Question

I have a class which is a subclass of tuple. I want to use instances of that class as elements of a set, but I get the error that it is an unhashable type. I guess this is because I’ve overridden the __eq__ and __ne__ methods. What should I do to restore my type’s hashability? I’m using Python 3.2.

Answer 1

you will need to majke your type hashable, which means implementing the __hash__() member function in your class deriving from tuple.

for example:

class test(tuple):
    def __eq__(self,comp):
        return self[0] == comp[0]
    def __ne__(self,comp):
        return self[0] != comp[0]
    def __hash__(self):
        return hash(self[0])

and this is what it looks like now:

>>> set([test([1,]),test([2,]),test([3,])])
{(1,), (2,), (3,)}
>>> hash(test([1,]))
1

note: you should absolutely read the documentation for the __hash__() function, in order to understand the relationship between the comparison operators and the hash computation.