Stefan, that proposal definitely looks like it is worth writing up as a PEP to me. One thing that I particularly like about it is that it should be possible to pluck out the first element of the {} entries fairly easily in order to get the ordinary Python signature to feed to Cython or a "from string" constructor for signature objects.

As far as the "positional-only" parameters problem goes, at one point Guido was kicking around the idea of allowing "/" as a separator in Python function declarations to indicate positional only arguments. So the signature of a function that didn't accept keyword arguments at all would look like:

    def addch(x, y, ch, attr, /):
        ...

He eventually dropped it because positional only arguments (and the need to avoid colliding with arbitrary keyword arguments) are relatively rare in Python code, and using an inner function together with *args is a reasonable way to get decent error messages. However, as a way of concisely indicating positional-only arguments in the signature of *C* functions, the idea may be worth reconsidering.