+1 we should make this work!

@dcherian is this a merge problem, or a combine_by_coords problem?

What do you think of:

1. If combine receives a list of unnamed DataArrays, we convert them to temp datasets and then pass datasets down to merge.

2. If combine receives a list of named DataArrays, then merge will do the right thing.

3. If combine receives a list containing both Datasets and unnamed DataArrays then merge will raise the right error.

So really it's just (1) that needs a special case in the combine functions?

Some additional information on the topic:

Combining named 1D data arrays works.

da1 = xr.DataArray(name='foo', data=np.random.randn(3), coords=[('x', [1, 2, 3])])
da2 = xr.DataArray(name='foo', data=np.random.randn(3), coords=[('x', [5, 6, 7])])
xr.combine_by_coords([da1, da2])

<xarray.Dataset>
Dimensions:  (x: 6)
Coordinates:
  * x        (x) int64 1 2 3 5 6 7
Data variables:
    foo      (x) float64 1.443 0.4889 0.9233 0.1946 -1.639 -1.455

However, when combining 2D gridded data...

da1 = xr.DataArray(name='foo', 
                  data=np.random.rand(3,3), 
                  coords=[('x', [1, 2, 3]),
                          ('y', [1, 2, 3])])

da2 = xr.DataArray(name='foo', 
                  data=np.random.rand(3,3), 
                  coords=[('x', [5, 6, 7]),
                          ('y', [5, 6, 7])])

xr.combine_by_coords([da1, da2])

...the method fails, despite passing a data variable name.

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-145-77ae89136c1f> in <module>
      9                           ('y', [5, 6, 7])])
     10 
---> 11 xr.combine_by_coords([da1, da2])

~/xarray/xarray/core/combine.py in combine_by_coords(datasets, compat, data_vars, coords, fill_value, join)
    580 
    581     # Group by data vars
--> 582     sorted_datasets = sorted(datasets, key=vars_as_keys)
    583     grouped_by_vars = itertools.groupby(sorted_datasets, key=vars_as_keys)
    584 

~/xarray/xarray/core/combine.py in vars_as_keys(ds)
    465 
    466 def vars_as_keys(ds):
--> 467     return tuple(sorted(ds))
    468 
    469 

~/xarray/xarray/core/common.py in __bool__(self)
    119 
    120     def __bool__(self: Any) -> bool:
--> 121         return bool(self.values)
    122 
    123     def __float__(self: Any) -> float:

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Again, converting to a dataset bypasses the issue.

ds1 = da1.to_dataset()
ds2 = da2.to_dataset()
xr.combine_by_coords([ds1, ds2])

<xarray.Dataset>
Dimensions:  (x: 6, y: 6)
Coordinates:
  * x        (x) int64 1 2 3 5 6 7
  * y        (y) int64 1 2 3 5 6 7
Data variables:
    foo      (x, y) float64 0.5078 0.8981 0.8707 nan ... 0.4172 0.7259 0.8431

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:

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?