Welcome to the Treehouse Community

The Treehouse Community is a meeting place for developers, designers, and programmers of all backgrounds and skill levels to get support. Collaborate here on code errors or bugs that you need feedback on, or asking for an extra set of eyes on your latest project. Join thousands of Treehouse students and alumni in the community today. (Note: Only Treehouse students can comment or ask questions, but non-students are welcome to browse our conversations.)

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and a supportive community. Start your free trial today.

Python Python Collections (2016, retired 2019) Lists Removing items from a list

Jacob Keeley
Jacob Keeley
2,033 Points

Why isn't .remove() deleting lists?

When I try to complete Task 2 of Removing Items from a List using the code below, Python outputs [1, 2, 3, [1, 2, 3]]. Why does the list within the list remain?

messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]

messy_list.insert(0, messy_list.pop(3))

for item in messy_list:
    if type(item) != int:
        messy_list.remove(item)
    else:
        continue

1 Answer

Steven Parker
Steven Parker
221,291 Points

This issue occurs when an iterable is altered while being used to control a loop. Disturbing the item order can cause items to be skipped over.

To prevent this, use a copy of the iterable for the loop (using the "copy" method or a slice with no arguments).