[Python-Dev] PEP 3144 review.

[R. David Murray]

On Mon, 28 Sep 2009 at 05:57, "Martin v. Löwis" wrote:
>>> Finally, to Stephen's point about seeing the other side of the
>>> argument, I wrote this offlist a week ago:
>>>
>>>   I *understand* what you're saying, I *understand* that
>>> 192.168.1.1/24 isn't a network,
>>
>> But you still want to treat it as one.
>>
>> 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?
>>
>
> It's fairly obvious to me why the library should support 192.168.1.1/24
> as an input, and return a network.
>
> End-users are likely going to enter such things (e.g. 82.94.164.162/29),
> as they will know one IP address in the network (namely, of one machine
> that they are looking at), and they will know the prefix length (more
> often, how large the network is - 8 or 16 or 32). So very clearly,
> end users should not be required to make the computation in their heads.
>
> 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.
>
> Does that answer your question?

The fundamental divide here is between two behaviors.

ipaddr:

     >>> x = IPv4Network('192.168.1.1/24')
     >>> y = IPv4Network('192.168.1.0/24')
     >>> x == y
     False
     >>> x.ip
     IPv4Address('192.168.1.1')

desired:

     >>> x = IPv4Network('192.168.1.1/24')
     >>> y = IPv4Network('192.168.1.0/24')
     >>> x == y
     True
     >>> x.ip
     Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
     AttributeError: 'IPv4Network' object has no attribute 'ip'

Everything else is pretty much bikeshedding and can be dealt with.  This
is fundamental and Peter has indicated he will not change it.

--David