Here are two examples, which I hope help us understand our options.

    class A:

        __keyfn__ = None

        def __setitem__(self, val, x=0, y=0, z=0):

            print((val, x, y, z))

    >>> a = A()

    >>> a[1, z=2] = 'hello'

    ('hello', 1, 0, 2)

Here's my understanding of Steven's proposal. (Please correct me if I've got something wrong.)

    class B:

        def __setitem__(self, argv, val, *, x=0, y=0, z=0):

            print((val, argv, x, y, z))

    >>> b = B()

    >>> b[1, z=2] = 'hello'

    ('hello', 1, 0, 0, 2)

By the way, I've not tested this code.

-- 

Jonathan