On Sat, 2 Mar 2019 at 19:15, Raymond Hettinger <raymond.hettinger@gmail.com> wrote:

> On Mar 1, 2019, at 11:31 AM, Guido van Rossum <guido@python.org> wrote:
> There's a compromise solution for this possible. We already do this for Sequence and MutableSequence: Sequence does *not* define __add__, but MutableSequence *does* define __iadd__, and the default implementation just calls self.update(other). I propose the same for Mapping (do nothing) and MutableMapping: make the default __iadd__ implementation call self.update(other).

Usually, it's easy to add methods to classes without creating disruption, but ABCs are more problematic.  If MutableMapping grows an __iadd__() method, what would that mean for existing classes that register as MutableMapping but don't already implement __iadd__?  When "isinstance(m, MutableMapping)" returns True, is it a promise that the API is fully implemented? Is this something that mypy could would or should complain about?

Just to clarify the situation, currently Mapping and MutableMapping are not protocols from both runtime and mypy points of view. I.e. they don't have the structural __subclasshook__() (as e.g. Iterable), and are not declared as Protocol in typeshed. So to implement these (and be considered a subtype by mypy) one needs to explicitly subclass them (register() isn't supported by mypy). This means that adding a new method will not cause any problems here, since the new method will be non-abstract with a default implementation that calls update() (the same way as for MutableSequence).

The only potential for confusion I see is if there is a class that de-facto implements current MutableMapping API and made a subclass (at runtime) of MutableMapping using register(). Then after we add __iadd__, users of that class might expect that __iadd__ is implemented, while it might be not. This is however OK I think, since register() is already non type safe. Also there is a simple way to find if there are any subclassses of MutableMapping in typeshed that don't have __iadd__: one can *try* declaring MutableMapping.__iadd__ as abstract, and mypy will error on all such subclasses.