[Python-Dev] Explorations on new-style classes (original) (raw)

Kevin Jacobs jacobs@penguin.theopalgroup.com
Wed, 6 Mar 2002 09:47:06 -0500 (EST)

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On Tue, 5 Mar 2002, David Abrahams wrote:

Although I understand the appeal, using slots to constrain the attributes might be misguided (depending on your goals):

How about something even more misguided? I've implemented a metaclass that works around the problem of layout conflicts. e.g.:

class A(object): metaclass = LazySlotBuilder slots = 'a'

class B(object): metaclass = LazySlotBuilder slots = 'b'

class C(B): slots = 'c'

class AB(A,B): slots = ['ab']

class ABC(A,B,C,AB): slots = ['abc']

abc=ABC() print type(abc).slots

['a', 'c', 'b', 'ab', 'abc']

An implementation of LazySlotBuilder metaclass is attached. It works by creating 2 types instead of 1 if slots are present. The first class is an abstract version of the desired class with no slots declared in itself or its base classes. A concrete version is then built that inherits from the abstract version, adding only the slot definitions and a reference to the abstract class.

The current version has one significant limitation: it requires that all base classes with slots have the LazySlotBuilder metaclass. This can be worked around, but makes the implementation needlessly complex.

I'm very much interested in what people think about these ideas about using (or mis-using) slots, metaclasses, new-style classes, and other cutting-edge Python features. I am intentionally trying to push the limits and take things beyond what some of these features were originally intended to do. It is my hope that such examples will educate, spur discussion and help determine exactly which behaviors are intentional and supported, and which are implementation artifacts that should not be taunted lightly.

I am also aware that a sizeable fraction of python-dev may not care much about this topic, so I would not be offended if the consensus was that this discussion is not topical enough for the list. In that case, I would be happy to move it elsewhere, maybe to its own mailing list. Just keep in mind that these features are potentially the most confusing and powerful language extensions that Python has ever had, and that people (like myself) are currently writing code that is beginning to make full use of them. There are a lot of dusty corners to explore, and it is important for the principal Python developers to be aware of and to deal with any gremlins hiding in them.

It would also be nice if some of the examples I'm working on could eventually become part of the official Python test suite. If for no other reason, because the current set of tests is somewhat sparse.

Regards, -Kevin

-- Kevin Jacobs The OPAL Group - Enterprise Systems Architect Voice: (216) 986-0710 x 19 E-mail: jacobs@theopalgroup.com Fax: (216) 986-0714 WWW: http://www.theopalgroup.com

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