[Python-Dev] PEP 3144 review.

[Peter Moody]

On Wed, Sep 16, 2009 at 8:21 PM, Andrew McNamara
<andrewm at object-craft.com.au> wrote:
>>> I think we're in a painful middle ground now - we should either go back
>>> to the idea of a single class (per protocol), or make the distinctions
>>> clear (networks are containers and addresses are singletons).
>>>
>>> Personally, I think I would be happy with a single class (but I suspect
>>> that's just my laziness speaking). However, I think the structure and
>>> discipline of three classes (per protocol) may actually make the concepts
>>> easier to understand for non-experts.
>>
>>I think this is where we disagree. I don't think the added complexity
>>does make it any easier to understand.
>
> I argue that we're not actually adding any complexity: yes, we add
> a class (per protocol), but we then merely relocate functionality to
> clarify the intended use of the classes.

And I argue the moving this functionality to new classes (and adding
new restrictions to existing classes) doesn't buy anything in the way
of overall functionality of the module or a developer's ability to
comprehend intended uses.

>>> A particular case in point - if you want to represent a single IP address
>>> with netmask (say an interface), you use a Network class, not an Address
>>> class. And the .network attribute returns a Address class!
>>
>>Right, and I don't see where the confusion lies.
>
> I suggest you are too close to the implementation to be surprised by it. 8-)

touche :)

>>You have an address + netmask. ergo, you have a Network object.
>
> In a common use case, however, this instance will not represent a
> network at all, but an address. It will have container-like behaviour,
> but it should not (this is a property of networks, not addresses). So
> the instance will be misnamed and have behaviours that are, at best,
> misleading.
>
>>The single address that defines the base address (most commonly referred
>>to as the network address) is an Address object. there is no netmask
>>associated with that single address, ergo, it's an Address object.
>
> I would argue that a Network never has a single address - by definition,
> it has two or more nodes. A .network attribute should return a Network
> instance. If you want the base address, this probably should be called
> .base_address or just .address (to parallel the .netmask attribute).

.network is shorthand for network address. are .network_address and
.broadcast_address less confusing?  I have to say, though,
.network/.broadcast are fairly common (IPy uses .net, netaddr and ipv4
use, IIRC .network...)

>>> The reason I suggest having the Network class assert that masked bits be
>>> zero is two-fold:
>>>
>>> * it ensures the correct class is being used for the job
>>> * it ensures application-user errors are detected as early as possible
>>>
>>> I also suggest the AddressWithMask classes not have any network/container
>>> behaviours for a similar reason. If the developer needs these, the
>>> .network attribute is only a lookup away.
>>
>>the problem I have with this approach is that it seems like a long way
>>to go for a shortcut (of checking if addr.ip != addr.network: raise
>>Error).
>
> This isn't about shortcuts, but about correctness... having the Network
> object represent a network, and having Address objects represent
> end-points, and having errors discovered as early as possible.

Then what I don't see is the purpose of your
network-only-network-object. essentially identical functionality can
be obtained with the module as is w/o the added complexity of new
classes.

Cheers,
/peter

> What I'm arguing here is that singletons should not simultaneously be
> containers - it's not pythonic, and it leads to ambiguity. The underlying
> IP concepts don't require it either: an IP address is a singleton, a
> network is a container, and there is no overlap. Yes, an address may be a
> member of a network, and having a reference to that network on the address
> object is valuable, but the address should not behave like a network.
>
> --
> Andrew McNamara, Senior Developer, Object Craft
> http://www.object-craft.com.au/
>