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

Guido van Rossum guido at python.org
Mon Sep 28 22:43:44 CEST 2009

On Mon, Sep 28, 2009 at 1:36 PM, R. David Murray <rdmurray at bitdance.com> wrote:

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.

I say the case has been argued extensively, let's wait for Peter to respond.

I would say that there certainly are precedents in other areas for keeping the information about the input form around. For example, occasionally it would be handy if parsing a hex integer returned an object that was compatible with other integers but somehow kept a hint that would cause printing it to use hex by default.

I see keeping around the IP address used to create a network object as a similar edge case. I would probably define the eq method to implement equivalency (ignoring the form of the input), just as I would in the case of the (hypothetical) hex integers. If you wanted to do a comparison that includes the input IP address, you could use (a, a.ip) == (b, b.ip).

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

More information about the Python-Dev mailing list