On Thu, Apr 25, 2019 at 12:46 PM Marten van Kerkwijk <m.h.vankerkwijk@gmail.com> wrote:
It seems we are adding to the wishlist!  I see four so far:
1. Exposed in API, can be overridden with __array_ufunc__
2. One that converts everything to ndarray (or subclass); essentially the current implementation;
3. One that does asduckarray
4. One that assumes all arguments are arrays.

Maybe handiest would be if there is a method to coerce all relevant arguments with a function of one's choice? I.e., in the example of Stephan, one would have
if function in JUST_COERCE:
    coerced_args, coerced_kwargs = function.__coerce__(np.asanyarray, *args, **kwargs)
    return function.__implementation__(*coerced_args, **coerced_kwargs)
Actually, this might in fact work with the plan proposed here, if we allow for an extra, optional kwarg that contains the coercion function, that is
    return function.__implementation__(*args, coercion_function=np.asanyarray, **kwargs)

The possible advantage of this over yet more dunder methods is that one can fine-tune the extent to which something has to mimic an array properly (e.g., run `asanyarray` only if `shape` is not present).

I do like the look of this, but keep in mind that there is a downside to exposing the implementation of NumPy functions -- now the implementation details become part of NumPy's API. I suspect we do not want to commit ourselves to never changing the implementation of NumPy functions, so at the least this will need careful disclaimers about non-guarantees of backwards compatibility.

But for now, I would love to pick a name for "essentially the current implementation", which is something that would make a big difference for near-term NEP-18 use cases. Some options:

The last two are not quite right, since there is some legacy dispatching to methods. Maybe __skipping_array_function__ is the best?

Whatever we pick, we can always make it an alias later, e.g., for func.__implementation__(*args, coercion_function=np.asanyarray, **kwargs).
It would be nice, though, if we could end up with also option 4 being available, if only because code that just can assume ndarray will be easiest to read.

This could perhaps just be coercion_function=None? Or maybe we want to keep around coercion_function=None for "do whatever ad-hoc coercion NumPy current does"?

All the best,

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