Learning Python, 5th 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 look at a handful of commonly used overloading methods. Although we won’t 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 expression operators.

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

• Overloading makes class instances act more like built-in types.

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

In other words, when certain specially named methods are provided in a class, Python automatically calls them when instances of the class appear in their associated expressions. Your class provides the behavior of the corresponding operation for instance objects created from it.

As we’ve learned, operator ...