Assignment

Assignment of individual elements / subsets seems to be only partially supported, but I have not found clear documentation on it. After playing a bit around with xarray I believe the current support boils down to the following (I list some issues I see with the current documentation and how things seem to work).

DataSet

• Change a element in a associated DataArray
  • ds.loc[selector_dict].NAME_OF_VARIABLE = -1 Doesn't work but at least throws an error (Use __setitem__ style assignment (e.g., ds['name'] = ...) instead to assign variables.)
  • ds.loc[selector_dict]["NAME_OF_VARIABLE"] = -1 Silently fails
    • Is it possible to throw an exception as well? I believe this usage variant is "more common", as NAME_OF_VARIABLE may only be known at runtime. #974 May fix this (?).
  • Same goes for ds.sel

DataArray
(Can be obtained via ds['name'])

• Using .sel (which is documented to return a new DataArray, so arguably it's expected not to work)
  • ds["NAME_OF_VARIABLE"].sel(**selector_dict) = -1 Invalid python syntax
  • ds["NAME_OF_VARIABLE"].sel(**selector_dict).values = -1 Silently fails
• Using .loc
  • ds["NAME_OF_VARIABLE"].loc[selector_dict] = -1 works

Is this the recommended behaviour? Then I believe the wrong pattern of ds.loc[selector_dict]["NAME_OF_VARIABLE"] = -1 should be documented and ds[NAME_OF_VARIABLE].loc[selector_dict] = -1 instead suggested.

Start by comparing the assignment behavior described for Dataset.loc, Dataset.sel, DataArray.sel, and DataArray.loc, then check the existing xarray documentation for these entry points. Done means the recommended assignment pattern and the behavior of silent failures are clearly documented, or the requested exception behavior is explicitly resolved.