`array_api` usage for `Buffer`

[ilan-gold]

Discussed in https://github.com/zarr-developers/zarr-python/discussions/2197

^{Originally posted by **ilan-gold** September 17, 2024} Hello all, I have been looking into the core code here (teehee) and I noticed the recent #1967. It made me wonder why `array_api` + dlpack is not used **for the `Buffer` class** (in general, and also specifically in that PR). I am guessing "no one has done it yet" is the answer, which is fair considering we all have other things going on, but then I am curious what #1967 gives that `array_api` + dlpack wouldn't. I think `cupy`, `jax` (mentioned in the PR), and `numpy` all support `array_api` if memory serves correctly. Sorry for the short question, with probably a long answer, but don't want to presume too much. I also did a quick search of the issues and found nothing relevant to the array api, but might have missed something: https://data-apis.org/array-api/latest/index.html

cc: @akshaysubr @madsbk @d-v-b @dcherian

[akshaysubr]

@ilan-gold This is a good discussion worth having. #1967 was meant to be a first pass at adding device support and it exposed many areas of the code that were relying implicitly on numpy and CPU backed arrays in general. In https://github.com/zarr-developers/zarr-python/issues/1751, I proposed supporting both the __cuda_array_interface__ as well as dlpack but left out the dlpack support in #1967 mainly for simplicity. It would certainly be useful to add a dlpack based Buffer implementation.

I was not aware of array_api and it does seem like a really useful standard that we can use for a generic Buffer implementation. The entrypoints based autodiscovery is a useful feature as well. From a quick search, here are the libraries that currently do/do not support the array_api specification:

1. Numpy: supported
2. CuPy: experimental
3. PyTorch: not supported, but work in progress
4. JAX: supported

Given the current state of array library compliance, might it make sense to add an array_api based Buffer implementation but keep the existing explicit NumPy and CuPy based implementations with a view to move to the array_api based implementation down the line?

[ilan-gold]

@akshaysubr I would say that sounds fine, but perhaps not keep the explicit numpy one since it is officially supported as array_api. This would just leave cupy for which there is this: https://github.com/data-apis/array-api-compat if I am not mistaken. I think the goal should really just be one implementation. I imagine it's doable modulo one or two extra if-then checks for things that might be broken. In fact, it seems like we won't have to wait too long if we don't want the compat layer: https://github.com/cupy/cupy/issues/8470

I really think we should strive for one implementation - simpler, and if there are bugs, maybe we can upstream the fixes since array api should be a one-stop-shop!

[ilan-gold]

Also re: pytorch, I think it's ok if zarr doesn't support it at first. This could be a good impetus to get its developers back on board re: array_api. Also the data interchange with cupy is quite good and anyone using zarr with torch for deep learning would probably have to write a data loader of some sort, no? So custom code already there as a layer around every read. Just some quick thoughts on that, but not as familiar.

Alternatively we could make the buffer class customizable to the point where you have an array-api compliant array as the "base" buffer, and then we decorate all functions that return arrays to return a dlpack representation in a different, less-compliant library i.e., cupy as base and then torch on top. Same thing would probably enable tensorflow (experimentally, maybe, just from a quick google): https://www.tensorflow.org/api_docs/python/tf/experimental/dlpack/from_dlpack + https://docs.cupy.dev/en/stable/user_guide/interoperability.html#dlpack