Python Pocket Reference, Second Edition

Object-Oriented Programming

Classes are Python’s main OOP tool. They support multiple instances, attribute inheritance, and operator overloading.

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 arguments 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.^[5]

Pseudo-Private Attributes

By default, all attribute names in modules and classes are visible everywhere. Special conventions allow some limited data hiding, but are mostly designed ...