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

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

    def pop(self):
        if self.is_empty():
            raise IndexError("pop from an empty stack")
        return self._items.pop()

    def peek(self):
        if self.is_empty():
            raise IndexError("peek at an empty stack")
        return self._items[-1]

    def is_empty(self):
        return len(self._items) == 0

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


stack = Stack()
stack.push(1)
stack.push(2)
stack.push(3)
print(stack.pop())   # 3
print(stack.peek())  # 2
print(stack.size())  # 2