list.append
mutates the list itself and returns None
. List comprehensions are for storing the result, which isn’t what you want in this case if you want to just change the original lists.
>>> x = [[1, 2], [3, 4], [5, 6]]
>>> for sublist in x:
... sublist.append('a')
...
>>> x
[[1, 2, 'a'], [3, 4, 'a'], [5, 6, 'a']]