non-deterministic ordering within "coordinates" attribute written by .to_netcdf

[itcarroll]

What is your issue?

Under the assumption that deterministic output is preferred whenever feasible, I'd like to point out that the variable names written into "coordinates" attributes with .to_netcdf are not ordered deterministically. For pipelines that depend on file hashes to validate dependencies, this can be a real headache.

Consider the dataset xarray.Dataset({"x": ((), 0)}, coords={"a": 0, "b": 0}). The NetCDF file XArray writes will include either:

variables:
	int64 x ;
		x:coordinates = "a b" ;
	int64 a ;
	int64 b ;

or

variables:
	int64 x ;
		x:coordinates = "b a" ;
	int64 a ;
	int64 b ;

My review of _encode_coordinates leads me to think the behavior results from collecting names in a set. I'd be happy to offer a PR to make the coordinates attribute deterministic. I am not aware of a CF convention regarding any ordering, but would research and follow if it exists. If not, then I would probably sort at L701 and L722.

[dcherian]

Your proposal sounds good to me. A PR would be welcome!

[trexfeathers]

By coincidence I was just looking at this corner of the Iris code. Here is how we achieve ordering (link):

            for element in sorted(
                coordlike_elements, key=lambda element: element.name()
            ):