[Numpy-discussion] Creating a subclass that never propagates

[Charles R Harris]

On Tue, Jul 16, 2019 at 3:44 AM Kevin Sheppard <kevin.k.sheppard at gmail.com>
wrote:

> I am trying to make a subclass that never propagates so that when
> interacted with another ndarray, or even itself so that the return type is
> always ndarray.  Is this possible?
>
> I got pretty far with
>
> def __array_wrap__(self, out_arr, context=None):
>     if out_arr.shape == ():
>         return out_arr.item()  # if ufunc output is scalar, return it
>     else:
>         out = super(ArrayLike, self).__array_wrap__(out_arr, context)
>         # Never return ArrayLike
>         if isinstance(out, ArrayLike):
>             out = out.view(np.ndarray)
>         return out
>
> Which works well for ufuncs.  However, when I try other functions like
> `dot` I get my subclass type returned.
>
> If there a reasonable way to ensure that my subclass doesn't propagate? I
> think I would need some way to override the behavior when .view(MySubClass)
> is called.
>
>
I think you will be able to do that with `__array_function__` in the
upcoming 1.17 release. It is also in 1.16, but you need an environmental
variable to activate it. Some documentation can be found at
https://www.numpy.org/devdocs/reference/arrays.classes.html#special-attributes-and-methods
.

Chuck
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