elliano123
cara_elliano123

Reputation: 13

How do I append values to different lists at one time, in one line?

I'm a beginner to python, but I can't find anything on the internet to help me with this problem:

I want to append different values to 2 different lists at once (the actual problem is more complex than this but I just need to know the right syntax so I'm simplifying it)

test1= []
test2= []
[test1.append(1) and test2.append(2) for i in range (10)]

this doesn't append to both lists, how do I make that happen?

Upvotes: 0

Views: 161

Answers (2)

jamylak
jamylak

Reputation: 133724

Firstly I don't get why you need it in 1 line, there seems to be no good reason for this, anyway I guess you could do this:

test1, test2 = [], []
for _ in range(10): test1.append(1); test2.append(2)

Upvotes: 1

Mad Physicist
Mad Physicist

Reputation: 114518

If your specific example is what you want, use argument unpacking to assign multiple values at once:

test1, test2 = [1] * 10, [2] * 10

You can also use the zip(*x) idiom to transpose a 2-column list:

test1, test2, map(list, zip(*[[1, 2]] * 10))

If you're OK with having tuples instead of lists, you can omit the map(list, ...) wrapper.

The best way I know of having a comprehension with side-effects is using collections.deque with a size of zero, so it runs the iterator but does not store any results:

from collections import deque

test1, test2 = [], []
deque(((test1.append(1), test2.append(2)) for _ in range(10)), maxlen=0)

The generator creates tuples (None, None) every time (since list.append returns None), which the deque promptly discards.

Upvotes: 0

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