[Numpy-discussion] __numpy_ufunc__

[Stephan Hoyer]

On Wed, Feb 22, 2017 at 6:31 AM, Marten van Kerkwijk <
m.h.vankerkwijk at gmail.com> wrote:

> It seems to me entirely logical (but then it would, I suggested it
> before...) that we allow opting out by setting `__array_ufunc__` to
> None; in that case, binops return NotImplemented and ufuncs raise
> errors. (In addtion, or alternatively, one could allow setting
> `__array__` to None, which would generally disable something to be
> turned into an array object).
>

This is indeed appealing, but I recall this was still a point of contention
because it leaves intact two different ways to override arithmetic
involving numpy arrays. Mimicking all this logic on classes designed to
wrap well-behaved array-like classes (e.g., xarray, which can wrap NumPy or
Dask arrays) could be painful -- it's easier to just call np.add and let it
handle all the dispatching rather than also worrying about NotImplemented.
That said, I think the opt-out is probably OK, as long we make it clear
that defining __array_ufunc__ to do arithmetic overriding is the preferred
solution (and provide appropriate Mixin classes to make this easier).

Just to be clear: if __array__ = None but __array_ufunc__ is defined, this
would be a class that defines array-like operations but can't be directly
converted into a NumPy array? For example, a scipy.sparse matrix?
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