Fix fill_value handling during unstack (#10901)

* Use get method for fill_value in unstack_once

* Add unit test for default fill_value in stack()

Author: peanutfun

xarray/core/dataset.py

@@ -5433,7 +5433,7 @@ def _unstack_once(
             if name not in index_vars:
                 if dim in var.dims:
                     if isinstance(fill_value, Mapping):
-                        fill_value_ = fill_value[name]
+                        fill_value_ = fill_value.get(name, xrdtypes.NA)
                     else:
                         fill_value_ = fill_value
 

xarray/tests/test_dataset.py

@@ -4150,6 +4150,10 @@ def test_unstack_fill_value(self) -> None:
         expected3 = ds.unstack("index").fillna({"var": -1, "other_var": 1}).astype(int)
         assert_equal(actual3, expected3)
 
+        actual4 = ds.unstack("index", fill_value={"var": -1})
+        expected4 = ds.unstack("index").fillna({"var": -1, "other_var": np.nan})
+        assert_equal(actual4, expected4)
+
     @requires_sparse
     def test_unstack_sparse(self) -> None:
         ds = xr.Dataset(