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  • setting variables named in CF attributes as coordinate variables · 2 ✖

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id html_url issue_url node_id user created_at updated_at ▲ author_association body reactions performed_via_github_app issue
658329779 https://github.com/pydata/xarray/issues/4215#issuecomment-658329779 https://api.github.com/repos/pydata/xarray/issues/4215 MDEyOklzc3VlQ29tbWVudDY1ODMyOTc3OQ== DWesl 22566757 2020-07-14T18:07:05Z 2020-07-14T18:07:05Z CONTRIBUTOR

formula_terms is another attribute with variable names, although it requires a bit more parsing.

Question: Should we allow decode_coords to control whether variables mentioned in these attributes are set as coordinate variables?

I don't think this is necessary. It's easy to explicitly set or reset coordinates afterwards if desired.

Is that "putting the variables in these attributes in coords is out of scope for XArray" or "putting the variables in these attributes in coords is out of scope for decode_coords" or something else?

I would say no however to ancillary_variables, since those are not really about coordinates and instead about linked data variables (like uncertainties).

I tend to think of uncertainties and status flags as important for the interpretation of the associated variables that should stay with the data variables unless a decision is explicitly made to drop them. On the other hand, since XArray seems to associate coordinates with dimensions rather than with variables, I can see why this might be less than desirable. This argument would also apply to grid_mapping.

My one concern with #2844 is clarifying the role of encoding vs. attrs.

I think we should probably ensure that xarray always propagates encoding exactly like how it propagates attrs.

Should this be part of #2844 or should preserving encoding be a separate PR?

 
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   setting variables named in CF attributes as coordinate variables 654889988
656789017 https://github.com/pydata/xarray/issues/4215#issuecomment-656789017 https://api.github.com/repos/pydata/xarray/issues/4215 MDEyOklzc3VlQ29tbWVudDY1Njc4OTAxNw== jthielen 3460034 2020-07-10T17:17:59Z 2020-07-10T17:41:31Z CONTRIBUTOR

I agree with #3689 that it makes the most sense to have decode_coords set those variables referenced in bounds as coordinates. By the same reasoning, I would think the other special variable-linked attrs of CF Section 7 should be treated similarly:

  • cell_measures
  • climatology
  • geometry
  • node_coordinates
  • node_count
  • part_node_count
  • interior_ring

grid_mapping and ancillary_variables were also brought up. grid_mapping definitely makes sense to be interpreted as a coordinate variable, since it is inherently linked to the CRS of the data. I would say no however to ancillary_variables, since those are not really about coordinates and instead about linked data variables (like uncertainties).

My one concern with #2844 is clarifying the role of encoding vs. attrs. I don't have any good conclusions about it, but I'd want to be very cautious about not having these "links" defined by the CF conventions disappear unexpectedly because they were decoded by decode_coords, moved to encoding, and then erased due to some xarray operation clearing encoding on the returned data. I'd hope to keep them around in some fashion so that they are still usable by libraries like cf-xarray and MetPy, among others.

 
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   setting variables named in CF attributes as coordinate variables 654889988

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