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Class definitions

A class definition defines a class object.

Syntax:

class name (object):
    suite

or

class name ( inheritance expression list ):
    suite

or

class name :
    suite

A class definition is an executable statement. It first evaluates the inheritance list, if present. Each item in the inheritance list (an ordinary expression list) should evaluate to a class object or class type which allows subclassing.

The class definition then sets up a new execution frame (see naming-and-binding), using a newly created local namespace and the original global namespace, and uses that to execute the statements in the suite. (Usually, the suite contains only function definitions.)

When the suite finishes execution, the 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. Finally, 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.