issues: 870266283

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
870266283 MDExOlB1bGxSZXF1ZXN0NjI1NDg5NTQx 5231 open_mfdataset: Raise if combine='by_coords' and concat_dim=None 35968931 closed 0     1 2021-04-28T19:16:19Z 2021-04-30T12:41:17Z 2021-04-30T12:41:17Z MEMBER   0 pydata/xarray/pulls/5231

Fixes bug which allowed incorrect arguments to be passed to open_mfdataset without complaint.

The combination open_mfdataset(files, combine='by_coords', concat_dim='t') should never have been permitted, and in fact it wasn't permitted until the last part of the deprecation process from the old auto_combine. It makes no sense to pass this combination because the combine_by_coords function does not have a concat_dim argument at all!

The effect was pretty benign - the concat_dim arg wasn't really used for anything in that case, and the result of passing dodgy datasets would just be a less informative error. However there were multiple tests which assumed this behaviour was okay - I had to remove that particular parametrization for a bunch of your join tests @dcherian because they now fail with a different (clearer) error.

I also noticed a related issue which I fixed - internally open_mfdataset was performing a rearrangement of the input datasets that it needs for the case combine='nested', even in the case combine='by_coords'. I hadn't previously realised that we can just skip this rearrangement without issue, so open_mfdataset(combine='by_coords') should be a little bit faster now.

  • [x] Closes #5230
  • [x] Tests added
  • [x] Passes pre-commit run --all-files
  • [x] User visible changes (including notable bug fixes) are documented in whats-new.rst
 
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