In Python the function can have 4 kinds of parameters:

    def f(a, b=1, *, c, d=2):

1. a is a required positional-or-keyword parameter.
2. b is an optional positional-or-keyword parameter.
3. c is a required keyword-only parameter.
4. d is an optional keyword-only parameter.

But PyArg_ParseTupleAndKeywords() supports only 3 of them. Keyword-only parameters can only be optional.

    PyArg_ParseTupleAndKeywords(args, kwds, "O|O$O", kwlist, &a, &b, &d)

This makes impossible using PyArg_ParseTupleAndKeywords() for haslib.scrypt(). Currently it parses all keyword-only arguments as optional, and checks that they are specified specially, but this does not allow to use the full power of the PyArg_Parse* API.