AttributeError: 'NoneType' object has no attribute 'remove' issue

Question

I want to remove certain values from a list if they are elements in a specified other list, as below. Ideally this would return list2 = ["e","f","g"], but instead it returns the above error as it seems once one of the elements is removed list1 becomes a None type variable…

list1 = ["a","b","c", "d", "e", "f", "g"]

list2 = ["a","b","c","d"]

for x in list1:
    if x in list2:
       list1 = list1.remove(x)

print(list1)

Asked By: Max Olivier

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Source

Answer 1

The method list.remove() is performed in place. So instead of:

list1 = list1.remove(x)

It should be like this:

list1.remove(x)

The complete code

The complete and working code should look as follows.

list1 = ["a", "b", "c", "d", "e", "f", "g"]
list2 = ["a", "b", "c", "d"]

temp = list1.copy()

for x in list1:
    if x in list2:
       temp.remove(x)

print(temp)

More explanations

Because list.remove() is performed in place, it does not return anything, or in other words, it returns None.

A simpler approach using list comprehension

As CrazyChucky pointed out, removing items from a list while iterating is not a good thing to do. You would have to make a copy of the list first before adding or removing items from the list. Or you can choose another option, which is to use list comprehension instead.

list1 = ["a", "b", "c", "d", "e", "f", "g"]
list2 = ["a", "b", "c", "d"]

print([i for i in list1 if i not in list2])

Answered By: Troll

AttributeError: 'NoneType' object has no attribute 'remove' issue

Question:

Answers:

The complete code

More explanations

A simpler approach using list comprehension