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Understanding python object ownership for sets

If I understand correctly, the __cmp __ () function of an object is called to evaluate all the objects in the collection when determining whether the object is a member or "in" a collection. However, this is not like many:

class MyObject(object):
    def __init__(self, data):
        self.data = data

    def __cmp__(self, other):
        return self.data-other.data

a = MyObject(5)
b = MyObject(5)

print a in [b]          //evaluates to True, as I'd expect
print a in set([b])     //evaluates to False

How is the membership of an object in a set checked, then?

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>>> xs = []
>>> set([xs])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

. , dicts. ( - O (1), ), : , , (, ) .

__hash__ id ( imho), , object __hash__, (, sizeof ).

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__hash__ :

class MyObject(object):
    def __init__(self, data):
        self.data = data

    def __cmp__(self, other):
        return self.data - other.data

    def __hash__(self):
        return hash(self.data)


a = MyObject(5)
b = MyObject(5)

print a in [b] # True
print a in set([b]) # Also True!
+5

, __hash__, id , , , __hash__, , , .

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The set uses a dict backstage, so the in operator checks to see if the object exists as a key in the dict. Since your object does not implement a hash function, the default hash function for objects uses the object identifier. Thus, even if a and b are equivalent, they are not the same object, and that is being tested.

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