queue.py

"""
queue.py
author: James heliotis
description: A linked queue (FIFO) implementation
"""

from node import LinkedNode

class Queue:

    __slots__ = "front", "back"

    def __init__( self ):
        """ Create a new empty queue.
        """
        self.front = None
        self.back = None

    def __str__( self ):
        """ Return a string representation of the contents of
            this queue, oldest value first.
        """
        result = "Queue["
        n = self.front
        while n != None:
            result += " " + str( n.value )
            n = n.link
        result += " ]"
        return result

    def is_empty( self ):
        return self.front == None

    def enqueue( self, newValue ):
        newNode = LinkedNode( newValue )
        if self.front == None:
            self.front = newNode
        else:
            self.back.link = newNode
        self.back = newNode

    def dequeue( self ):
        assert not self.is_empty(), "Dequeue from empty queue"
        self.front = self.front.link
        if self.front == None:
            self.back = None

    def peek( self ):
        assert not self.is_empty(), "peek on empty stack"
        return self.front.value

    insert = enqueue
    remove = dequeue

def test():
    s = Queue()
    print( s )
    for value in 1, 2, 3:
        s.enqueue( value )
        print( s )
    print( "Dequeueing:", s.peek() )
    s.dequeue()
    print( s )
    for value in 15, 16:
        s.insert( value )
        print( s )
    print( "Removing:", s.peek() )
    s.remove()
    print( s )
    while not s.is_empty():
        print( "Dequeueing:", s.peek() )
        s.dequeue()
        print( s )
    print( "Trying one too many dequeues... ", end="" )
    try:
        s.dequeue()
        print( "Problem: it succeeded!" )
    except Exception as e:
        print( "Exception was '" + str( e ) + "'" )


if __name__ == "__main__":
    test()