[Python-3000] super(), class decorators, and PEP 3115

Guido van Rossum guido at python.org
Mon Apr 30 21:17:53 CEST 2007

Hm... Where *is* the class decorators PEP? I was expecting it.

Assuming class decorators are added, can't you do all of this using a
custom metaclass?

I'm not sure that your proposal for implementing an improved super has
anything over the currently most-favored proposal by Timothy Delaney.


On 4/30/07, Phillip J. Eby <pje at telecommunity.com> wrote:
> While working on the generic function PEP, I came across an unexpected (to
> me, anyway) consequence of PEP 3115: it breaks current methods (circa
> Python 2.2+) of implementing class decorators.  However, there does not
> appear to be a class decorator PEP outstanding.
> In current Python versions, the only way to implement class decorators is
> to call a "magic" function in the body of the class, that inserts a special
> (temporary) metaclass to handle the decoration.  This is implemented in the
> DecoratorTools package as well as in zope.interface (where its most common
> use case is to supply interface information about a class).
> Of course, in Py 3.x, some of these features could be handled using a
> metaclass with keyword arguments, due to PEP 3115.  However, the other use
> cases cannot.  Notably, any code that relies on in-class decorators or
> conditional class decorators (such as method decorators that add class
> decorators), will not be able to work.
> However, I have an idea for a small modification to PEP 3115 that would
> re-enable support for both static and dynamic class decorators, and make it
> possible to implement an improved super() as well.
> Suppose that we reserved a special name, '__decorators__', that would be
> contained in the mapping object returned by '__prepare__', and initialized
> to contain any static decorators defined outside the class, e.g., this code:
>      @implements(IFoo)
>      @otherdecorator
>      class X:
>          print __decorators__
> would print a list equal to [implements(IFoo), otherdecorator].  The idea
> here is that following the execution of metaclass(name, bases, classdict,
> **kw), the __decorators__ list would be repeatedly popped to yield
> decorators to be invoked on the "finished" class.  In this way, code that
> needs dynamic access to the finished class can simply add objects to
> __decorators__.
> As a result, we could then implement a super() enhancement of the form
> "super(__class__, self).methodname(args)", where "__class__" would be a
> compiler-defined variable (ala "__debug__").  If any function inside a
> class body uses "__class__", a preamble is added to the class body that
> defines an anonymous callback function that sets a closure variable to
> point to the class, and returns the class.  This anonymous callback
> function is then appended to __decorators__.
> So, this approach could be used to enhance super() directly (using
> __class__ instead of a specific class), or as a behind-the-scenes part of
> the implementation of a more "sugary" syntax.  It would also continue to
> enable dynamic class decorators for other uses.
> Thoughts?
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--Guido van Rossum (home page: http://www.python.org/~guido/)

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