open_mfdataset() on a single file drops the concat_dim

[WeatherGod]

When calling xr.open_mfdataset() on a 1 element list of filenames, the concat dimension is never added.

This isn't a MWE at the moment (will make one soon enough), just wanted to get my thoughts down.

from datetime import datetime
import xarray as xr

time_coord = xr.DataArray([datetime.utcnow()], name='time', dims='time')
radmax_ds = xr.open_mfdataset(['foobar.nc'], concat_dim=time_coord)
print(radmax_ds)

<xarray.Dataset>
Dimensions:    (latitude: 5650, longitude: 12050)
Coordinates:
  * latitude   (latitude) float32 13.505002 13.515002 13.525002 13.535002 ...
  * longitude  (longitude) float32 -170.495 -170.485 -170.475 -170.465 ...
Data variables:
    RadarMax   (latitude, longitude) float32 dask.array<shape=(5650, 12050), chunksize=(5650, 12050)>
Attributes:
    start_date:   03/07/2017 01:00
    end_date:     03/07/2017 01:55
    elapsed:      60

Problem description

If there are two files, then there is a time coordinate, and the data array becomes 3D.

Output of xr.show_versions()

I am currently on a recent-ish master of xarray.

[jhamman]

This does seem inconsistent to me. Not that the behavior of concat already works with a single object:

In [1]: import xarray as xr

In [2]: da = xr.DataArray([1, 2], dims='x', name='foo')

In [3]: xr.concat([da], dim='y')
Out[3]:
<xarray.DataArray 'foo' (y: 1, x: 2)>
array([[1, 2]])
Dimensions without coordinates: y, x

The offending line is here:

xarray/xarray/core/combine.py

Lines 342 to 344 in 9261601

	def _auto_concat(datasets, dim=None, data_vars='all', coords='different'):
	if len(datasets) == 1:
	return datasets[0]

Based on this, I'm surprised my little example with concat works the way it does. In either event, it would be great if someone could spend some time normalizing the behavior here.

[shoyer]

Yes, this seems like a bug. open_mfdataset() should always concatenate if a dim argument is provided explicitly.

[WeatherGod]

Could the fix be as simple as if len(datasets) == 1 and dim is None:?

[shoyer]

@WeatherGod Possibly! As usual, tests are the hard part :)

[WeatherGod]

I'll give it a go tomorrow. My work has gotten to this point now, and I have some unit tests that happen to exercise this edge case.

On a somewhat related note, would a allow_missing feature be welcomed in open_mfdataset()? I have written up some code that expects a concat_dim, and a list of filenames. It will then pass to open_mfdataset() only the files (and corresponding concat_dim values) that exists, and then calls reindex() with the original concat_dim to have a nan-filled slab where-ever there was a missing file.

Any interest?

[WeatherGod]

Yup... looks like that did the trick (for auto_combine and open_mfdataset). I even have a simple test to demonstrate it. PR coming shortly.