Learning Python, 4th Edition

Chapter 29. 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 class 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 class methods.

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. As we’ve learned, operator overloading methods are never required and generally don’t have defaults; if you don’t code or inherit one, ...