Removing items inside a for loop

Last Updated: February 3, 2018 12:20 PM UTC

Last edited by:

Joseph <joseph@josephbanks.me>

Initial Commit

THIS IS FAKE DEMO DATA

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!