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

Guido van Rossum guido@digicool.com
Wed, 02 May 2001 09:29:03 -0500

[Greg Ward, welcome back!]

I was just doing some gedanken with various ways to spell "super", and I think my favourite is the same as Java's (as I remember it):

class MyClass (BaseClass): def foo (self, arg1, arg2): super.foo(arg1, arg2)

I'm sure that's everybody's favorite way to spell it! It's mine too. :-)

Since I don't know much about Python's guts, I can't say how implementable this is, but I like the spelling. The semantics would be something like this (with adjustments to the reality of Python's guts):

* 'super' is a magic object that only makes sense inside a 'def' inside a 'class' (at least for now; perhaps it could be generalized to work at class scope as well as method scope, but let's keep it simple)

Yes, that's about the only way it can be made to work. The compiler will have to (1) detect that 'super' is a free variable, and (2) make it a local and initialize it with the proper magic. Or, to relieve the burden from the symbol table, we could make super a keyword, at the cost of breaking existing code.

I don't think super is needed outside methods.

* super's notional getattr() does something like this: - peek at the calling stack frame and fetch the calling function (MyClass.foo) and the first argument to that function (self) - [is this possible?] ensure that callingfunction is a bound method, and that it's bound to the self object we just plucked from the stack; raise a "misuse of super object" exception if not

I don't think you can make that test, but making it a 'magic local' as I suggested above would avoid the problem.

- walk the superclass tree starting at self.class.bases (ie. skip self's class), looking for an object with the name passed to this getattr() call -- 'foo' - when found, return it - if not found, raise AttributeError

Yup, that's the easy part. :-)

The ability to peek at the calling stack frame is essential to this scheme, in order to fetch the "current object" (self) without needing to have it explicitly passed. Is this as bothersome from C as it is from Python?

No, in C it's easy. The problem is that there is no information in the frame that tells you where the currently executing function was defined -- all you have is the code object, which is context-independent.

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