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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
RRP £12.95 ($19.95)

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7.7 Class definitions

A class definition defines a class object (see section 3.3):

classdef "class" classname [inheritance] ":"
inheritance "(" [ expression_list ] ")"
classname identifier

A class definition is an executable statement. It first evaluates the inheritance list, if present. Each item in the inheritance list should evaluate to a class object or class type which allows subclassing. The suite of the class is then executed in a new execution frame (see section 4.1), using a newly created local namespace and the original global namespace. (Usually, the suite contains only function definitions.) When the suite finishes execution, its execution frame is discarded but its local namespace is saved. A class object is then created using the inheritance list for the base classes and the saved local namespace for the attribute dictionary. The class name is bound to this class object in the original local namespace.

Programmer's note: Variables defined in the class definition are class variables; they are shared by all instances. To define instance variables, they must be given a value in the __init__() method or in another method. Both class and instance variables are accessible through the notation "self.name", and an instance variable hides a class variable with the same name when accessed in this way. Class variables with immutable values can be used as defaults for instance variables. For new-style classes, descriptors can be used to create instance variables with different implementation details.

ISBN 0954161785Python Language Reference ManualSee the print edition