Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

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

Posted by Vladyslav Koroviakovskyi
Vladyslav Koroviakovskyi
5,049 Points

the condition i used to remove all elements except ints from the list does not remove a list inside the list

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)

in the preview, I can see that the output of the function is a list with a list in the end: [1, 2, 3, [1, 2, 3]]

why my condition does not remove a inside list from a list?

lists.py 
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)








# Your code goes below here
Vladyslav Koroviakovskyi
Vladyslav Koroviakovskyi
5,049 Points

ok, i just figured out, I changed the line: for item in messy_list.copy() so, it iterates though a copy, and it worked. can someone explain why i need copy() to make this work, please

1 Answer

Steven Parker
Steven Parker
229,732 Points
Steven Parker
Steven Parker
229,732 Points

Internally, the loop keeps track of which item it is working with by its index. Each time through the loop, the index is incremented and the next item is retrieved.

If you remove an item from the list while the loop is running, the index numbers of the remaining items changes; and that causes the loop to skip the next one in the sequence.

The copy does not change as the loop runs, so every item is checked.