[Python-Dev] PEP 3144: IP Address Manipulation Library for the Python Standard Library

[Antoine Pitrou]

Le Wed, 19 Aug 2009 08:35:15 -0700, Peter Moody a écrit :
>>
>> does it mean that the objects are mutable? Would it make sense to make
>> them immutable and therefore hashable (such as, e.g., datetime
>> objects)?
> 
> that's a good point. I'll implement __hash__ in the BaseIP class.

It is a common practice that only immutable objects define a meaningful 
__hash__ method. The reason is that dicts and sets (and perhaps other 
structures) cache the hash value instead of calling __hash__ again and 
again. If you stick a mutable with a meaningful __hash__ in a dict, and 
then modify the mutable object, lookups will give the wrong results (they 
will be based on the old, stale hash value).

It seems to me that hashability is a more desireable property of IP 
objects than modifiability. I don't see any reason to modify an IP object 
after having created it (rather than creating a new object).

Regards

Antoine.