Document implementation notes for priority queues
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.. moduleauthor:: Kevin O'Connor
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.. sectionauthor:: Guido van Rossum <guido@python.org>
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.. sectionauthor:: François Pinard
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.. sectionauthor:: Raymond Hettinger
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.. versionadded:: 2.3
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@ -151,6 +152,68 @@ values, it is more efficient to use the :func:`sorted` function. Also, when
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functions.
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Priority Queue Implementation Notes
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-----------------------------------
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A `priority queue <http://en.wikipedia.org/wiki/Priority_queue>`_ is common use
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for a heap, and it presents several implementation challenges:
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* Sort stability: how do you get two tasks with equal priorities to be returned
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in the order they were originally added?
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* In the future with Python 3, tuple comparison breaks for (priority, task)
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pairs if the priorities are equal and the tasks do not have a default
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comparison order.
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* If the priority of a task changes, how do you move it to a new position in
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the heap?
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* Or if a pending task needs to be deleted, how do you find it and remove it
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from the queue?
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A solution to the first two challenges is to store entries as 3-element list
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including the priority, an entry count, and the task. The entry count serves as
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a tie-breaker so that two tasks with the same priority are returned in the order
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they were added. And since no two entry counts are the same, the tuple
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comparison will never attempt to directly compare two tasks.
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The remaining challenges revolve around finding a pending task and making
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changes to its priority or removing it entirely. Finding a task can be done
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with a dictionary pointing to an entry in the queue.
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Removing the entry or changing its priority is more difficult because it would
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break the heap structure invariants. So, a possible solution is to mark an
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entry as invalid and optionally add a new entry with the revised priority::
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pq = [] # the priority queue list
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counter = itertools.count(1) # unique sequence count
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task_finder = {} # mapping of tasks to entries
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INVALID = 0 # mark an entry as deleted
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def add_task(priority, task, count=None):
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if count is None:
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count = next(counter)
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entry = [priority, count, task]
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task_finder[task] = entry
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heappush(pq, entry)
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def get_top_priority():
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while True:
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priority, count, task = heappop(pq)
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del task_finder[task]
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if count is not INVALID:
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return task
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def delete_task(task):
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entry = task_finder[task]
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entry[1] = INVALID
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def reprioritize(priority, task):
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entry = task_finder[task]
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add_task(priority, task, entry[1])
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entry[1] = INVALID
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Theory
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------
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