A generic priority queue for Python
You can use Queue.PriorityQueue.
Recall that Python isn't strongly typed, so you can save anything you like: just make a tuple of (priority, thing)
and you're set.
When using a priority queue, decrease-key is a must-have operation for many algorithms (Dijkstra's Algorithm, A*, OPTICS), I wonder why Python's built-in priority queue does not support it. None of the other answers supply a solution that supports this functionality.
A priority queue which also supports decrease-key operation is this implementation by Daniel Stutzbach worked perfectly for me with Python 3.5.
from heapdict import heapdict
hd = heapdict()
hd["two"] = 2
hd["one"] = 1
obj = hd.popitem()
print("object:",obj[0])
print("priority:",obj[1])
# object: one
# priority: 1
I ended up implementing a wrapper for heapq
, adding a dict for maintaining the queue's elements unique. The result should be quite efficient for all operators:
class PriorityQueueSet(object):
"""
Combined priority queue and set data structure.
Acts like a priority queue, except that its items are guaranteed to be
unique. Provides O(1) membership test, O(log N) insertion and O(log N)
removal of the smallest item.
Important: the items of this data structure must be both comparable and
hashable (i.e. must implement __cmp__ and __hash__). This is true of
Python's built-in objects, but you should implement those methods if you
want to use the data structure for custom objects.
"""
def __init__(self, items=[]):
"""
Create a new PriorityQueueSet.
Arguments:
items (list): An initial item list - it can be unsorted and
non-unique. The data structure will be created in O(N).
"""
self.set = dict((item, True) for item in items)
self.heap = self.set.keys()
heapq.heapify(self.heap)
def has_item(self, item):
"""Check if ``item`` exists in the queue."""
return item in self.set
def pop_smallest(self):
"""Remove and return the smallest item from the queue."""
smallest = heapq.heappop(self.heap)
del self.set[smallest]
return smallest
def add(self, item):
"""Add ``item`` to the queue if doesn't already exist."""
if item not in self.set:
self.set[item] = True
heapq.heappush(self.heap, item)