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Mon Sep 28 22:36:10 CEST 2009 · https://mail.python.org/pipermail/python-dev/2009-September/092262.html

On Mon, 28 Sep 2009 at 22:11, "Martin v. LÃ¶wis" wrote:
>> Martin v. LÃ¶wis <martin <at> v.loewis.de> writes:
>>>> Could you explain what benefit there is for allowing the user to create
>>>> network objects that don't represent networks? Is there a use-case
>>>> where these networks-that-aren't-networks are something other than a
>>>> typo? Under what circumstances would I want to specify a network as
>>>> 192.168.1.1/24 instead of 192.168.1.0/24?
>>>>
>> [...]
>>> So Python code has to make the computation, and it seems most natural
>>> that the IP library is the piece of code that is able to compute a
>>> network out of that input.
>>
>> The thing is, it doesn't create a network, it creates a hybrid "network plus
>> host" which retains knowledge about the original host (that is, 192.168.1.1
>> rather than simply 192.168.1.0, if you enter "192.168.1.1/24").
>>
>> That's what the OP meant with "networks-that-aren't-networks", and that's what
>> people are objecting to.
>
> That's not the question that was asked, though - the question asked
> was "Under what circumstances would I want to specify...". I hope
> most people agree that it is desirable to be able to specify a network
> not just by its network address.

But then it's not a network, it is an ipaddress-plus-mask.  It is exactly
that conflation that we are objecting to.  There is no question about
the use case of _specifying_ a network ip plus a netmask and deriving
a network object from that.  That is unquestionably required by any
useful API.  The argument is about whether the object returned is a
Network object, or a hybrid object representing _both_ an IP address
and a network.  It is the latter, which ipaddr does, which many of us
find problematic and likely to lead to hard to read programs that will
probably produce maintenance errors.

I observe that this line in the current PEP rationale:

     IP addresses and IP networks are distinct.

is not in fact achieved by the reference implementation.  Peter, however,
clearly thinks it is, since it is listed as a design goal of ipaddr.
This is a language disconnect I don't understand, which I think has
been the source of a lot of the difficulties in this thread.

--David

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[Python-Dev] PEP 3144 review.