Python heapq module, heapify method on an object

Question

Since I’m trying to be efficient in this program I’m making, I thought I’d use the built in heapq module in python, but some of my objects have multiple attributes, like name and number. Is there a way to use the heapify method to heapify my objects based on a certain attribute? I don’t see anything in the documentation.

Asked By: Trim

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Source

Answer 1

Right after I posted, I figured you could make a list of the objects by the attribute needed before using heapify which would take O(n) linear time. This wouldn’t affect the runtime of heapify or other heapq methods.

Answered By: Trim

Answer 2

@vsekhar and @ all the others wondering about the accepted answer.

Assumption:

class SomeObject():
    def __init__(self,name, number):
        self.name = name
        self.number = number

a_list = []
obj_1 = SomeObject("tim", 12)
obj_2 = SomeObject("tom", 13)

Now, instead of creating a heap with the objects only as elements:

heapq.heappush(a_list, obj_1)
heapq.heappush(a_list, obj_2)

you actually want to create the heap with a tuple of 2 values as heap elements – The idea is to have the attribute you want to sort with as first value of the tuple and the object (as before) as the second element of the tuple:

# Sort by 'number'.
heapq.heappush(a_list, (obj_1.number, obj_1))
heapq.heappush(a_list, (obj_2.number, obj_2))

The heap considers this first value of the tuple as the value to sort by.

In case the element pushed to the heap is not of a simple data type like int or str, the underlying implementation needs to know how to compare elements.
If the element is an iterable the first element is considered to contain the sort value.
Have a look at the examples here: https://docs.python.org/3/library/heapq.html#basic-examples (search for tuple)

Heap elements can be tuples. This is useful for assigning comparison values (such as task priorities) alongside the main record being tracked:

Another option might be to make comparison work with your custom class – This can be implemented so the object itself can be used as the heap element (as in the first example).

Have a look here for reference and an example: "Enabling" comparison for classes
Have a look at the enahnced class SomeObject:

class SomeObject():
    def __init__(self,name, number):
        self.name = name
        self.number = number
    def __eq__(self, obj): 
        return self.number == obj.number 
    def __lt__(self, obj): 
        return self.number < obj.number 
    def __hash__(self): 
        return hash(self.number)

This way you can create the heap with the objects only as elements:

heapq.heappush(a_list, obj_1)
heapq.heappush(a_list, obj_2)

Answered By: rocksteady

Python heapq module, heapify method on an object

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