Comparison of two methods to append to a list in Python

A colleague recently told me that the append method on a Python list is more efficient than using the += operator but provided no justification. Curious, I investigated whether this was true.

String validation as an example

Consider the following function as an example. It checks whether a string is semantically correct, i.e. whether it satisifies some set of requirements that are dictated by the needs of the application. In this example, the requirements are

  1. The string cannot be longer than 10 characters

  2. The string cannot contain a number

import re

def validate(input: str) -> str:
    reasons = []

    if len(input) > 10:
        reasons += ["Input is too long"]

    if bool("\d", input)):
        reasons += ["Input contains a number"]

    if reasons:
         raise Exception(" | ".join(reasons))

    return input

If either of these conditions are violated, an exception is raised containing a message. The message describes which condition(s) is(are) violated. The reasons list contains zero, one, or two elements, and it is built by appending to the list using the += operator. I could have instead used the append method of list.

+= vs. append

To test the speed of the two methods, I use the timeit package from the Python standard library in the program below. The test consists of the following:

  1. Create an empty list

  2. Append error strings to the list one at a time

import string
import timeit

reasons = [
    "This is an error message",
    "This is another error message",
    "Let's add another for good measure",
def test_plus_equals():
    result = []
    for reason in reasons:
        result += [reason]

def test_append():
    result = []
    for reason in reasons:

number = 1000000
repeat = 5
results_plus_equals = min(
        setup="from __main__ import test_plus_equals"

results_append = min(
        setup="from __main__ import test_append")

if __name__ == "__main__":
    print("1E+6 loops per test")
    print(f"+= (best of five tests):\t{results_plus_equals:0.4f} s")
    print(f"append (best of five tests):\t{results_append:0.4f} s")


1E+6 loops per test
+= (best of five tests):     0.2392 s
append (best of five tests): 0.2241 s

In this test the append method of Python's list does appear to be faster by a factor of 6% or 7%. append took about 0.224 microseconds per loop, whereas the += operator took 0.239 microseconds.

The advantage of the append method is probably only noticeable if you need to append to a list many millions of times per second.


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