Some additional abstractions

We'll look at some other interesting ABC classes that are less widely extended. It's not that these abstractions are less widely used. It's more that the concrete implementations rarely need extensions or revisions.

We'll look at the iterator, which is defined by collections.abc.Iterator. We'll also look at the unrelated idea of a context manager. This isn't defined with the same formality as other ABC classes. We'll look at this in detail in Chapter 5, Using Callables and Contexts.