The following methods only apply when an instance of the class
containing the method (a so-called descriptor class) appears in
the class dictionary of another new-style class, known as the
owner class. In the examples below, ``the attribute'' refers to
the attribute whose name is the key of the property in the owner
class' __dict__.  Descriptors can only be implemented as
new-style classes themselves.
| self, instance, owner) | 
None when
the attribute is accessed through the owner.  This method should
return the (computed) attribute value or raise an
AttributeError exception.
| self, instance, value) | 
| self, instance) | 
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