[Python-3000] Metaclasses in Py3K (original) (raw)

Talin talin at acm.org
Fri Dec 8 10:54:34 CET 2006

I'd like to see metaclasses have additional capabilities in Py3K, and one thing I'd like is for metaclasses to be able to have access to the order of declarations within a class.

If I understand correctly, in the current Python a class definition is essentially a suite with ordinary local variables; once that suite finishes executing, the locals() dict becomes the class's dict. The metaclass slips in between these two phases of construction, in that it can examine or modify the dict before it becomes a class.

Suppose, however, that the metaclass could hook into the get/set operations of the locals() dict itself. In other words, the metaclass statement would have a side-effect of replacing the locals() dict with a mapping object supplied by the metaclass.

This would allow the metaclass to intercept individual variable assignments as they occur, rather than taking the whole dict in at once.

I can think of a ton of applications for this. For example, you could implement the functionality of slots as a metaclass, allowing you to define the slots using regular Python syntax rather than having to pass in a list of name strings. You could define C structures using regular Python syntax. You could declare CORBA or other IDL interfaces using Python syntax and appropriate decorators. You might even be able to do things like enumerations using metaclasses.

-- Talin

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