Copying nested lists in Python

Question

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

For a one-dimensional 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 a two-dimensional list:

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

If I modify b, a gets modified as well.

How do I fix this?

Solution 1

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

import copy
b = copy.deepcopy(a)

Solution 2

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