Python: deepcopy(list) vs new_list = old_list[:]

You asked two questions:

Deep vs. shallow copy

matrix[:]

is a shallow copy -- it only copies the elements directly
stored in it, and doesn’t recursively duplicate the elements of arrays or
other references within itself. That means:

a = [[4]]b = a[:]a[0].append(5)print b[0] # Outputs [4, 5], as a[0] and b[0] point to the same array

The same would happen if you stored an object in

a

.

deepcopy()

is, naturally, a deep copy -- it makes copies of each of its
elements recursively, all the way down the tree:

a = [[4]]c = copy.deepcopy(a)a[0].append(5)print c[0] # Outputs [4], as c[0] is a copy of the elements of a[0] into a new array

Returning

return final.append(li)

is different from calling

append

and returning

final

because list.append does not return the list object itself, it returns

None