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

Phillip J. Eby pje at telecommunity.com
Fri Mar 9 17:15:15 CET 2007

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.

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.

More information about the Python-3000 mailing list