Question

I have a list a and a list b which is (should be) a copy of list a.

a = [[['a'], ['b'], ['c']], [['A'], ['B'], ['C']]]
b = a[:][:]
b[0][1], b[0][2] = b[0][2], b[0][1]

If I now look at a and b I get the following:

a = [[['a'], ['c'], ['b']], [['A'], ['B'], ['C']]]
b = [[['a'], ['c'], ['b']], [['A'], ['B'], ['C']]]

Why does the swap in list b also affects the original list a?

Thank you.

Answer 1

b = a[:][:] is just b = (a[:])[:] or a copy of a copy of the original list. The lists inside the original list are still referenced and when you change them it shows in both lists.

You can do

b = [l[:] for l in a] # a new list, consisting of copies each sublist

or

from copy import deepcopy
b = deepcopy(a)