No chunk warning if empty

[jaicher]

• Closes Unnecessary warning when specifying chunks opening dataset with empty dimension #6401
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[max-sixty]

This looks very reasonable, @jaicher, thanks a lot.

I don't know this area well at all, so I'll leave open so someone who knows it better can check, or for a couple of days. Hope that's OK.

[dcherian]

@stanwest Can you suggest how to fix this merge conflict please?

[stanwest]

@stanwest Can you suggest how to fix this merge conflict please?

Sure. I recommend the following before the return statement in xarray.core.dataset._get_chunk:

# Warn where requested chunks break preferred chunks, provided that the variable
# contains data.
if var.size:
    for dim, size, chunk_sizes in zip(dims, shape, chunk_shape):
        try:
            preferred_chunk_sizes = preferred_chunks[dim]
        except KeyError:
            continue
        # Determine the stop indices of the preferred chunks, but omit the last stop
        # (equal to the dim size).  In particular, assume that when a sequence
        # expresses the preferred chunks, the sequence sums to the size.
        preferred_stops = (
            range(preferred_chunk_sizes, size, preferred_chunk_sizes)
            if isinstance(preferred_chunk_sizes, Number)
            else itertools.accumulate(preferred_chunk_sizes[:-1])
        )
        # Gather any stop indices of the specified chunks that are not a stop index
        # of a preferred chunk.  Again, omit the last stop, assuming that it equals
        # the dim size.
        breaks = set(itertools.accumulate(chunk_sizes[:-1])).difference(
            preferred_stops
        )
        if breaks:
            warnings.warn(
                "The specified Dask chunks separate the stored chunks along "
                f'dimension "{dim}" starting at index {min(breaks)}. This could '
                "degrade performance. Instead, consider rechunking after loading."
            )

[max-sixty]

@stanwest how's that? (feel free to correct if I missed something, forgive the delay on merging)

Thanks!

[stanwest]

@stanwest how's that?

That looks great to me.

[dcherian]

Thanks all!