On Aug 17, 5:09 pm, Bruno Desthuilliers
<[EMAIL PROTECTED]> wrote:
> akonsu a écrit :> hello,
>
> > i need to add properties to instances dynamically during run time.
> > this is because their names are determined by the database contents.
> > so far i found a way to add methods on demand:
>
> > class A(object) :
> > def __getattr__(self, name) :
> > if name == 'test' :
> > def f() : return 'test'
> > setattr(self, name, f)
> > return f
> > else :
> > raise AttributeError("'%s' object has no attribute '%s'" %
> > (self.__class__.__name__, name))
>
> > this seems to work and i can invoke method test() on an object.
>
> Nope. This adds per-instance *function* attributes - not *methods*.
>
> class A(object) :
> def __getattr__(self, name) :
> if name == 'test' :
> def f(self) :
> return "%s.test" % self
> setattr(self, name, f)
> return f
> else :
> raise AttributeError(
> "'%s' object has no attribute '%s'" \
> % (self.__class__.__name__, name)
> )
>
> a = A()
> a.test()
> => Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: f() takes exactly 1 argument (0 given)
>
> To add methods on a per-instance basis, you have to manually invoke the
> descriptor protocol's implementation of function objects:
>
> class A(object) :
> def __getattr__(self, name) :
> if name == 'test' :
> def f(self) :
> return "%s.test" % self
> m = f.__get__(self, type(self))
> setattr(self, name, m)
> return m
> else :
> raise AttributeError(
> "'%s' object has no attribute '%s'" \
> % (self.__class__.__name__, name)
> )
>
> > it
> > would be nice to have it as property though. so i tried:
>
> > class A(object) :
> > def __getattr__(self, name) :
> > if name == 'test' :
> > def f() : return 'test'
> > setattr(self, name, property(f))
> > return f
> > else :
> > raise AttributeError("'%s' object has no attribute '%s'" %
> > (self.__class__.__name__, name))
>
> > but this does not work, instance.test returns a callable but does not
> > call it.
>
> Properties must be class attributes. The only way (the only way I know)
> to get them to work as instance-attributes is to overload
> __getattribute__, which is tricky and may have pretty bad impact on
> lookup perfs - and ruins the whole point of using properties FWIW.
>
> > i am not an expert in python, would someone please tell me what i am
> > doing wrong?
>
> Wrong solution to your problem, I'd say. Let's start again:
>
> """
> > i need to add properties to instances dynamically during run time.
> > this is because their names are determined by the database contents.
> """
>
> Care to elaborate ? I may be wrong, but I suspect you're trying to roll
> your own python/database mapper. If so, there are quite a couple Python
> ORMs around. Else, please tell us more.
I posted this to another thread, but...
You can dynamically add properties (or anything else) to a CLASS just
before returning the
instance using __new__():
class AClass(object):
def __new__(cls):
setattr(cls,"propName", property(fget = ...,
fset = ...,
fdel = ...,
doc = ...) )
obj = super(AClass, cls).__new__(cls)
return obj
- Rafe
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