[Python-Dev] Request for comments: [issue22941] IPv4Interface arithmetic changes subnet mask

[Søren Løvborg]

Thanks for the feedback, and apologies for my late reply. I have to
say, I'm not entirely sold on the argument for raising an exception on 
subnet "overflow".

First, I'll note what happens if we overflow an IPv4Address:

     >>> ipaddress.IPv4Address('255.255.255.255') + 1
     Traceback (most recent call last):
     ...
     ipaddress.AddressValueError: 4294967296 (>= 2**32) is not permitted
     as an IPv4 address

Now, I used "IPv4Interface() + 1" to mean "Give me the IP next to the
current one in the current subnet", knowing from the context that the
address would be valid and available.

     >>> host = ipaddress.IPv4Interface('10.0.0.2/24')
     >>> peer = host + 1

In this context, I would welcome an exception, as it would certainly be 
an error if I overflowed the subnet.

However, there are also situations in which overflowing would be valid 
and expected, e.g. as a way to skip to the "same" IP in the next subnet:

     >>> ip = ipaddress.IPv4Interface('10.0.0.42/24')
     >>> ip + ip.network.num_addresses
     IPv4Interface('10.0.1.42/24')

It's not even a hypothetical example; I've been working on a distributed 
embedded system where all the hosts have two (redundant) addresses 
differing only by their subnet; this could be a convenient way calculate 
one address from the other.

There's an additional issue with raising an exception, and that is that
it still won't catch overflow errors in my example use case:

     >>> host = ipaddress.IPv4Interface('10.0.0.254/24')
     >>> peer = host + 1
     >>> peer
     IPv4Interface('10.0.0.255/24')

This doesn't overflow and does not trigger an exception, but the
resulting peer address is still invalid (it's the subnet broadcast
address, not a host address). As such, the exception isn't even useful
as an error detection tool. (I'll not suggest raising an exception when 
hitting the broadcast or network address; that way lies madness.)

As for consistency with IPv4Address, I can argue either way:

Pro-exception:

"Overflowing an IPv4Interface raises AddressValueError just like with
IPv4Address."

Contra-exception:

"An IPv4Interface behaves exactly like an IPv4Address, except that it
also has an associated subnet mask." (This is essentially how the type
is currently documented).

Thoughts?

Best regards,
Søren Løvborg