combine_by_coords fails with DataArrays

MCVE Code Sample

da1 = xr.DataArray([1, 2, 3], dims='x', coords={'x': [0, 1, 2]})
da2 = xr.DataArray([3, 4, 5], dims='x', coords={'x': [2, 3, 4]})
xr.combine_by_coords([da1, da2])

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-133-d27216ba5688> in <module>
      1 da1 = xr.DataArray([1, 2, 3], dims='x', coords={'x': [0, 1, 2]})
      2 da2 = xr.DataArray([3, 4, 5], dims='x', coords={'x': [2, 3, 4]})
----> 3 xr.combine_by_coords([da1, da2])

~/work/python/xarray/xarray/core/combine.py in combine_by_coords(datasets, compat, data_vars, coords, fill_value, join)
    619         compat=compat,
    620         fill_value=fill_value,
--> 621         join=join,
    622     )
    623 

~/work/python/xarray/xarray/core/merge.py in merge(objects, compat, join, fill_value)
    588             )
    589 
--> 590         obj = obj.to_dataset() if isinstance(obj, DataArray) else obj
    591         dict_like_objects.append(obj)
    592 

~/work/python/xarray/xarray/core/dataarray.py in to_dataset(self, dim, name)
    478             return self._to_dataset_split(dim)
    479         else:
--> 480             return self._to_dataset_whole(name)
    481 
    482     @property

~/work/python/xarray/xarray/core/dataarray.py in _to_dataset_whole(self, name, shallow_copy)
    426         if name is None:
    427             raise ValueError(
--> 428                 "unable to convert unnamed DataArray to a "
    429                 "Dataset without providing an explicit name"
    430             )

ValueError: unable to convert unnamed DataArray to a Dataset without providing an explicit name

To get what I want, I need xr.combine_by_coords([da1.to_dataset(name='a'), da2.to_dataset(name='a')]).a

I think the issue is that the code uses to_dataset instead of _to_temp_dataset

[dcherian]

Thanks @friedrichknuth .

It looks like all the combine functions expect lists of Datasets, not DataArrays (see docstrings).

It should be relatively easily to make this work like merge which can take a list of Datasets or DataArrays. See this code in merge: https://github.com/pydata/xarray/blob/756c94164840e8c070bcd26681b97c31412909ae/xarray/core/merge.py#L593-L602

to_dataset will fail for unnamed DataArrays, but _to_temp_dataset() will succeed. It seems to me like we want lists of unnamed DataArrays to work (like in the first example), so call _to_temp_dataset when needed. Maybe @shoyer has an idea on how to implement that cleanly.

Can you send in a PR?

[aijams]

It looks like this issue has been neglected for a while. @TomNicholas, can you take a look at this PR?