[Python-Dev] PEP 3144 review. (original) (raw)

R. David Murray rdmurray at bitdance.com
Mon Sep 28 22:36:10 CEST 2009

On Mon, 28 Sep 2009 at 22:11, "Martin v. Löwis" wrote:

Martin v. Löwis <martin v.loewis.de> writes:

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?

[...] 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. The thing is, it doesn't create a network, it creates a hybrid "network plus host" which retains knowledge about the original host (that is, 192.168.1.1 rather than simply 192.168.1.0, if you enter "192.168.1.1/24"). That's what the OP meant with "networks-that-aren't-networks", and that's what people are objecting to. That's not the question that was asked, though - the question asked was "Under what circumstances would I want to specify...". I hope most people agree that it is desirable to be able to specify a network not just by its network address.

But then it's not a network, it is an ipaddress-plus-mask. It is exactly that conflation that we are objecting to. There is no question about the use case of specifying a network ip plus a netmask and deriving a network object from that. That is unquestionably required by any useful API. The argument is about whether the object returned is a Network object, or a hybrid object representing both an IP address and a network. It is the latter, which ipaddr does, which many of us find problematic and likely to lead to hard to read programs that will probably produce maintenance errors.

I observe that this line in the current PEP rationale:

 IP addresses and IP networks are distinct.

is not in fact achieved by the reference implementation. Peter, however, clearly thinks it is, since it is listed as a design goal of ipaddr. This is a language disconnect I don't understand, which I think has been the source of a lot of the difficulties in this thread.

--David

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