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
203999231,MDU6SXNzdWUyMDM5OTkyMzE=,1238,`set_index` converts string-dtype to object-dtype,500246,open,0,,,10,2017-01-30T12:37:05Z,2023-03-13T14:09:21Z,,CONTRIBUTOR,,,,"'Dataset.set_index' apparently changes a `<U1` dtype into an `object`-dtype, as illustrated below: 

```
In [108]: ds = xarray.Dataset({""x"": ((""a"", ""b""), arange(25).reshape(5,5)+100), ""y"": (""b"", arange(5)-100)}, {""a"": arange(5), ""b"": arange(5)*2, ""c"": ((""a"",), list(""ABCDE""))})

In [109]: print(ds)
<xarray.Dataset>
Dimensions:  (a: 5, b: 5)
Coordinates:
  * b        (b) int64 0 2 4 6 8
    c        (a) <U1 'A' 'B' 'C' 'D' 'E'
  * a        (a) int64 0 1 2 3 4
Data variables:
    x        (a, b) int64 100 101 102 103 104 105 106 107 108 109 110 111 ...
    y        (b) int64 -100 -99 -98 -97 -96

In [110]: print(ds.set_index(a='c'))
<xarray.Dataset>
Dimensions:  (a: 5, b: 5)
Coordinates:
  * b        (b) int64 0 2 4 6 8
  * a        (a) object 'A' 'B' 'C' 'D' 'E'
Data variables:
    x        (a, b) int64 100 101 102 103 104 105 106 107 108 109 110 111 ...
    y        (b) int64 -100 -99 -98 -97 -96
```","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1238/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue