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Python Python Collections (Retired) Lists Redux Manipulating Lists

Geoff Ruddock
Geoff Ruddock
2,573 Points
Posted by Geoff Ruddock
Geoff Ruddock
2,573 Points

Removing non-ints from a list

Trying to remove non-ints using the code below, but getting an unexpected result.

Code:

the_list = ["a", 2, 3, 1, False, [1, 2, 3]] for item in the_list: if type(item) is not int: the_list.remove(item) print(the_list)

Gives me the result: [2, 3, 1, [1, 2, 3]]

Does anyone know why this happens? Although I can get around it for the quiz question, I am curious why this approach is failing.

2 Answers

Fable Turas
Fable Turas
9,405 Points
Fable Turas
Fable Turas
9,405 Points

The reason you cannot remove items in a list while iterating over it is because once you remove an item the index of all the remaining items changes and so the item immediately following the removed item will be skipped when the code loops around for the next iteration.

For example, at the start of your code "a" is at index 0, and 2 is at index 1, but as soon as "a" is removed the index point for 2 becomes 0. But the for loop doesn't know this and only knows to move on to the next item, which is now 3 and 2 is never evaluated. This becomes more important when False is removed and [1, 2, 3] gets bumped up an index.

There is, however, another way around this aside from creating 2 lists, and that is to iterate through the list backwards. 

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

for item in reversed(the_list):
    if type(item) is not int:
        the_list.remove(item)

print(the_list)
Geoff Ruddock
Geoff Ruddock
2,573 Points
Geoff Ruddock
Geoff Ruddock
2,573 Points

Very clever! Thanks for this workaround!

Chris Bennett
Chris Bennett
7,499 Points
Chris Bennett
Chris Bennett
7,499 Points

So in Python you can't remove items from a list while you are iterating over it.

(you technically can but you get funky results)

A simple solution would be to create a temporary list and append all the ints to that list then make the_list = temp_list. There's shorthand to do this by the way but I think this code is easier to understand.

the_list = ["a", 2, 3, 1, False, [1, 2, 3]]
temp_list = []

for item in the_list:
    if type(item) == int:
        temp_list.append(item)

the_list = temp_list

print the_list
Geoff Ruddock
Geoff Ruddock
2,573 Points
Geoff Ruddock
Geoff Ruddock
2,573 Points

Thanks for your help. This is a very clear and logical way to solve it.