Python Cookbook, 3rd Edition

Chapter 8. Classes and Objects

The primary focus of this chapter is to present recipes to common programming patterns related to class definitions. Topics include making objects support common Python features, usage of special methods, encapsulation techniques, inheritance, memory management, and useful design patterns.

8.1. Changing the String Representation of Instances

Problem

You want to change the output produced by printing or viewing instances to something more sensible.

Solution

To change the string representation of an instance, define the __str__() and __repr__() methods. For example:

class Pair:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __repr__(self):
        return 'Pair({0.x!r}, {0.y!r})'.format(self)
    def __str__(self):
        return '({0.x!s}, {0.y!s})'.format(self)

The __repr__() method returns the code representation of an instance, and is usually the text you would type to re-create the instance. The built-in repr() function returns this text, as does the interactive interpreter when inspecting values. The __str__() method converts the instance to a string, and is the output produced by the str() and print() functions. For example:

>>> p = Pair(3, 4)
>>> p
Pair(3, 4)         # __repr__() output
>>> print(p)
(3, 4)             # __str__() output
>>>

The implementation of this recipe also shows how different string representations may be used during formatting. Specifically, the special !r formatting code indicates that the output of __repr__() should be used instead of __str__(), the default. ...