We're back after a server migration that caused effbot.org to fall over a bit harder than expected. Expect some glitches.

Descriptors

A descriptor is a special object that controls access to attributes of another object. Both objects must be new-style classes, and the descriptor must be attached to the owner’s class object, rather than to an instance.

In the following example, the my_descriptor instance will control access to the attribute member of my_class instances:

class my_descriptor(object):
    ...

class my_class(object):
    attribute = my_descriptor()

c = my_class()
v = c.attribute # calls descriptor's __get__ method
c.attribute = v # calls descriptor's __set__ method

The descriptor interface contains the following methods:

These 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. Descriptors can only be implemented as new-style classes themselves.

For details, see invoking-descriptors.