issue_comments: 1063859715

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https://github.com/pydata/xarray/issues/6329#issuecomment-1063859715 https://api.github.com/repos/pydata/xarray/issues/6329 1063859715 IC_kwDOAMm_X84_aTYD 6574622 2022-03-10T09:44:59Z 2022-03-10T09:44:59Z CONTRIBUTOR

Sure, no problem. I believe, this page has a good summary:

mode ({"w", "w-", "a", "r+", None}, optional) – Persistence mode: “w” means create (overwrite if exists); “w-” means create (fail if exists); “a” means override existing variables (create if does not exist); “r+” means modify existing array values only (raise an error if any metadata or shapes would change). The default mode is “a” if append_dim is set. Otherwise, it is “r+” if region is set and w- otherwise.

So the difference between "a" and "r+" roughly codifies the intended behaviour for sequential access (it's ok to modify everything) and parallel access to independent chunks (where modifying metadata would be bad).

So probably that message was suggesting that you have to use "a" if you want to modify metadata (e.g. by expanding the shape), which is true. But to me, it's unclear how one would do that safely with (potentially) parallel region writes, so it's kind of reasonable that region writes don't like to modify metadata.

 
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