Object-Oriented Programming

Classes are Python’s main object-oriented programming (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 are inherited object state and behavior.

  • Class methods are nested defs, with special first arguments to receive the implied subject 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, and uses the first version of an attribute name found. Superclasses are searched depth-first and then left-to-right (but new-style classes search across before proceeding up in diamond pattern trees).

Pseudoprivate Attributes ...

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