I am receiving on server objects from clients ( every object has same structure and have field self.utc_time which contains time of creation of that object). I need to store in some structure so I always have sorted in ascending so when I pop I pop the oldest object by utc_time, not by time when I receive. I thought to use priority queue from heapq but how to say to heaify by utc_time field from custom objects ? Is there better solution ?
Add the magic __cmp__
comparison method to your class to avoid needing to do the tuple-decoration that Maksim describes:
>>> import heapq
>>> class MyObject(object):
... def __init__(self, val):
... self.val = val
... def __cmp__(self, other):
... return cmp(self.val, other.val)
...
...
...
>>> q = []
>>> heapq.heappush(q, MyObject(50))
>>> heapq.heappush(q, MyObject(40))
>>> heapq.heappush(q, MyObject(30))
>>> heapq.heappush(q, MyObject(20))
>>> heapq.heappush(q, MyObject(200))
>>> obj = heapq.heappop(q)
>>> print obj.val
20
Note: Override __lt__
in Python 3, __cmp__
only in Python 2
Python documentation implicitly provides the following solution:
Heap elements can be tuples. This is useful for assigning comparison values (such as task priorities) alongside the main record being tracked:
h = []
heappush(h, (5, 'write code'))
heappush(h, (7, 'release product'))
heappush(h, (1, 'write spec'))
heappush(h, (3, 'create tests'))
heappop(h)
=> (1, 'write spec')
You can do it in a similar fashion - store tuples where the first element will contain a utc_time
of an object being placed to the PQ and the second - a reference to object itself.
In a similar SO question it's proposed to create an easy-to-use wrapper that would allow a cleaner way of working with a priority queue.
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