Using standard library mixin classes

The standard library makes use of mixin class definitions. There are several modules that contain examples, including io, socketserver, urllib.request, contextlib, and collections.abc.

When we define our own collection based on the collections.abc abstract base classes, we're making use of mixins to assure that cross-cutting aspects of the containers are defined consistently. The top-level collections (Set, Sequence, and Mapping) are all built from multiple mixins. It's very important to look at section 8.4 of the Python Standard Library to see how the mixins contribute features, as the overall structure is built up from pieces.

Looking at just one line, the summary of Sequence, we see that it inherits from Sized, Iterable, and Container. These mixin classes lead to methods of __contains__(), __iter__(), __reversed__(), index(), and count().

Using the context manager mixin class

When we looked at context managers in Chapter 5, Using Callables and Contexts...