[Python-Dev] PEP 3144 review.

[Scott Dial]

Peter Moody wrote:
> but that's what's being suggested here for networks.
> 
>>>> ipaddr.IPv4Network('1.1.1.1/24')[0][1][2][3]...

This example here solidifies my support of RDM's suggestion of there
being 3 types:

    IPv4Address
    IPv4AddressWithNetwork (or as he called it: IPv4AddressWithMask)
    IPv4Network

The primary difference between IPv4Network and IPv4AddressWithNetwork
would be that IPv4AddressWithNetwork would not support
iteration/indexing. The example above makes no sense logically and my
original suggestion unknowingly opened that can of worms. If indexing a
IPv4Network returned IPv4AddressWithNetwork, then that would remove that
oddity.

This would also solve the weirdness that Stephen brought up in another
branch of this discussion:

Stephen J. Turnbull wrote:
> Scott Dial writes:
>  > ipaddr.IPv4Network('1.1.1.0/24')[0] ==
>  >     ipaddr.IPv4Network('1.1.1.0/24')
>
> So foo returns True?
>
> def foo():
>     a = ipaddr.IPv4Network('1.1.1.0/24')
>     return a[0] == a
>
> That seems ... weird.  Maybe you committed a typo?

The root of the weirdness is that my proposition would appear to make
IPv4Network play double-duty. However, it already does! That you can
instantiate a IPv4Network object with a non-zero host is a clue, and
that it is apparently the only way to input a host with a mask is the
second clue.

If I have an IP (10.2.3.4) and I know the netmask (say, 255.255.0.0),
then how do I get the network that it is on? As it stands, the only way
I see to do this is to do:

>>> net = ipaddr.IPv4Network("10.2.3.4/255.255.0.0")

Which is strange, because I didn't input a *network*, I inputted an
*address* on a network. Moreover, it's strange because:

>>> net[0] == net.ip
False

If I needed that identity to hold true, then I have to do:

>>> net = ipaddr.IPv4Network("%s/%s" % (net[0], net.netmask)

Although it is unclear why a "network" has an "ip" attribute. As far as
I am concerned, IPv4Network objects should *only* have a net.network.

Hopefully at this point, I have made the case that IPv4Network already
is playing double-duty as a IPv4Network and IPv4AddressWithNetwork. And
that the API would be well-served by splitting that role and that it
would be considerably less confusing.

-- 
Scott Dial
scott at scottdial.com
scodial at cs.indiana.edu