Question

If I want to print out values in a PriorityQueue, how can I do it? I extended PriortiyQueue, and got access to the underlying list queue. However when I do a for loop, I get the order in which items are added

# the ADT PriorityQueue
class ReadyQueue(PriorityQueue):
    ...

    def listAll(self):
        print("PID \t Name \t Status \t Priority")
        print("=" * 42)
        for _, _, pcb in self.queue:
            print(pcb)

# ADT 
class PCB:
    ...
    def __lt__(self, other):
        selfPriority = (self.priority, self.creationTime)
        otherPriority = (other.priority, other.creationTime)
        return selfPriority < otherPriority

# in the main function
q = ReadyQueue()
q.enqueue(PCB("p1", 0, None, q))
q.enqueue(PCB("p8", 6, None, q))
q.enqueue(PCB("p2", 1, None, q))
q.enqueue(PCB("p0", 1, None, q))
q.enqueue(PCB("p6", 1, None, q))
q.enqueue(PCB("p4", 1, None, q))
q.enqueue(PCB("p3", 2, None, q))

q.listAll()

The output:

---

 PID     Name    Status      Priority
 ==========================================
 0   p1      ready_s     0
 3   p0      ready_s     1
 2   p2      ready_s     1
 1   p8      ready_s     6
 4   p6      ready_s     1
 5   p4      ready_s     1
 6   p3      ready_s     2

Notice priority is not right. Though they dont appear in creation order either … did I miss something?

Answer 1

PriorityQueue use heapq to push and pop elements, http://docs.python.org/library/heapq.html

so, the queue is a heap, not a sorted list.