Newbie - chapter 19 in "How to think like a CS in python"

[MooMaster]

Well, being a lowly CS student myself this sounded like fun, so I went
ahead and took the challenge. The first thing you have to consider is
how to add the new element to the list. Obviously you have to iterate
through the list, find the first element that the new one is greater
than, and insert this one before it while keeping the list structure
intact. You must also remember to update the head if necessary, and add
at the end if the element to be added is less than what is already in
the list...So this is what I came up with:

First of all, however, add this to your Improved Queue Class-----
 def printQueue(self):
            if(self.isEmpty()):
                print "Empty"
            else:
                pointer = self.head
                while(pointer != None):
                    print str(pointer.cargo)
                    pointer = pointer.next

Then------
class PriorityQueueList(ImprovedQueue):
        def __init__(self):
            ImprovedQueue.__init__(self)

        def insert(self, cargo):
            newNode = Node(cargo)
            if self.length == 0:
                self.head = self.last = newNode
            else:
                #iterate through the list, keeping a reference to a
node and the node before it
                tracker = self.head
                while(tracker !=None):
                    copy2 = tracker.prev
                    if(newNode.cargo > tracker.cargo):
                    #add the new node by updating links, then update
head while keeping list intact
                            tracker.prev = newNode
                            newNode.next = tracker
                            self.head = newNode
                            newNode.prev = copy2
                            if(copy2 !=None):
                                copy2.next = newNode
                            break
                    else:
                        tracker = tracker.next
                #if a repeat element is added
                #we want to add to end to preserve the queue structure,
thus if the element hasn't
                #been added at this point it should be added at the end

                if(newNode.prev ==None and newNode.next ==None):
                        last = self.last
                        last.next = newNode
                        self.last = newNode
            self.length = self.length + 1

This produces the following output-----
>>> l = PriorityQueueList()
>>> l.insert(1)
>>> l.insert(2)
>>> l.insert(1)
>>> l.printQueue()
2
1
1
>>> l.insert(10)
>>> l.printQueue()
10
2
1
1

Hope this helps!