Queue practice with Python 3.

queue.py

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

class Queue:
    """An ADT that stores items in the order in which they
    were added. Items are added to the back and removed
    from the front - FIFO.
    """
    def __init__(self):
        self.items = []

    def enqueue(self, item):
        """Takes in an item and inserts that item into the zeroeth
        index of the list that is representing the Queue.

        The runtime is 0(n), or linear time, because inserting into
        the zeroeth index of a list forces all other items to move
        one index to the right.
        """
        self.items.insert(0, item)

    def dequeue(self):
        """Removes and returns the front most item from Queue, which
        is represented by the last item in list.

        The runtime is 0(1), or constant time, because indexing to
        the end of a list happens in constant time.
        """
        if self.items:
            return self.items.pop()
        else:
            return None

    def peek(self):
        """Returns the last item in list, which represents the font
        most item in the Queue.

        The runtime is 0(1), or constant time, because indexing the
        last item in a list happens in constant time.
        """
        if self.items:
            return self.items[-1]
        else:
            return None
        
    def size(self):
        """Returns size of Queue, which is represented by the length
        of the list.

        The runtime is 0(1), or constant time, because returning list
        length happens in constant time.
        """
        return len(self.items)

    def is_empty(self):
        """Returns Boolean value expressing wheter or not the list
        representing Queue is empty.

        A simple check for equality happens in 0(1), or constant time.
        """
        return self.items == []