How do sets work in Python

Question

I have a list of objects which I want to turn into a set. My objects contain a few fields that some of which are o.id and o.area. I want two objects to be equal if these two fields are the same. ie: o1==o2 if and only if o1.area==o2.area and o1.id==o2.id.

I tried over-writing __eq__ and __cmp__ but I get the following error:

TypeError: unhashable instance.

Please tell me what should I over-write?

Answer 1

Define the __hash__ method to return a meaningful hash based on the id and area fields.

For example, you can try this:

def __hash__(self):
    return hash(self.id) ^ hash(self.area)