Descriptor puzzlement

[Peter Otten]

John Roth wrote:

> Using Python 2.2.3, I create this script:
> 
> [beginning of script]
> 
> class AnObject(object):
>     "This might be a descriptor, but so far it doesn't seem like it"
>     def __init__(self, somedata):
>         self.it = somedata
>     def __get__(self, obj, type=None):
>         print "in get method"
>         return self.it
>     def __set__(self, obj, it):
>         print "in set method"
>         self.somedata = it
>         return None
> ##    def foo(self):
> ##        return 1
> 
> class AnotherObject(object):
>     def __init__(self):
>         self.prop = AnObject("snafu")
> 
> myClass = AnotherObject()
> print myClass.prop
> myClass.prop = "foobar"
> print myClass.prop
> 
> [end of script]
> 
> Then I execute it:
> 
> C:\Documents and Settings\John\My Documents\Projects\AstroPy4>b
> 
> C:\Documents and Settings\John\My Documents\Projects\AstroPy4>python b.py
> <__main__.AnObject object at 0x0086D248>
> foobar
> 
> It doesn't look like the descriptor protocol is getting
> invoked at all.
> 
> What's happening here?

The descriptor protocol works on the class, not the instance, so 

class AnotherObject(object):
    prop = AnObject("snafu")

or something similar should work. This means in particular that you have to
store the property's state in the AnotherObject rather than the AnObject
instance.

Peter