[Python-Dev] PEP 3144 review.

[Peter Moody]

On Tue, Sep 15, 2009 at 10:16 AM, Scott Dial
<scott+python-dev at scottdial.com> wrote:
> R. David Murray wrote:
>> On Tue, 15 Sep 2009 at 14:28, Antoine Pitrou wrote:
>>> Andrew McNamara <andrewm <at> object-craft.com.au> writes:
>>>>
>>>>    >>> ipaddr.IPv4Network('192.168.1.1/16').network
>>>>     IPv4Address('192.168.0.0')
>>>
>>> Er, does this mean that taking the `network` attribute from a network
>>> object
>>> actually gives an address object (not a network)?
>>> It looks horribly misleading to me.
>>
>> That was my opinion, too.  I've always called that number the 'zero'
>> of the network, but other people said that it is usually called the
>> 'network'.
>>
>> I also find it odd to see a 'network' with an IP address of 192.168.1.1.
>> That looks like a host-address-plus-netmask to me, not a network
>> address.
>>
>
> It just happened that I needed a tool today to calculate the gateway IP
> for an interface, and I took it as an opportunity to try out this ipaddr
> module. I'll attempt to relate my experience below...
>
> I have to concur with the opinions above. I was very confused by the
> following error:
>
>>>> addr = ipaddr.IPAddress("10.1.2.3/255.255.240.0")
> ...
> ipaddr.IPAddressIPValidationError: '98.223.189.24/255.255.240.0' is not
> a valid address (hint, it's probably a network)

are you actually getting '98.223.189.24/255.255.240.0' back? that
doesn't look right.

> Because, it *is* a address of a host on a network. I gave in and said:
>
>>>> net = ipaddr.IPNetwork("10.1.2.3/255.255.240.0")
>
> But then, I was dumbfounded as to how I could get the gateway IP from
> this IPNetwork object. It took me a while to figure out that you can
> iterate over IPNetwork instances:
>
>>>> gateway = net[1]
>
> I was then confused, because:
>
>>>> print(type(gateway))
> <class 'ipaddr.IPv4Address'>
>
> Which sorta blew my mind.. I fully expected to receive an IPNetwork back
> from that operation. It is unclear to me why the network information
> gets chucked by that operation. I foresee having to work around that in
> real applications by doing something obnoxious like:
>
>>>> actual_gateway = ipaddr.IPNetwork("%s/%s" % (gateway, addr.netmask))
>
> But since I only actually needed the IP address for the gateway, it
> fulfilled my needs.
>
> In the end, I found the names IPNetwork/IPAddress and their
> instantiations confusing. ISTM that IPNetwork is overloaded and plays
> two roles of being an IPNetwork and being an IPAddressWithNetwork. And
> finally, it's unclear to me why iterating over a IPNetwork would not
> produce a sequence of IPNetwork(/IPAddressWithNetwork) objects.

What you're describing, at least for the networks, is basically what
ipaddr was.  It seemed, after much heated discussion, that the
community felt that

ipaddr.IPv4Network('1.1.1.0/24')[0]

was actually an individual address. ie, each of the addresses in
1.1.1.0/24 are individual addresses, rather than networks with /32
prefix lengths.

having said that, I understand your confusion; I spent a year+
treating addresses exactly as you've mentioned. It didn't seem as
though the majority of the community felt the same, though, so ipaddr
evolved.

Cheers,
/peter

> ISTM that if I started with an IPNetwork object, the API should always
> return IPNetwork objects. If I want just an IPAddress object, then I can
> always fetch the "ip" attribute to get that. Perhaps there is some
> compelling conceptual argument as to why this is not correct, but it
> seems like the API destroys information needlessly.
>
> Just my opinion..
>
> --
> Scott Dial
> scott at scottdial.com
> scodial at cs.indiana.edu
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