Fastest way to check if a list contains an item in Python

The Problem

Using Python, what is the fastest way to check if an item is present in a list?

The Solution

This depends on the size of the list and the number of items to check for. When checking for a single item in a list, the fastest way is to use Python’s in operator:

my_list = [1, 2, 3, 4, 5]

if 4 in my_list:
    print("4 is present.")

If we’re checking for multiple items in a large, unordered list, it can be faster to convert the list and the items to sets before performing the check.

import random

my_list = list(range(1,10001))      # list of number 1 to 10000
random.shuffle(my_list)             # put my_list in a random order
items_to_find = list(range(1,1001)) # find the numbers 1 to 1000

my_list_set = set(my_list)
items_to_find_set = set(items_to_find)
for item in items_to_find_set.intersection(my_list_set):
    print(f"{item} is present.")

Performance can vary at lower scales and for different lists. When searching for a single item in a list, in should be sufficient; when searching for common items between two very long lists with thousands of items, set conversion is probably the way to go.