[Numpy-discussion] __numpy_ufunc__

[Marten van Kerkwijk]

HI Stephan,

Indeed, `__array_ufunc__` is None would be for classes that interact
with arrays only as if they were any other numeric type, and thus have
no use for ufuncs, but may need normal operations (astropy's `Unit`
class is a reasonably good example).

Your example also makes clear that, indeed, setting __array__ or
__array_ufunc__ to None implies different things, so concretely here
the proposal is that if `__array_ufunc__` is None, ndarray methods
will return `NotImplemented`.

As an aside, I think that if we do not have something like that, we'll
be stuck with supporting `__array_priority__`. (Which is OK by me too,
but it might as well be a conscious choice.)

All the best,

Marten