[Python-Dev] Classes and Metaclasses in Smalltalk (original) (raw)

Guido van Rossum guido@digicool.com
Tue, 01 May 2001 21:48:43 -0500

Guido:

> If both are defined, I propose the following, clumsy but backwards > compatible rule: if DictType.dict['foo'] describes a method, it > wins. Otherwise, TypeType.dict['foo'] wins.

Greg Ewing:

Yeek! I think that's far too confusing a rule. I suppose it might do in the meantime, but we'd better have a long term solution in mind before going too far down this route.

I agree 100%. I had to do something quick to be able to make progress with my PEP 252 project, but it's a clear indication that there's a problem!

Ultimately it seems like we'll have to introduce a separate namespace for methods and default instance attributes, say classdict. Then lookup of x.foo would look first in x.dict, then x.class.classdict, etc up the inheritance chain.

Except that sometimes you really do want x.class.classdict to have priority (e.g. for "guarded" attributes).

Then we'll have to resolve the ambiguity of the class.foo syntax. The bravest way would be simply to change the syntax for getting unbound methods.

Agreed again.

The most common use for these seems to be for calling inherited methods, so perhaps something like

inherited MyBaseClass.foo(arg, ...) which would be equivalent to getmethod(MyBaseClass, 'foo')(self, arg, ...) where getmethod() is a new builtin like getattr() except that it looks in the classdict, and 'self' is really whatever the first argument of the containing method was.

The second most common use is to reference class variables (e.g. imagine a class that keeps counters of how many instances have been created and deleted in C.initcount and C.delcount). But these should not have to change, since they really are class attributes.

Now that we have future, would such a change be contemplatable? Or is it too radical to even think about?

If we can find a way to spell "super.method", we should be ready for the future. I can't think of something right off the bat unfortunately.

But the issue of backwards compatibility is a big one here: the idioms for calling base class methods and using class variables as defaults for instance variables are so common that we will have to support these for many future versions! (Two things I am not looking forward to: fixing all the Zope code that uses this, and telling the author of Programming Python, 2nd. ed.)

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