Add array storage helpers

[d-v-b]

This PR adds nchunks, nbytes, and nchunks_initialized functionality from 2.x.

closes #2027 depends on #2064

details

Adds the following to array.py:

• (AsyncArray / Array).nchunks : deprecated, the total number of chunks in the array. exists for 2.xx compatibility.
• (AsyncArray / Array).cdata_shape : deprecated, the shape of the chunk grid. exists for 2.xx compatibility.
• (AsyncArray / Array).nbytes : the total number of bytes that the array can store
• (AsyncArray / Array)._iter_chunk_coords : an iterator over tuples of ints which represent positions in the chunk grid
• (AsyncArray / Array)._iter_chunk_regions : an iterator over slices which represent the contiguous array region spanned by each chunk
• (AsyncArray / Array)._iter_chunk_keys : an iterator over strings which represent the paths in storage for all the chunks
• chunks_initialized(array): a function that takes an array and returns a tuple of the chunk keys for that array that exist in storage. this also has tests.
• nchunks_initialized(array): deprecated, a function that calls len(chunks_initialized(array)). this exists for 2.xx compatibility.

All of the above _iter_chunk_* methods should be considered private and provisional. I added them because their functionality is valuable, but eventually I think we will have a better array API that renders these methods obsolete. If we think these are cluttering the array API, I'd be happy splitting them off into stand-alone functions.

• adds a function iter_grid to indexing.py, this just provides lexicographic iteration over the elements of a bounded N-dimensional, positive grid (e.g., a grid of chunks).

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)

[d-v-b]

@tomwhite let me know if this looks workable for you

[tomwhite]

Thanks @d-v-b this looks great!

I wondered why you deprecated nchunks (and nchunks_initialized) though? The number of chunks in an array is something that should always be well-defined. Also, deprecating something usually means there's a better alternative, but I don't see one here.

[d-v-b]

I wondered why you deprecated nchunks (and nchunks_initialized) though? The number of chunks in an array is something that should always be well-defined. Also, deprecating something usually means there's a better alternative, but I don't see one here.

my thinking for this is twofold:

• with the new chunks_initialized function that gives the names of the initialized chunks, one can easily do len(chunks_initialized(...)), i.e. we don't need a separate function to express the composition of chunks_initialized and len. similarly, nchunks is merely len(array._iter_chunk_keys). If this logic is unsound, or these deprecation warnings are a problem, then we can remove them, but see the second point:
• we haven't yet figured out how we are going to express sharded arrays in the top-level array API, and I think those decisions might require rethinking how we express chunking more broadly. see conversations happening in this discussion. Until we solve that problem, I don't feel comfortable committing to any "here's what your chunks are like" APIs, especially if they are APIs that developed pre-sharding. hence adding private methods in this PR, and the deprecation warnings.

does this check out? I'm sorry if the warnings are inconvenient, but I really would like to find a proper expression of v3 semantics on the Array class and I worry that a blanket policy of forward-propagating v2-isms could be a hindrance to that effort.

[d-v-b]

The number of chunks in an array is something that should always be well-defined.

to expand on this: v3 introduces two kinds of chunks, read-chunks and write chunks. the number of read chunks may not equal the number of write chunks. so where we had 1 nchunks quantity in v2, v3 has two possible answers to nchunks. that's why it is not straightforward to commit to this aspect of the array API.