Python Pocket Reference

Object-Oriented Programming

Classes are Python’s main OOP tool. They support multiple copies, 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 the first argument (“self”) 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 they are generated from, plus all its superclasses.

• Inheritance searches the instance, then its class, then all accessible superclasses (depth-first, left-to-right) and uses the first version of an attribute name found.

Special Overloading Methods

Classes intercept and implement built-in operations by providing specially-named method functions, which all start and end with two underscores. Python calls a class’s overloading methods automatically when ...