setting up dynamic descriptors in Python

Gabriel Genellina gagsl-py2 at
Sat Sep 26 02:37:21 CEST 2009

En Thu, 24 Sep 2009 12:14:39 -0300, brian huggins <bghugg at>  

> I want to set up descriptors at runtine, but it isn't working the way
> i would expect.  Does anybody know if this is possible?
> class TestDesc (object):
>     x=Descriptor ("x")
>     def __init__ (self):
>         self.y=Descriptor("y")
>>>> test=TestDesc()
>>>> test.y
> <descriptor.Descriptor object at 0x00B3A5F0>
> When i access y, it appears to be a regular object and not a
> descriptor type object like x.  Is there a way to make y work the same
> as x but setting it up during object creation?

The descriptor "magic" only happens when the attribute is looked up on the  
class; when it's found in the instance itself, nothing special happens.
You cannot attach descriptors to a specific instance, but you can alter  
its class at runtime. Basically you have to create a new class (that  
inherits from the "real" class and adds the desired descriptor) and set it  
as the instance __class__ attribute.
(This was discussed some time ago, try searching the list archives. Uh, I  
found this:
but you should be able to find other threads)

Gabriel Genellina

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