Automatic Attribute Assignment during Class Inheritance
steven at REMOVE.THIS.cybersource.com.au
Thu Sep 10 06:00:21 CEST 2009
On Wed, 09 Sep 2009 20:14:33 -0700, gizli wrote:
> I do not want to force the consumers of this framework to write this
> obscure line of code. I was wondering if it is possible (at class
> definition time) to capture the fact that MyTask extends Task and
> automatically insert the polymorphic_identity key into the
> __mapper_args__ class attribute (with value set to __name__).
> So far, I have not been able to do anything because during class
> definition time, there is no a lot of variables I can access. *self*
> obviously does not work. __name__ and __class__ are also useless.
Class definitions are controlled by the metaclass, which is fairly deep
magic but not entirely impenetrable. Once you've defined a metaclass to
use, the consumers will only need to say:
__metaclass__ = MyMetaclass
which is less obscure than the alternative. You may even be able to have
Task use the metaclass, in which case MyTask doesn't need to do anything
special at all. (I think.)
Another alternative is to use a class decorator:
# You write this and provide it in your API.
cls.__mapper_args__ = dict(polymorphic_identity=cls.__name__)
# The consumer writes this.
Decorator syntax for classes only works for Python 2.6 or better. In 2.5,
the consumer would need to write:
MyTask = mapper(MyTask)
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