Copying 2D lists in python

Question

Hi I want to copy a 2D list, so that if I modify 1 list, the other is not modified.

For 1 D list, I just do this:

a = [1,2]
b = a[:]

And now if I modify b, a is not modified.

But this doesn't work for 2D list:

a = [[1,2],[3,4]]
b = a[:]

If I modify b, a gets modified as well.

How do I fix this?

Answer 1

b = [x[:] for x in a]

Answer 2

For a more general solution that works regardless of the number of dimensions, use copy.deepcopy():

import copy
b = copy.deepcopy(a)