Learning Python, 6th Edition

Chapter 30. Operator Overloading

This chapter continues our in-depth survey of class mechanics by focusing on operator overloading. We looked briefly at operator overloading in prior chapters. Here, we’ll fill in more details and explore a handful of commonly used overloading methods, most of which we haven’t yet encountered. Although we don’t have space to demonstrate each of the many operator-overloading methods available, those we will code here are a representative sample large enough to uncover the possibilities of this Python class feature.

The Basics

Really “operator overloading” simply means intercepting built-in operations in a class’s methods—Python automatically invokes your methods when instances of the class appear in built-in operations, and your method’s return value becomes the result of the corresponding operation. Here’s a review of the key ideas behind overloading:

• Operator overloading lets classes intercept normal Python operations.

• Classes can overload all Python built-in expression operators.

• Classes can also overload other built-in operations such as printing, function calls, and attribute access.

• Overloading is implemented by providing specially named methods in a class.

• Python predefines the special method names that correspond to built-in operations.

In other words, when methods of predefined special names are provided in a class, Python automatically calls them when instances of the class appear in their associated built-in operations or expressions. ...