Python: How do sets work

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 error: TypeError: unhashable instance.

What should I over-write?

Answer 1

"TypeError: unhashable instance." error is probably due to old-style class definition i.e.:

class A:
  pass

Use new style instead:

class A(object):
  pass

If you override __cmp__ function you should override __hash__ for using your object in sets. In the other case hash considers all object instances as unequal and __cmp__ function will never be called.

Answer 2

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

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