119 – Efficiently extending a list

The method extend is the efficient way of adding multiple elements to a list in one go:

my_list = [...]

my_list.extend(other_list)

This also works with other iterables, even if they’re not lists.

A common alternative that also works with non-list iterables is using a for loop that calls the method append repeatedly:

my_list = [...]

for value in other_list:
    my_list.append(value)

However, this is inefficient because the repeated calls to append will waste time constantly resizing the list.

Another alternative that may sound like a good idea but actually wastes time and memory computing a third list is concatenating the two lists:

my_list = [...]

my_list = my_list + other_list

CPython implementation detail: if you use augmented assignment += then it’s efficient again because it defers to the method extend at the C level.