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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
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3.4.2.2 Implementing Descriptors

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 __dict__ of the owner class. Descriptors can only be implemented as new-style classes themselves.

__get__(self, instance, owner)
Called to get the attribute of the owner class (class attribute access) or of an instance of that class (instance attribute access). owner is always the owner class, while instance is the instance that the attribute was accessed through, or None when the attribute is accessed through the owner. This method should return the (computed) attribute value or raise an AttributeError exception.
__set__(self, instance, value)
Called to set the attribute on an instance instance of the owner class to a new value, value.
__delete__(self, instance)
Called to delete the attribute on an instance instance of the owner class.
ISBN 0954161785Python Language Reference ManualSee the print edition