DeprecationWarning -> FutureWarning

The `DeprecationWarning` is ignored by default for `.assign_coords` due
to the
stacklevel (pydata#6798 (comment))

Use `FutureWarning` instead to show the warning for both `.assign` and
`.assign_coords`.

xarray/core/coordinates.py

@@ -442,7 +442,7 @@ def drop_coords(
                 f"other variables: {list(maybe_midx.index.names)!r}. "
                 f"This will raise an error in the future. Use `.drop_vars({idx_coord_names!r})` before "
                 "assigning new coordinate values.",
-                DeprecationWarning,
+                FutureWarning,
                 stacklevel=4,
             )
             for k in idx_coord_names:

xarray/tests/test_dataarray.py

@@ -1501,9 +1501,7 @@ def test_assign_coords(self) -> None:
 
     def test_assign_coords_existing_multiindex(self) -> None:
         data = self.mda
-        with pytest.warns(
-            DeprecationWarning, match=r"Updating MultiIndexed coordinate"
-        ):
+        with pytest.warns(FutureWarning, match=r"Updating MultiIndexed coordinate"):
             data.assign_coords(x=range(4))
 
     def test_coords_alignment(self) -> None:

xarray/tests/test_dataset.py

@@ -4136,14 +4136,10 @@ def test_assign_multiindex_level(self) -> None:
 
     def test_assign_coords_existing_multiindex(self) -> None:
         data = create_test_multiindex()
-        with pytest.warns(
-            DeprecationWarning, match=r"Updating MultiIndexed coordinate"
-        ):
+        with pytest.warns(FutureWarning, match=r"Updating MultiIndexed coordinate"):
             data.assign_coords(x=range(4))
 
-        with pytest.warns(
-            DeprecationWarning, match=r"Updating MultiIndexed coordinate"
-        ):
+        with pytest.warns(FutureWarning, match=r"Updating MultiIndexed coordinate"):
             data.assign(x=range(4))
 
         # https://github.com/pydata/xarray/issues/7097 (coord names updated)