list vs UserList and dict vs UserDict

Question

Coding this day, which of the above is preferred and recommended (both in Python 2 and 3) for subclassing?

I read that UserList and UserDict

Answer 1

Depending on your usecase, these days you'd either subclass list and dict directly, or you can subclass collections.MutableSequence and collections. MutableMapping; these options are there in addition to using the User* objects.

The User* objects have been moved to the collections module in Python 3; but any code that used those in the Python 2 stdlib has been replaced with the collections.abc abstract base classes. Even in Python 2, UserList and UserDict are augmented collections.* implementations, adding methods list and dict provide beyond the basic interface.

The collections classes make it clearer what must be implemented for your subclass to be a complete implementation, and also let you implement smaller subsets (such as collections.Mapping, implementing a read-only mapping, or collections.Sequence for a tuple-like object).

The User* implementations should be used when you need to implement everything beyond the basic interface too; e.g. if you need to support addition, sorting, reversing and counting just like list does.

For anything else you are almost always better off using the collections abstract base classes as a basis; the built-in types are optimised for speed and are not that subclass-friendly. For example, you'll need to override just about every method on list where normally a new list is returned, to ensure your subclass is returned instead.

Only if you need to build code that insists on using a list or dict object (tested by using isinstance() is subclassing the types an option to consider. This is why collections.OrderedDict is a subclass of dict, for example.