Add expand_dims

[Illviljan]

This allows adding a dimension of an array. Which is relevant when for example using keepdims in NamedArray.reduce.

Normally you can do this with None but that's not really allowed in the getitem documentation currently https://data-apis.org/array-api/2022.12/API_specification/generated/array_api.array.__getitem__.html

Should follow the standard but with an additional dim argument, dim should be preferred over the axis argument.
https://data-apis.org/array-api/draft/API_specification/generated/array_api.expand_dims.html

xref #8344, #8406

[dcherian]

Should follow the standard but with an additional dim argument, dim should be preferred over the axis argument.

Thanks @Illviljan we've been leaning toward not providing Array API specifically because there's quite some potential for confusing mismatches like this one.

[Illviljan]

What mismatch? This is only expanding on the standard which in my view should be fine.

How do you plan on handling expanding dims (arrayapi[None]) without this function?

We have to handle axis in all our methods, I don't see why we can't use those helper functions in the array_api namespace as well.
I don't mind our methods (NamedArray.mean()) being more restrictive though.

The current _array_api is not public yet either, at this point it's just solving issues that we aren't yet testing for since we barely test any arrayapi compliant arrays at the moment.

[andersy005]

ks @Illviljan we've been leaning toward not providing Array API specifically because there's quite some potential for confusing mismatches like this one.

after reading data-apis/array-api#698 (comment), i support the idea of implementing the same (or expanded) syntax and semantics as the array API standard for a subset of the array API spec when possible.

it appears that as long as we don't add a __array_namespace__ method to xarray.NamedArray to declare array API compatibility, trying to align with the array API would be worth of our time and effort.

to proceed in the right direction, i'm curious to hear what your take is, @dcherian / @Illviljan

[andersy005]

@Illviljan, since we do not plan on declaring xarray.NamedArray as an array API compliant/compatible object, would it be possible to directly implement these methods on NamedArray instead of going through the private module?

[Illviljan]

Considering how much xarray has pushed upstream array-libraries to follow various standards, ufunc/array_function/array_api, I think it's bad form to not make an effort to respect the standards ourselves. Whether we start supporting it officially in the future or not.

I wouldn't mind adding NamedArray.expand_dims that just calls xp.expand_dims afterwards though, that would just be an extension for array api users rather than a weird quirk of NamedArray.

[andersy005]

thank you, @Illviljan