class Stack(object):

    def __init__(self):
        self._items = []

    def is_empty(self):
        return not bool(self._items)

    def push(self, item):
        self._items.append(item)

    def pop(self):
        return self._items.pop()

    def peek(self):
        return self._items[-1]

    def size(self):
        return len(self._items)

class Stack:

    def __init__(self):
        self._items = []

    def is_empty(self):
        return not bool(self._items)

    def push(self, item):
        self._items.append(item)

    def pop(self):
        return self._items.pop()

    def peek(self):
        return self._items[-1]

    def size(self):
        return len(self._items)

Make the two Stack implementations consistent
Make the two Stack implementations consistent

In terms of whether they inherit from `object` (now they both don't).