How to flatten a list of lists in Python

If you have a list of lists like this:

lol = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]

And you want to flatten it into a single list like this:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

You can use a list comprehension like this:

l = [item for sublist in lol for item in sublist]

Or you can use itertools.chain() like this:

from itertools import chain
l = list(chain(*lol))