Removing items inside a for loop

Last Updated: April 16, 2024 5:14 PM UTC

Last edited by:

Galen Rice <[email protected]>

GitHub <[email protected]>

fix: "in the example", less terse and less confusing

Avoid adding to or removing from a collection, such as a list, as you iterate that collection in a for loop:

data = [1, 2, 3, 4]
for item in data:
    data.remove(item)
print(data)  # [2, 4] <-- every OTHER item was removed!

Inside the loop, an index tracks the current position. If the list is modified, this index may no longer refer to the same element, causing elements to be repeated or skipped.

In the example above, 1 is removed, shifting 2 to index 0. The loop then moves to index 1, removing 3 (and skipping 2!).

You can avoid this pitfall by: - using a list comprehension to produce a new list (as a way of filtering items):

data = [x for x in data if x % 2 == 0]

- using a while loop and .pop() (treating the list as a stack):

while data:
    item = data.pop()

- considering whether you need to remove items in the first place!