Push, Pop & Display Operations for Stack

stack.py

stack = []
top = None if stack == [] else len(stack) - 1

def push(stack, element):
    stack.append(element)
    global top
    top = len(stack) - 1
    return stack # optional

def pop(stack):
    if len(stack) == 0:
        return "Underflow"
    else:
        stack.pop()
        global top
        top = len(stack) - 1
        return stack # optional

def display(stack):
    return stack[::-1]