Creating a namedtuple with a custom hash function

Question

Say I have a namedtuple like this:

FooTuple = namedtuple("FooTuple", "item1, item2") 

And I want the following function to be used for hashing:

foo_hash(self):     return hash(self.item1) * (self.item2) 

I want this because I want the order of item1 and item2 to be irrelevant (I will do the same for the comparison-operator). I thought of two ways to do this. The first would be:

FooTuple.__hash__ = foo_hash 

This works, but it feels hacked. So I tried subclassing FooTuple:

class EnhancedFooTuple(FooTuple):     def __init__(self, item1, item2):         FooTuple.__init__(self, item1, item2)      # custom hash function here 

But then I get this:

DeprecationWarning: object.__init__() takes no parameters 

So, what can I do? Or is this a bad idea altogether and I should just write my own class from scratch?

Answer 1

I think there is something wrong with your code (my guess is that you created an instance of the tuple with the same name, so fooTuple is now a tuple, not a tuple class), because subclassing the named tuple like that should work. Anyway, you don't need to redefine the constructor. You can just add the hash function:

In [1]: from collections import namedtuple  In [2]: Foo = namedtuple('Foo', ['item1', 'item2'], verbose=False)  In [3]: class ExtendedFoo(Foo):    ...:     def __hash__(self):    ...:         return hash(self.item1) * hash(self.item2)    ...:   In [4]: foo = ExtendedFoo(1, 2)  In [5]: hash(foo) Out[5]: 2