How to make heapq evaluate the heap off of a specific attribute?

Question

I wish to hold a heap of objects, not just numbers. They will have an integer attribute in them that the heap can sort by. The easiest way to use heaps in python is heapq, but how do I tell it to sort by a specific attribute when using heapq?

Answer 1

According to the example from the documentation, you can use tuples, and it will sort by the first element of the tuple:

>>> 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') 

So if you don't want to (or can't?) do a __cmp__ method, you can manually extract your sorting key at push time.

Note that if the first elements in a pair of tuples are equal, further elements will be compared. If this is not what you want, you need to ensure that each first element is unique.

Answer 2

heapq sorts objects the same way list.sort does, so just define a method __cmp__() within your class definition, which will compare itself to another instance of the same class:

def __cmp__(self, other):     return cmp(self.intAttribute, other.intAttribute) 

Works in Python 2.x.

In 3.x use:

def __lt__(self, other):     return self.intAttribute < other.intAttribute