Update signature of ConfigParser.get()

[elazarg]

Issue #492

This signature seems to break the protocol of MutableMappting[str, _section]

[gvanrossum]

Thanks for trying to tackle this one!

I've thought about this more, and I think the right thing to do (for now) is to add # type: ignore to the definition of get(). (The overload games won't work.) This will shut up mypy's complaint about get not being incompatible with MutableMapping (#492) but calls of it will still correctly be type-checked with the (str, str) -> str signature (IOW the # type: ignore does not make the definition of get equal to (*Any) -> Any).

The problem here is quite deep, you can read more about it at python/mypy#1237 and python/typing#241. But as a short-term solution, the # type: ignore should work just fine.

Of course if you pass a ConfigParser object to a function that requires a Mapping and that function happens to call get, it's a problem, and the type checker doesn't warn you. But no matter what solution we choose that will be a problem, unless we decide to not inherit from [Mutable]Mapping after all -- but that goes against the public documentation. (If we end up deciding that the Mapping inheritance is only "implementation inheritance" then the right thing to do in the stub would indeed be to not declare MutableMapping as a base class, but that might have other unpleasant consequences. Life's rarely fully consistent! :-)

I think there are other problems with configparser.pyi too (e.g. read() takes a string too, and RawConfigParser is not there at all) but those can wait until another PR.

[elazarg]

I can't find the documentation about type: ignore on function definition.

[gvanrossum]

It should look like this:

def get(blah blah) -> blah: # type: ignore
    ...

[gvanrossum]

You should still specify the correct signature (according to configparser docs, ignoring MutableMapping).

[gvanrossum]

Please don't start this comment with exactly # type: ignore since that may trigger other tools looking for # type: ignore on a new line. Just prefix with "Uses" or something.

[gvanrossum]

Thanks! Phew!