stacks.py

class Stack:
    def __init__(self):
        self.theItems = list()

    def isEmpty(self):
        return len(self) == 0
    
    def __len__(self):
        return len(self.theItems)
    
    def pop(self):
        assert not self.isEmpty()
        return self.theItems.pop()

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

PROMPT = "Enter an int value : "
myStack = Stack()
value = int(input(PROMPT))
while value >= 0:
    myStack.push(value)
    value = int(input(PROMPT))

while not myStack.isEmpty():
    value = myStack.pop()
    print(value)