On Dec 9, 2006, at 2:17 AM, Phillip J. Eby wrote: > At 01:20 PM 12/9/2006 +1300, Greg Ewing wrote: >> Talin wrote: >>> In other words, the >>> __metaclass__ statement would have a side-effect of replacing the >>> locals() dict with a mapping object supplied by the metaclass. >> >> __metaclass__ isn't a statement, it's just an >> attribute that is interpreted in a special way >> *after* the class namespace has been populated, >> by which time it's too late to do what you >> suggest. > > Note that if we simply make locals() set or get attributes on the > class > instance, assigning to __class__ in the body of the class would > actually > set the metaclass immediately. > > The principal hurdle that would need to be overcome to do this, is > that you > can't change a builtin type's __class__ currently:
Another hurdle is that you can add slots the the type object (currently only in C code), how would your scheme deal with that? PyObjC implements types that correspond with Objective-C classes and add a (hidden) slot to type objects that refers to those classes. Classic classes are a simular problem: they aren't instances of type: >>> class C: pass >>> isinstance(C, type) False >>> isinstance(object, type) True I know classic classes are going away, but this does seem to point to a problem with your scheme: metaclasses can change the type of the object created by a class statement. Ronald _______________________________________________ Python-3000 mailing list Python-3000@python.org http://mail.python.org/mailman/listinfo/python-3000 Unsubscribe: http://mail.python.org/mailman/options/python-3000/archive%40mail-archive.com
