custom "coordinates" attribute

[dcherian]

This set of lines prevents users from writing files with a handcrafted coordinates attribute. This is useful when not all non-dimensional coordinates in a Dataset are appropriate for every data variable in a dataset.

xarray/xarray/conventions.py

Lines 667 to 675 in 16fdac9

	# These coordinates are saved according to CF conventions
	for var_name, coord_names in variable_coordinates.items():
	attrs = variables[var_name].attrs
	if "coordinates" in attrs:
	raise ValueError(
	"cannot serialize coordinates because variable "
	"%s already has an attribute 'coordinates'" % var_name
	)
	attrs["coordinates"] = " ".join(map(str, coord_names))

It seems to me like we should only automatically set "coordinates" when "coordinates" not in attrs. We could raise a warning saying that setting attrs["coordinates"] prevents full roundtripping of Datasets.

What do you think?