在python中使用heapq作为优先级列表的问题

Question

I can't understand why my below code raise an error.

I'm trying to build a priority list based on heapq module of Python. The only difference with a basic example of the module is that want to use it with custom objects in it it, instead of simple (int,int) or (int,str) tuples.

import heapq

class MyObject():

 def __init__(self,a=0,name='toto'):

     self.a = a
     self.name = name

if __name__ == '__main__':

 priority_list = []
 heapq.heappush(priority_list,(1,MyObject()))
 heapq.heappush(priority_list,(1,MyObject()))

This is the error I have:

heapq.heappush(priority_list,(1,MyObject()))

TypeError: '<' not supported between instances of 'MyObject' and 'MyObject'

The error is not raised if I use a different key to insert in the heap, but isn't heapq supposed to deal with same keys? I don't understand very well this behaviour.

Thanks a lot

Answer 1

The operator < is not defined for your class. That way heapq can't define priority.

ob1 = MyObject()
ob1 < ob1

raises

TypeError: unorderable types: MyObject() < MyObject()

You must then define the logical operators. See this for more info.

class MyObject():
    def __init__(self,a=0,name='toto'):
        self.a = a
        self.name = name

    def __lt__(ob1, ob2):
        return ob1.a < ob2.a

ob1 = MyObject()
ob1 < ob1 # returns False

Answer 2

A heap has the property that the smallest object is always on top. In order for Python to preserve that invariant, it must have some way of determining which object is smaller. Your MyObject class does not provide this.

You can define __gt__ or __lt__ to enable this.