Create nested list using list comprehension

Question

I have two lists:

L1 = [3, 5, 7, 8, 9, 5, 6, 7, 4, 3]
L2 = [1, 4, 5, 8, 3, 6, 9, 3, 5, 9]

And I need to create sub-list for each item in L2 that is smaller than 4, add it to all the numbers in L1 that are smaller than 4. I tried doing this:

result = [(x+y) for x in L2 if x < 4 for y in L1 if y < 4]

But it resulted me this:

[4, 4, 6, 6, 6, 6]

While the outcome should look like this:

[[4, 4], [6, 6], [6, 6]]

any idea on how should I nest it in the right way?

Answer 1

Create a nested list comprehension

>>> [[(x+y) for y in L1 if y < 4] for x in L2 if x < 4]
[[4, 4], [6, 6], [6, 6]]

Here the inner list comprehension creates the inner lists which are then appended to a single list by the outer comprehension.

Answer 2

The numbers below 4 in L1 are:

L1_below_4 = [x for x in L1 if x < 4]

And for L2:

L2_below_4 = [y for y in L2 if y < 4]

Now it's easy:

[[x + y for x in L1_below_4] for y in L2_below_4]

Or as a one-liner:

[[x + y for x in L1 if x < 4] for y in L2 if y < 4]