[Python-Dev] PEP 3144: IP Address Manipulation Library for the Python Standard Library
solipsis at pitrou.net
Wed Aug 19 18:29:08 CEST 2009
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
> 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).
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