We plan to introduce something similar to CPython's argument clinic eventually. I.e., you will be able to declare what is the C signature of your impl function and HPy will generate the argument parsing code automatically.

One nice effect of such an API is that under certain conditions, implementations will be allowed to bypass argument parsing entirely: e.g., if I have a function which takes two C longs, the PyPy JIT will be able to emit a call directly to it, without having to box/unbox the arguments just to do the call as it happens right now.

However, on the other hand we will need to provide an API similar to the existing PyArg_ParseTuple, because we want to make it easy to migrate existing extensions