[Python-Dev] PEP 3144 review.

Nick Coghlan ncoghlan at gmail.com
Wed Sep 30 13:06:09 CEST 2009


Mark Dickinson wrote:
> On Wed, Sep 30, 2009 at 10:52 AM, Paul Moore <p.f.moore at gmail.com> wrote:
>> 2009/9/30 Mark Dickinson <dickinsm at gmail.com>:
>>> Please could someone who understands the uses of IPNetwork better than
>>> I do explain why the following wouldn't be a significant problem, if __eq__
>>> and __hash__ were modified to disregard the .ip attribute as suggested:
>>>
>>>>>> linus = IPv4Network('172.16.200.1/24')
>>>>>> snoopy = IPv4Network('172.16.200.3/24')
>>>>>> fqdn = {linus: 'linus.peanuts.net', snoopy: 'snoopy.peanuts.net'}
>>>>>> fqdn[linus]  # expecting 'linus.peanuts.net'
>>> 'snoopy.peanuts.net'
>> I certainly don't understand IPv4Network better than you :-) But that
>> just looks wrong to me - linus and snoopy are hosts not networks, so
>> making them IPv4Network classes seems wrong. I'd instinctively make
>> them IPv4Address objects (which, I believe, would work).
> 
> Okay, so maybe this is an abuse of IPv4Network.  But I'd (mis?)understood
> that the retention of the .ip attribute was precisely a convenience to allow
> this sort of use.  If not, then what's it for?  I've read the PEP and almost
> all of this thread, but I can't help feeling I'm still missing something.  If
> someone could point out the obvious to me I'd be grateful.

You're not missing anything that I'm aware of - unlike the use case for
accepting a denormalised network definition in the IPNetwork constructor
(which has been made quite clear in the list discussion, even if it is
still a bit vague in the PEP), the use case for *retaining* the host
information on the network object hasn't been well articulated at all.

The closest anyone has come to describing a use case is an indirect
comment via Guido that leaving out the attribute would involve real code
having to find somewhere else to stash the original address details
(e.g. by passing around an IPAddres/IPNetwork tuple rather than just an
IPNetwork object).

However, while I'd still be a little happier if the .ip attribute went
away all together and another means was found to conveniently associate
an IPAddress and an IPNetwork, keeping it doesn't bother me anywhere
near as much as having network equivalence defined in terms of something
other than the network ID and the netmask.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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