Review comments.

Author: dcherian

xarray/core/computation.py

@@ -43,11 +43,11 @@
 
 
 def _first_of_type(args, kind):
-    """" Returns either first object of type 'kind' or None if not found. """
+    """ Return either first object of type 'kind' or raise if not found. """
     for arg in args:
         if isinstance(arg, kind):
             return arg
-    raise NotImplementedError("This should be unreachable.")
+    raise ValueError("This should be unreachable.")
 
 
 class _UFuncSignature:

xarray/tests/test_dataset.py

@@ -4491,6 +4491,14 @@ def test_propagate_attrs(self, func):
             assert func(ds).attrs != ds.attrs
             assert func(ds).a.attrs != ds.a.attrs
 
+        with set_options(keep_attrs=True):
+            assert func(ds).attrs == ds.attrs
+            assert func(ds).a.attrs == ds.a.attrs
+
+        with set_options(keep_attrs=True):
+            assert func(ds).attrs == ds.attrs
+            assert func(ds).a.attrs == ds.a.attrs
+
     def test_where(self):
         ds = Dataset({"a": ("x", range(5))})
         expected = Dataset({"a": ("x", [np.nan, np.nan, 2, 3, 4])})