[Python-Dev] @decoration of classes

Jack Diederich jack at performancedrivers.com
Tue Mar 29 02:55:50 CEST 2005

On Sat, Mar 26, 2005 at 12:36:08PM -0800, Josiah Carlson wrote:
> Eric Nieuwland <eric.nieuwland at xs4all.nl> wrote:
> > 
> > Given the ideas so far, would it possible to:
> > 
> > def meta(cls):
> > 	...
> > 
> > @meta
> > class X(...):
> > 	...
> It is not implemented in Python 2.4.  From what I understand, making it
> happen in Python 2.5 would not be terribly difficult.  The question is
> about a "compelling use case".  Is there a use where this syntax is
> significantly better, easier, etc., than an equivalent metaclass?  Would
> people use the above syntax if it were available?
For compelling, I think the code smell put off by the "no conflict" metaclass
generator recipe (which also appeared in Alex Martelli's PyCon talk) is fairly
compelling from a duck typing point of view.  

# would you rather
class K:
  __metaclass__ = no_conflict(MetaA, MetaB)
# or
class K: pass

Unless you actually want a class two inherit magic methods from two different
types you don't need two metaclasses.  You just need the class manipulations 
that are done in two different metaclasses.

I get around this[1] by defining a function that calls things that manipulate
classes, the metaclass's init will make the 'register' function static if it
is defined in the __dict__ and then call it with (name, cls).
If I called that method 'decorate' instead and spelled it @decorate I'd be
a happy camper.


[1] "Register" metatype, define the register method to screw around with
    your class definition or leave it out to let your parent class do its thing

class Register(type):
  def __init__(cls, name, bases, dict):
    if ('register' in dict):
      setattr(cls, 'register', staticmethod(dict['register']))
    cls.register(name, cls)

I call it Register because largely I just use it to check the __dict__ for
special methods and put classes in one or more global buckets.  I have cron
jobs that operate on the different buckets.

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