friendlier error messages for missing chunk managers

[keewis]

The current error message when trying to use a chunked-array related method without actually having a chunk manager available is:

unrecognized chunk manager dask - must be one of: []

That's pretty confusing, so this catches the case where no chunk manager is available and raises an error with guidance on how to fix that.

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[max-sixty]

Much better!

[mathause]

Thanks - I had a PR on this but don't mind closing mine in favor of this one. There is also #7963 which seems related.

• closes missing chunkmanager: update error message #8803

[keewis]

wow, I don't know how I missed two open PRs that aim to do something similar in different ways. Which one do we take?

If we merge this one your PR might still be valuable since it also changes the error message if there are chunk managers but not the one that was requested.

[dcherian]

Shall we merge?

[TomNicholas]

wow, I don't know how I missed two open PRs that aim to do something similar in different ways. Which one do we take?

Sorry for dropping the ball on reviewing / merging these guys 😞

Let's merge this one.

If we merge this one your PR might still be valuable since it also changes the error message if there are chunk managers but not the one that was requested.

This change would also be useful but is much less likely to come up.

[TomNicholas]

While this could be further improved in cases like cubed being installed but dask not (when there would not be 0 chunkmanagers installed), this PR on it's own addresses the confusing error that 99% of users are encountering so should be merged asap.

[TomNicholas]

If you change the other ValueError to ImportError on line 109 of parallelcompat.py the failing test should pass.

[dcherian]

I'm planning to release on Friday US time. Would be good to wrap this up

[TomNicholas]

Trying to summarize what we discussed in the meeting on Wednesday:

at least, without maintaining a list of known chunk managers and suggesting sufficiently "close" names

This would be okay, because right now there are only 3 chunkmanagers we know of, one which ships with xarray. Outside of these 3 a slightly less helpful error message is fine.

This allows our error messages to explicitly point to the correct package to install, e.g. dask/cubed-xarray/arkouda-xarray.

We questioned the wisdom of even creating the whole entrypoint system in the first place, but also said that removing it is a separable issue for later, and the priority should be to improve the error messages first.

I can't really remember what we decided about ValueError vs ImportError though...

[keewis]

Thanks for the summary.

I can't really remember what we decided about ValueError vs ImportError though...

I don't think we decided anything, but it would make sense to me to raise a ImportError for all known-but-missing chunk managers, and raise a ValueError for everything else where we don't really know whether that's because of a missing package/one that fails to import or a typo

[keewis]

we probably also want to prefer the more specific error over the generic n_chunkmanagers == 0 error (which would also allow passing a chunkmanager object if no chunk manager is available via the entrypoints)

[keewis]

@dcherian, @TomNicholas, this should be ready for a final review (and can hopefully still make it into the release looks like I was late by a couple minutes)

[dcherian]

we can always release more!

[keewis]

the failing zarr tests were because of the automatic chunking in open_zarr (chunks="auto")

[TomNicholas]

@keewis I made some very minor changes - feel free to merge this.

[keewis]

re-reading this, it looks like only this error message is only describing the issue, not offering advice. What do you think about extending it?

Suggested change

	f"unrecognized chunk manager {manager} - must be one of: {list(available_chunkmanagers)}"
	f"unrecognized chunk manager {manager} - must be one of: {list(available_chunkmanagers)}"
	" (you can extend this list by installing more packages that provide a chunk manager)."

[TomNicholas]

Sounds good. You could also change it to say "must be one of the installed chunkmanagers: {{list(available_chunkmanagers)}}" which would be a bit clearer about what the list represents.