Possibly useful for subclasses? Clearly not the most important stub, but the default good reason of "it exists at runtime" applies. Maybe @natemcmaster remembers if they had a specific use case in mind when they contributed #4784 :-)

Yes, I added it for subclassing. I have a type in my project that overrides ChainMap.__getitem__ and calls __missing__

It seems fine to have this method, although we could consider changing the return type to NoReturn.

Yes, I added it for subclassing. I have a type in my project that overrides ChainMap.__getitem__ and calls __missing__

Thanks! Makes sense :)

Changing to NoReturn would break my usecase since my subclass returns a default value when __missing__ is called, similar to defaultdict. https://docs.python.org/3/library/collections.html#collections.defaultdict.missing