How do sets work in Python?

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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?
Apr 12 in Python by ana1504.k
• 7,590 points
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1 answer to this question.

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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)
answered Apr 12 by SDeb
• 12,600 points

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