% cat stack.py
class Stack():
    def __init__(self):
        self.__items = []

    def __repr__(self):
        return str(self.__items)

    def push(self, x):
        self.__items.append(x)

    def pop(self):
        self.__items.pop()


% python3
Python 3.7.4 (default, Sep  7 2019, 18:27:02)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from stack import Stack
>>> s = Stack()
>>> s.push(42)
>>> s
[42]
>>> s.__items
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Stack' object has no attribute '__items'
>>>