[Python] How does a Python set([]) check if two objects are equal? What methods does an object...

I need to create a 'container' object or class in Python, which keeps a record of other objects which I also define. One requirement of this container is that if two objects are deemed to be identical, one (either one) is removed. My first thought was to use a set([]) as the containing object, to complete this requirement.

However, the set does not remove one of the two identical object instances. What must I define to create one?

Here is the Python code.

class Item(object):
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
def __repr__(self):
return "Item(%s, %s)" % (self.foo, self.bar)
def __eq__(self, other):
if isinstance(other, Item):
return ((self.foo == other.foo) and (self.bar == other.bar))
else:
return False
def __ne__(self, other):
return (not self.__eq__(other))


Interpreter

>>> set([Item(1,2), Item(1,2)])
set([Item(1, 2), Item(1, 2)])


It is clear that __eq__(), which is called by x == y, is not the method called by the set. What is called? What other method must I define?

Note: The Items must remain mutable, and can change, so I cannot provide a __hash__() method. If this is the only way of doing it, then I will rewrite for use of immutable Items.

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