Priority queue with higher priority first in Python

Question

I need a priority queue that gets the item with the highest priority value first. I'm currently using the PriorityQueue Class from the Queue library. However, this function only returns the items with the lowest value first. I tried some ugly solutions like (sys.maxint - priority) as the priority, but was just wondering if a more elegant solution exists.

score 43 · Accepted Answer · answered Feb 27 '13 at 22:53

43

Use a negative priority instead, no need to subtract from sys.maxint.

queue.put((-priority, item))

An item with priority -10 will be returned before items with priority -5, for example.

answered Feb 27 '13 at 22:53

Martijn Pieters

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4

While this works, it honestly bothers me because that implies all other associated logic has be reversed in your head. – onesiumus Jan 26 '18 at 06:06
2

@blueman: then subclass the queue class and override the methods to invert the priority for you. – Martijn Pieters Jan 26 '18 at 12:23

anask · Answer 2 · 2019-07-10T17:42:12.103

You can extend the Priority Queue to keep the logic unchanged:

from Queue import PriorityQueue

class DualPriorityQueue(PriorityQueue):
    def __init__(self, maxPQ=False):
        PriorityQueue.__init__(self)
        self.reverse = -1 if maxPQ else 1

    def put(self, priority, data):
        PriorityQueue.put(self, (self.reverse * priority, data))

    def get(self, *args, **kwargs):
        priority, data = PriorityQueue.get(self, *args, **kwargs)
        return self.reverse * priority, data


minQ = DualPriorityQueue()
maxQ = DualPriorityQueue(maxPQ=True)

minQ.put(10, 'A')
minQ.put(100, 'A')


maxQ.put(10, 'A')
maxQ.put(100,'A')

print "Min DQ: {}".format(minQ.get())
print "Max DQ: {}".format(maxQ.get())

Output:

Min DQ: (10, 'A')
Max DQ: (100, 'A')

Priority queue with higher priority first in Python

2 Answers2