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ENH: at: add __setitem__ fancy indexing fallback #395

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ENH: at: add __setitem__ fancy indexing fallback #395

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@amacati amacati commented Aug 21, 2025

Fancy indexing is currently not supported for __setitem__, which blocks some PRs in scipy (scipy/scipy#23425).

As discussed in data-apis/array-api#864 (comment), all frameworks of the array api already implement this feature, but not necessarily in a consistent manner for duplicate indices. It is currently not part of the standard. This PR adds a workaround for array api strict to allow fancy indexing in xpx.at(x, ...).

@amacati amacati changed the title Add __setitem__ fancy indexing support for array-api-strict ENH: Add __setitem__ fancy indexing support for array-api-strict Aug 21, 2025
@lucascolley lucascolley added the enhancement New feature or request label Aug 22, 2025
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lucascolley reviewed Aug 22, 2025
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thanks @amacati !

@lucascolley lucascolley changed the title ENH: Add __setitem__ fancy indexing support for array-api-strict ENH: Add __setitem__ fancy indexing fallback Aug 22, 2025
@lucascolley lucascolley changed the title ENH: Add __setitem__ fancy indexing fallback ENH: at: add __setitem__ fancy indexing fallback Aug 22, 2025
@lucascolley lucascolley mentioned this pull request Aug 23, 2025
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amacati commented Aug 23, 2025

The fallback is now implemented as error handling for __setitem__ and the behaviour for data races is documented. I also fixed the handling of negative indices and the assignment of scalar values.

One remaining concern is out-of-bounds indices. For anything negative or positive > len(x), the fallback currently does not throw an error. We would need to add value-based checks to catch that.

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For anything negative or positive > len(x), the fallback currently does not throw an error. We would need to add value-based checks to catch that.

Seems fine to leave that for now. I guess the pattern would be to hide things behind not is_lazy_array(x) like SciPy?

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lucascolley approved these changes Aug 23, 2025
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looks good to me, thanks @amacati !

Would you like to take a look @crusaderky ?

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the codecov failure is not a worry

@lucascolley lucascolley added this to the 0.9.0 milestone Aug 23, 2025
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lint failures are real but trivial

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I'll release 0.8.1 soon, and merge this for 0.9.0 by the end of the week if there is no further feedback

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