[Python-Dev] RFC: readproperty

Wed Sep 28 19:28:45 CEST 2005

Michael Hudson <mwh <at> python.net> writes:
>> "Phillip J. Eby" <pje <at> telecommunity.com> writes:
>> > Unfortunately, finding out a descriptor's name is non-trivial; it'd be nice 
> > if there were a descriptor hook __bind__(cls,name) that was called by 
> > classes during cls.__new__ or assignment to a class attribute, and which 
> > you could define to return a replacement descriptor. It seems like one of 
> > the first metaclasses I end up writing in any new project is something to 
> > do this, and I believe Ian Bicking has encountered the same thing in e.g. 
> > SQLObject.
>> I've done this many times too, but I've never really felt the need to
> propose a change to Python for it. I guess one could modify the
> descriptor protocol slightly, but is it worth it? Hmm, dunno.
>> Cheers,
> mwh
>
You can use something like this to find a descriptor's name:
class Private(property):
 def __init__(self, permission, fget=None, fset=None, fdel=None, doc=None):
 fget = fget or 'r' in permission and self.default_fget or None
 fset = fset or 'w' in permission and self.default_fset or None
 fdel = fdel or 'd' in permission and self.default_fget or None
 super(private, self).__init__(fget, fset, fdel, doc)
 def get_attribute_name(self, instance):
 my_name = None
 for cls in instance.__class__.mro():
 for attribute_name, attribute_object in cls.__dict__.iteritems():
 if attribute_object is self:
 my_name = attribute_name
 break
 if my_name is not None:
 class_name = cls.__name__
 break
 attribute_name = '_%s__%s' % (class_name, my_name)
 self.attribute_name = attribute_name
 return attribute_name
 def default_fget(self, instance):
 try:
 attribute_name = self.attribute_name
 except AttributeError:
 attribute_name = self.get_attribute_name(instance)
 return getattr(instance, attribute_name)
 def default_fset(self, instance, value):
 try:
 attribute_name = self.attribute_name
 except AttributeError:
 attribute_name = self.get_attribute_name(instance)
 setattr(instance, attribute_name, value)
 def default_fdel(self, instance):
 try:
 attribute_name = self.attribute_name
 except AttributeError:
 attribute_name = self.get_attribute_name(instance)
 delattr(instance, attribute_name)