Python Pocket Reference, 3rd Edition

Object-Oriented Programming

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57

Classes and Instances

Class objects provide default behavior

• The class statement creates a class object and assigns it

to a name.

• Assignments inside

class

statements create class attributes,

which export object state and behavior.

• Class methods are nested defs, with special first argu-

ments to receive the instance.

Instance objects are generated from classes

• Calling a class object like a function makes a new

instance object.

• Each instance object inherits class attributes and gets its

own attribute namespace.

• Assignments to attributes of the first argument (e.g.,

self.X = V) in methods create per-instance attributes.

Inheritance rules

• Inheritance happens at attribute qualification time: on

object.attribute, if object is a class or instance.

• Classes inherit attributes from all classes listed in their

class statement header line (superclasses). Listing more

than one means multiple inheritance.

• Instances inherit attributes from the class from which

they are generated, plus all that class’s superclasses.

• Inheritance searches the instance, then its class, then all

accessible superclasses (depth-first, and left-to-right),

and uses the first version of an attribute name found.