proposal.rst

Proposal: how to properly subclass dictionaries

I'd like to write a short article describing experience adding dictionary features to a class.

Background

I needed an object like the regular dictionary, but I wanted to alter how __str__ worked, and I wanted to restrict allowed values for some keys, by altering how __setitem__ works.

This was messier than it should have been:

1. subclassing dict was no good because methods like update and setdefault wouldn't use my __setitem__ method.

2. The UserDict.UserDict class is not a new-style class, so any

   sub-subclass (grandchild class?) couldn't use super, so this was out.

3. The UserDict.DictMixin is OK until you use python 2.6.
4. As of python 2.6, the collections module has a MutableMapping class which is recommended instead of UserDict.DictMixin. The collections.MutableMapping does everything UserDict.DictMixin does AND it has the nice benefit of inheriting from the abstract base class Mapping.

Outline

1. In TDD fashion, write out some unittest.TestCase classes for all the features I want in my class.
2. Go through the different approaches I listed above until I get to collections.MutableMapping.
3. Spend a few sentences explaining what is so great about using abstract base classes vs. doing duck typing by calling methods and then catching exceptions.