Stack practice with Python 3.

stack.py

# Exercise completed as part of Erin Allard's Lynda.com course,
# 'Python Data Structures: Stacks, Queues, and Deques'

class Stack:
    """An ADT that stores items in the order in which they were
    added. Items are added to and removed from the top of the   
    stack - LIFO.
    """
    def __init__(self):
        self.items = []

    def push(self, item):
        """Accepts an item as a parameter and appends it to end of list.
        It returns nothing.

        The run time for this method is O(1), or constant time, because
        appending to the end of a list happens in constant time. 
        """
        return self.items.append(item)

    def pop(self):
        """This method removes and returns the last item in list, which
        is also the top item of the Stack.

        The run time for this method is O(1), or constant time, because
        popping from the end of a list happens in constant time. 
        """
        if self.items:
            return self.items.pop()
        else:
            return None

    def peek(self):
        """This method returns the last item in list, which is also the item
        at the top of the Stack.

        The run time for this method is O(1), or constant time, because indexing
        a list happens in constant time.
        """
        if self.items:
            return self.items[-1]
        else:
            return None

    def size(self):
        """This method returns the length of list that is representing the Stack.

        The run time for this method is O(1), or constant time, because finding
        the length of a list happens in constant time.
        """
        return len(self.items)

    def is_empty(self):
        """This method simply returns a Boolean value describing whether or not
        list is empty.

        The run time for this method is O(1), or constant time, because testing for
        equality happens in constant time.
        """
        return self.items == []