How does a Python set([]) check if two objects are equal? What methods does an object need to define to customise this?

Question

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.

Answer 1

Yes, you need a __hash__()-method AND the comparing-operator which you already provided.

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