[Python-3000] metaclass syntax [was: Discussions with no PEPs] (original) (raw)

Guido van Rossum guido at python.org
Fri Mar 9 17:30:00 CET 2007

On 3/9/07, Phillip J. Eby <pje at telecommunity.com> wrote:

At 07:06 AM 3/9/2007 -0800, Guido van Rossum wrote: >Those look like use cases for metaclasses, but I don't see how they >require setting a custom dict *while the class suite is being >executed*.

The main use case for that is to know what order the items were defined in; currently there's no way to achieve that without say, abusing the debugger hook.

I agree, and I have a proposed solution (which Talin knows and will turn into a PEP). I was just noting that Jim, in a response to Talin's claim that the only use case for a custom dict was to know the declaration order, was replying with use cases for metaclasses that could be solved without a custom dict, hence he wasn't refuting Talin's claim (as he seemed to think he was).

There are other interesting use cases that could be achieved with a custom dictionary, too, now that I'm thinking about it, that currently require at least some level of bytecode hacking or string manipulation. For example, there's a Python Cookbook recipe that does propositional logic and works by exec'ing code with a custom dictionary after figuring out what names are being used by a function body. With a custom dictionary for the class body, that could be done without exec or bytecode inspection.

I personally can see the possibility of using the feature for implementing database schema definition forward references (where you need to be able to refer to objects that don't yet exist) in a class body.

Don't worry. This will be possible.

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

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