bruno.42.desthuilliers at wtf
Sep 12, 2007, 12:50 AM
Post #2 of 2
(105 views)
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Re: __getattr__ and static vs. instantiated
[In reply to]
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tvaughan a écrit :
> Hi,
>
> Let's say I have:
>
> class Persistable(object):
>
> __attrs__ = {}
>
> def __getattr__(self, name):
> if name in self.__attrs__:
> return self.__attrs__[name]['value']
> else:
> return Object.__getattr__(self, name)
>
> def __setattr__(self, name, value):
> if name in self.__attrs__:
> self.__attrs__[name]['value'] = value
> else:
> Object.__setattr__(self, name, value)
>
> And the I have:
>
> class Person(Persistable):
>
> __storm_table__ = 'person'
>
> id = Int(primary=True)
>
> __attrs__ = {
> 'name': {
> 'lang': 'en',
> 'value': Unicode(),
> },
> }
>
> def __init__(self):
> self.id = int(random.random() * 1000)
>
> I can do this:
>
> person = Person()
> person.name = 'Jane Smith'
>
> But I cannot do:
>
> Person.name = 'Jane Smith'
yes, you can. But it won't do the same thing.
> or use Person.name in a Storm query like:
>
> Person.name == 'Jane Smith'
It will obviously raise an AttributeError if the class Person doesn't
have a 'name' attribute.
> __getattr__ is only called when using an instantiated class,
A class's '__getattr__' method is only triggered when looking up
attributes on an instance of the class, yes.
> and
> never, it seems, in a static case. Why? How do I work around this?
Since classes are instances of their type, the obvious solution would be
to define a custom metaclass implementing __getattr__. I didn't try, but
it might work.
Now is this the best thing to do is another question. You seems not to
be aware of the descriptor protocol - which is used to implement methods
and properties. This lets you define 'smart' attributes. You'll find
relevant documentation here:
http://docs.python.org/ref/descriptors.html
and here:
http://users.rcn.com/python/download/Descriptor.htm
Also, you seem to be trying to roll your own ORM. There are already
quite a few existing ones - SQLAlchemy being probably the best one
already, and also a good base (no pun) for building your own solutions.
HTH
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