Two for loops in list comprehension produce weird output in Python

Question

I've written this code:

l1 = [1, 2, 3, 4]
l2 = [5, 6, 7, 8]

print([x**2 for x in l1 for y in l2])

which outputs this:

[1, 1, 1, 1, 4, 4, 4, 4, 9, 9, 9, 9, 16, 16, 16, 16]

My question is, what is happening inside this list comprehension? Why is the amount of items in l1 multiplied by the length of l2?

Does this answer your question? [List comprehension on a nested list?](https://stackoverflow.com/questions/18072759/list-comprehension-on-a-nested-list) — Passerby, Nov 23 '21 at 19:43
Because the comprehension goes through l1 for each element of l2, i.e. l2 times, so a total of l2*l1 results. — DisappointedByUnaccountableMod, Nov 23 '21 at 19:45
If this strikes you as "weird" -- what was the non-weird result that you were expecting? — John Coleman, Nov 23 '21 at 19:46
Note that the second list is useless, only its length matters. You would get the same with `print([x**2 for x in l1 for _ in range(4)])` — mozway, Nov 23 '21 at 19:49
Guys, I came across this code by accident, I'm not trying to achieve anything with it, I just wanted to see what's happening here. Hopefully got the answer already — Oleksandr Novik, Nov 23 '21 at 19:50

score 2 · Accepted Answer · answered Nov 23 '21 at 19:47

2

It's like you do:

for x in l1:
   for y in l2: #this loop has len(l2) iteration
      print(x**2)

In the y loop, you only use variable x, which doesn't change for len(l2) iterations.

answered Nov 23 '21 at 19:47

ma4strong

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score 1 · Answer 2 · answered Nov 23 '21 at 19:48

There are two things happening here.

First: print([x**2 for x in l1...

This is taking each of the elements in l1 and squaring them producing [1, 4, 9, 16]

Second: ... for y in l2])

This is repeating this process (squaring and printing the element in l1) one time for each value in l2 which is why you are getting 4 of each squared value.

score 0 · Answer 3 · answered Nov 23 '21 at 19:48

0

Your list comprehension is equivalent to the following for loops:

result = []
for y in l2:
    for x in l1:
        result.append(x**2)

From this you can see why the inner loop is repeating 4 times.

If you want to loop over the two lists in parallel, not nested, use zip():

[x**2 for x, y in zip(l1, l2)]

answered Nov 23 '21 at 19:48

Barmar

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53
500
612

I guess the order of for loops is swapped. It doesn't produce the same result as my comprehension :) – Oleksandr Novik Nov 23 '21 at 19:55

Two for loops in list comprehension produce weird output in Python

3 Answers3