Why is `collections.ChainMap.__missing__` defined in the stub?

[AlexWaygood]

collections.ChainMap.__missing__ is defined in typeshed here:

typeshed/stdlib/collections/__init__.pyi

Line 329 in 63489c3

def __missing__(self, __key: _KT) -> _VT: ...

But, at runtime, it always raises KeyError (and always has done), so there's no situation in which it would be safe to call this method (it's really just an implementation detail to give ChainMap a nicer error message for when the key isn't present in any of the underlying mappings, as far as I can tell).

The method was added to typeshed in a standalone PR (#4784)... Am I __missing__ something? Is there a good reason for it to be there?

[hauntsaninja]

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 :-)

[natemcmaster]

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

[srittau]

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

[AlexWaygood]

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

Thanks! Makes sense :)

[natemcmaster]

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