Python Random Fact: append vs extend

2026-01-15

Both operations are O(N), but extend runs in C, so it's faster in practice.

Quiz

What's the difference between the following code?

For loop:

for idx in range(numRows):
    for ch in each_rows[idx]:
        all_chars.append(ch)

Extend:

for idx in range(numRows):
    all_chars.extend(each_rows[idx])

Answer

Using extend is faster.

Here's a quick comparison:

+--------------------+----------------------+------------------+
| Perspective        | append + double loop | extend           |
+--------------------+----------------------+------------------+
| Big-O complexity   | O(N)                 | O(N)             |
| Python-level loops | Many                 | Few              |
| C-level processing | Less                 | More (faster)    |
| Element copying    | No                   | No               |
+--------------------+----------------------+------------------+

list_extend: https://github.com/python/cpython/blob/main/Objects/listobject.c