Difference between list.pop() and list = list[:-1]

Question

>>> a = [1,2,3]
>>> a.pop()
3
>>> a
[1, 2]
>>> a = [1,2,3]
>>> a = a[:-1]
>>> a
[1, 2]
>>>

Is there any difference between the above methods to remove the last element from a list?

Answer 1

Yes. pop is O(1) and mutates the original list, while slice is O(n) and creates a copy of the list. Less formally, the pop method is an operation on the element at the end of the list and is defined in CPython as a call to list_resize(self, Py_SIZE(self) - 1);. This doesn't traverse the entire structure.

On the other hand, list_slice allocates a new list and loops over the entries in the old list ranging from the beginning to the end - 1, copying references to each item to the new list.

If what you're trying to do is remove the last element of the list, use pop or del a[-1].