Special Methods and Operator Overloading

From Class Definition to New Type

A class in Python is more than a container for related code. The class keyword creates a new type: a new category of object that Python recognizes. When you create an instance from that class, the instance belongs to the new type in the same broad sense that 5 belongs to the built-in type int and a text value belongs to the built-in type str.

Operator Expressions and Special Methods

Special methods are among the methods that Python makes available on objects. They are listed by dir() along with user-defined attributes and methods. These methods are connected with the behavior Python can provide for objects. In operator overloading, an expression such as obj + other is associated with a special method such as __add__. The important inspection skill is to recognize that this special method is part of the object's available behavior, even though its name uses Python's double-underscore convention.

Inspecting the Instance Type

The same inspection rule applies to built-in and custom objects. type(5) identifies int, type('Alice') identifies str, and type(an) identifies PartyAnimal. The result shows the type of the object itself, not the type of every value stored inside it.

Reading dir() Results

The dir() function lists the attributes and methods available on an object. For a PartyAnimal instance named an, the result includes the user-defined attribute x and the user-defined method party. It also includes many special methods that Python automatically adds to every class. Therefore, dir(an) gives a broader view than a list of only the names you wrote yourself.

Separate Instances, Separate Data

A class can produce more than one instance. If an2 is created from PartyAnimal, both an and an2 have type PartyAnimal. They are nevertheless separate objects with separate data. Each instance has its own x attribute and its own party method available through the instance.

Mistakes in Object Inspection

• Thinking a class only organizes code

  The class keyword creates a new type, and instances created from it are objects of that type.

  Fix: Describe PartyAnimal as a custom type and an instance created from it as an object of type PartyAnimal.

• Expecting type(an) to report the type of an attribute

  type() reports the type of the object passed to it. The instance is PartyAnimal; the separate expression an.x has type int.

  Fix: Inspect type(an) and type(an.x) as two different questions.

• Assuming dir() lists only names written by the programmer

  dir() lists user-defined attributes and methods as well as special methods automatically available on the class.

  Fix: Expect dir(instance) to contain both your own members and special methods.

• Assuming two instances are the same object because their types match

  Instances can have the same type while remaining separate objects with separate data.

  Fix: Distinguish the shared class type from each instance's separate data.

Practice the Inspection Habit

1. A class definition introduces a new type. Instances created from that class are objects of that type, just as built-in values are objects of types such as int and str. Use type() to identify the class of a particular object. Use dir() to inspect the object's available attributes and methods, including special methods. When you see a special method such as __add__ in that inspection, connect it with the behavior associated with an operator expression such as obj + other.