How to implement Priority Queues in Python?
- by dragosrsupercool
Sorry for such a silly question but Python docs are confusing.. .
Link 1: Queue Implementation
http://docs.python.org/library/queue.html
It says thats Queue has a contruct for priority queue. But I could not find how to implement it.
class Queue.PriorityQueue(maxsize=0)
Link 2: Heap Implementation
http://docs.python.org/library/heapq.html
Here they says that we can implement priority queues indirectly using heapq
pq = [] # list of entries arranged in a heap
entry_finder = {} # mapping of tasks to entries
REMOVED = '<removed-task>' # placeholder for a removed task
counter = itertools.count() # unique sequence count
def add_task(task, priority=0):
'Add a new task or update the priority of an existing task'
if task in entry_finder:
remove_task(task)
count = next(counter)
entry = [priority, count, task]
entry_finder[task] = entry
heappush(pq, entry)
def remove_task(task):
'Mark an existing task as REMOVED. Raise KeyError if not found.'
entry = entry_finder.pop(task)
entry[-1] = REMOVED
def pop_task():
'Remove and return the lowest priority task. Raise KeyError if empty.'
while pq:
priority, count, task = heappop(pq)
if task is not REMOVED:
del entry_finder[task]
return task
raise KeyError('pop from an empty priority queue'
Which is the most efficient priority queue implementation in python? And how to implement it?
