Can someone explain to me why using pop() on a list equal to self.list also get self.list pop the last place

Question

I'm trying to build a game in pygame with OOP and I got into a problem.

The problem is that when I tried to pop the last item from the list, self.list got popped as well.

This is a simpler version of the problem:

class Example(object):
    def __init__(self):
        self.list_that_shouldnt_get_popped = [1,2,3,4,5]

    def pop_equal_list(self):
        list_that_should_get_popped = self.list_that_shouldnt_get_popped
        list_that_should_get_popped.pop()

And when you try to run it the outcome is this:

Ex = Example()
Ex.pop_equal_list()

print(Ex.list_that_shouldnt_get_popped.__len__())



Output : 4

I expect that the length of self.list_that_shouldnt_get_popped will be 5 and not 4 meaning I would like it to not pop.

They are the **same list**. Also, please always tag python questions with the generic [python] tag. As an aside, just use `len` — juanpa.arrivillaga, Apr 04 '19 at 18:39
pop is an in place operation and both variables refer to the same list. — Kyle G, Apr 04 '19 at 18:40

score -1 · Accepted Answer · answered Apr 04 '19 at 18:40

-1

In Python a list variable only holds a reference to the list, so when you do:

list_that_should_get_popped = self.list_that_shouldnt_get_popped

you are only copying the reference to the list that self.list_that_shouldnt_get_popped references to the variable list_that_should_get_popped.

You should instead assign to list_that_should_get_popped with a new list:

list_that_should_get_popped = self.list_that_shouldnt_get_popped[:]

answered Apr 04 '19 at 18:40

blhsing

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stop answering obvious duplicates, you should be using your dupe-hammer to close these questions, there's probably 100 of these a day. – juanpa.arrivillaga Apr 04 '19 at 18:41

Can someone explain to me why using pop() on a list equal to self.list also get self.list pop the last place

1 Answers1