creating mock of builtin functions in Python

mock_builtin_len.py

from unittest.mock import Mock
import builtins

len = builtins.len


def new_len(o):
    return builtins.len(o) + 1


def patch_len():
    global len
    len = Mock()
    len.side_effect = new_len


def test_new_len():
    length = len("a")
    assert len.call_count == 1
    assert length == 2

    length = len("abcd")
    assert len.call_count == 2
    assert length == 5

    print("Success!")


if __name__ == "__main__":
    patch_len()
    test_new_len()