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
1386596170,PR_kwDOAMm_X84_olQw,7085,solve a bug when the units attribute is not a string ,10563614,closed,0,,,2,2022-09-26T19:27:08Z,2022-09-28T19:13:11Z,2022-09-28T19:13:11Z,CONTRIBUTOR,,0,pydata/xarray/pulls/7085,"
<!-- Feel free to remove check-list items aren't relevant to your change -->

- [ ] Closes #xxxx
- [x] Tests added
- [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
- [ ] New functions/methods are listed in `api.rst`

We faced a sort of bug with a colleague of mine. It seems to be legal to set a numeric value to the units attributes in an xarray or a netcdf file. xarray accepts to save such an array to netcdf: xr.DataArray([1, 2, 3], attrs={'units': 1}, name='x').to_csv('tmp.nc'). Reading this netcdf file with xarray.open_dataset raises an error.

It is unlikely to have a scalar for the units, but at least it happened to us (the value was NaN) and this raised an exception very difficult to understand.

This raises an exception because ```""since"" in attrs[""units""]``` was called twice in xarray codebase (in coding_times.py and in conventions.py) without checking for the type of the attribute.

This PR solves this improbable bug
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705182835,MDExOlB1bGxSZXF1ZXN0NDg5OTU2NDQ2,4442,Fix DataArray.to_dataframe when the array has MultiIndex,10563614,closed,0,,,4,2020-09-20T20:45:12Z,2021-02-20T00:08:42Z,2021-02-20T00:08:42Z,CONTRIBUTOR,,0,pydata/xarray/pulls/4442,"<!-- Feel free to remove check-list items aren't relevant to your change -->

 - [X] Closes #3008
 - [x] Tests added
 - [x] Passes `isort . && black . && mypy . && flake8`
 - [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
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