>>> import ipaddress
>>> hash(ipaddress.ip_network(u'20.0.2.3/32')) == hash(ipaddress.ip_network(u'20.0.2.0/30'))
True

This shouldn't be a problem: there's no rule that says that different objects should have different hashes. Indeed, with a countable infinity of possible different hashable inputs, a deterministic hashing algorithm, and only finitely many outputs, such a rule would be a mathematical impossibility. For example:

>>> hash(-1) == hash(-2)
True

Are these hash collisions causing real issues in your code? While a single hash collision like this shouldn't be an issue, if there are many collisions within a single (non-artificial) dataset, that _can_ lead to performance issues.

Looking at the code, we could probably do a better job of making the hash collisions less predictable. The current code looks like:

    def __hash__(self):
        return hash(int(self.network_address) ^ int(self.netmask))

I'd propose hashing a tuple instead of using the xor. For example:

    def __hash__(self):
        return hash((int(self.network_address), int(self.netmask)))

Hash collisions would almost certainly still occur with this scheme, but they'd be a tiny bit less obvious and harder to find.

Concur with Mark.

> I'd propose hashing a tuple instead of using the xor.

To be clear; I'd propose this _only_ if there's evidence that the current hashing scheme is unsatisfactory for real-world use-cases. Otherwise, on an "if it's not broken, don't fix it" basis, I don't think there's any change to be made here.

Thanks for the analysis, I agree completely.

Actually the problem was coming from my code where one of the __eq__ method was implemented like this:
>>> def __eq__(self, other):
>>>   return hash(self) == hash(other)

so 2 instances with only a slight difference in their ip_network attribute (ip_network(u'20.0.2.3/32') and ip_network(u'20.0.2.0/30')) were having the same hash and being equal -> they could not be inserted both in the same collection.

I will just rewrite my __eq__ method properly.

Thanks, Francois. Indeed, comparing hashes isn't a good way to implement equality. :-)  Closing here.

messages: + msg319030
stage: resolved

messages: + msg318976