Detect target name in descriptor __set__ method

[Gabriel Genellina]

I have a class attribute 'foo' which is a data descriptor. I create an  
instance of such class. When I say instance.foo = value, the descriptor  
__set__ method is called. Is there any way to obtain the name being  
assigned to? ('foo' in this example). That is, I want to know the target  
name for the assignment that triggered the __set__ method call.

class descriptor(object):
   def __get__(self, instance, owner):
     print 'descriptor.__get__ self=%r instance=%r owner=%r' % (
       self, instance, owner)
     return self

   def __set__(self, instance, value):
     # I want to know the *name* this value is being assigned to
     print 'descriptor.__set__ self=%r instance=%r value=%r' % (
       self, instance, value)

   def __delete__(self, instance):
     print 'descriptor.__delete__ self=%r instance=%r' % (
       self, instance)

class X(object):
   foo = descriptor()

x = X()
x.foo = "value"


I can obtain the name descriptor() was assigned to at the time the X class  
was defined, using a custom metaclass:


class Meta(type):
   def __new__(meta, name, bases, dict):
     for key,value in dict.iteritems():
       if isinstance(value, descriptor):
         value.name = key
     return type.__new__(meta, name, bases, dict)

class Y(object):
   __metaclass__ = Meta
   foo = descriptor()

y = Y()
print y.foo.name # prints 'foo'


This is good enough for me (assuming no one modifies the class after  
defining it, no two names refer to the same descriptor, no two classes  
share the same descriptor instance...). But I would like to use a more  
direct/robust approach, if available.

Any ideas?

-- 
Gabriel Genellina