Python Cookbook, 3rd Edition

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

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. You ...