From: "Armin Ronacher" <armin.ronacher@active-4.com>
If the ABCs exists only for forward compatibility they will be mostly useless for every day usage.
The Mapping ABC is still useful as a mixin in code that will run on both 2.6 and 3.0. It is the easiest way to start coding with views.
Deque's do not support count(), insert() or __iadd__(). They should not be registered.
If it doesn't implement the MutableSequence protocol it still is a Sized container. However currently it's not registered as a container.
Seems useless to me. I don't think the intent of the ABC pep was to mandate that every class that defines __len__ must be registered as Sized.
Another issue is that builtin types don't accept ABCs currently. For example set() | SomeSet() gives a TypeError, SomeSet() | set() however works.
Pandora's Box -- sure you want to open it? Raymond