html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/5230#issuecomment-828596025,https://api.github.com/repos/pydata/xarray/issues/5230,828596025,MDEyOklzc3VlQ29tbWVudDgyODU5NjAyNQ==,35968931,2021-04-28T16:28:50Z,2021-04-28T16:29:34Z,MEMBER,"Okay so I found the problem. If you also pass `combine='nested'` then it should work - that's what you're supposed to do to open your two datasets. 

We currently get an unhelpful error because the default argument `combine='by_coords'` should be incompatible with passing any value for `concat_dim` (because the function `combine_by_coords` doesn't accept any `concat_dim` argument), but right now that possibility is not checked in any way (it [used to be](https://github.com/pydata/xarray/blob/795926a50c690cae20e928f2514b2269f09a9b91/xarray/core/combine.py#L597), but got lost in a deprecation cycle...)

To fix this I would simply add an input check that raises an error like 
`""passing a value for concat_dim can only be used with combine='nested', not combine='by_coords'""`","{""total_count"": 2, ""+1"": 2, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,869948050
https://github.com/pydata/xarray/issues/5230#issuecomment-828538649,https://api.github.com/repos/pydata/xarray/issues/5230,828538649,MDEyOklzc3VlQ29tbWVudDgyODUzODY0OQ==,57914115,2021-04-28T15:13:51Z,2021-04-28T15:14:16Z,CONTRIBUTOR,"Thank you for the comment @TomNicholas, if it can help i think this error comes from this particular line : https://github.com/pydata/xarray/blob/master/xarray/core/combine.py#L81
from a missing else statement. Indeed, the `concat_dims` is empty due to initialization and then raise the error. ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,869948050
https://github.com/pydata/xarray/issues/5230#issuecomment-828529986,https://api.github.com/repos/pydata/xarray/issues/5230,828529986,MDEyOklzc3VlQ29tbWVudDgyODUyOTk4Ng==,35968931,2021-04-28T15:03:24Z,2021-04-28T15:03:24Z,MEMBER,"Thanks for reporting this @antscloud - the error is from the `combine_by_coords` function that can be used inside `open_mfdataset`, but I think something in the logic has gone wrong to give you this particularly unhelpful error. I'll have a look now","{""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,869948050