Python Cookbook

Implementing the Null Object Design Pattern

Credit: Dinu C. Gherman

Problem

You want to reduce the need for conditional statements in your code, particularly the need to keep checking for special cases.

Solution

The usual marker for “there’s nothing here” is None, but we may be able to do better than that:

class Null:
    """ Null objects always and reliably "do nothing." """

    def _ _init_ _(self, *args, **kwargs): pass
    def _ _call_ _(self, *args, **kwargs): return self
    def _ _repr_ _(self): return "Null(  )"
    def _ _nonzero_ _(self): return 0

    def _ _getattr_ _(self, name): return self
    def _ _setattr_ _(self, name, value): return self
    def _ _delattr_ _(self, name): return self

Discussion

An instance of the Null class can replace the primitive value None. Using this class, you can avoid many conditional statements in your code and can often express algorithms with little or no checking for special values. This recipe is a sample implementation of the Null Object design pattern (see “The Null Object Pattern”, B. Woolf, Pattern Languages of Programming, PLoP 96, September 1996).

This recipe’s Null class ignores all parameters passed when constructing or calling instances and any attempt to set or delete attributes. Any call or attempt to access an attribute (or a method, since Python does not distinguish between the two and calls _ _getattr_ _ either way) returns the same Null instance (i.e., self, since there’s no reason to create a new one). For example, if you have a computation such as: