Stacks.py

# A Stack is a type of list where instead of accessing
# any item in the list at any time, you can only add
# or remove items from the top of the Stack.
#
# Adding a new item to the stack is called "pushing"
# the item onto the stack. Removing the top item on
# the stack is called "popping" the item off the stack.
# When an item is "popped" off the stack, it is removed
# from the list altogether.
#
# Write a class called Stack. Stack should have the
# following methods:
#
# - An __init__ method that initializes the empty list
#   that is the stack's contents.
# - A stack_push() method that takes one parameter (in
#   addition to self): an item to push onto the top
#   of the stack.
# - A stack_pop() method that returns the current top
#   item on the stack and removes it from the underlying
#   list. If the list is already empty, this returns
#   None.
#
# For example, the following code would print the
# numbers 3, 2, and 1 (in that order). Note that this
# is the opposite order of how they are pushed: the
# pop method will always return the elements in the
# reverse order that they were added in.
#
# new_stack = Stack()
# new_stack.stack_push(1)
# new_stack.stack_push(2)
# new_stack.stack_push(3)
# print(new_stack.stack_pop())
# print(new_stack.stack_pop())
# print(new_stack.stack_pop())

# Add your class here!
class Stack:
    def __init__(self):
        self.items = []

    def stack_push(self, num):
        self.items.append(num)

    def stack_pop(self):
        if len(self.items) == 0:
            return None
        else:
            n = self.items[-1]
            del self.items[-1]
            return n


# The following lines of code will test your class.
# If it works correctly, it will print 3, 2, and 1
# in that order, each on their own line.
new_stack = Stack()
new_stack.stack_push(1)
new_stack.stack_push(2)
new_stack.stack_push(3)
print(new_stack.stack_pop())
print(new_stack.stack_pop())
print(new_stack.stack_pop())