I have an implementation proposal that I believe is distinct from any of the ones mentioned in the PEP currently.
Pass keyword arguments as ordinary keyword arguments [which any particular __getitem__ implementation is free to handle as **kwargs, specific keywords, or simple named arguments]. When a single positional argument is passed, it's used directly; when zero or two or more are passed, they are bundled into a tuple and passed as a single positional argument. Having zero arguments result in an empty tuple allows for easy conceptual compatibility with numpy.
d : d.__getitem__(0)
d[0,1] : d.__getitem__((0, 1))
d[x=0]: d.__getitem__((), x=0)
d[0, y=1]: d.__getitem__(0, y=1)
d[0, 1, z=2]: d.__getitem__((0, 1), z=2)
That may not be in the PEP, but apart from the edge cases for d and d[x=0] it’s exactly what I and Steven have been proposing for quite a while.