ENH: Update units in .cf.differentiate

Author: DWesl

cf_xarray/accessor.py

@@ -2102,6 +2102,26 @@ def differentiate(
             (_single(_get_coords),), self._obj, coord, error=False, default=[coord]
         )[0]
         result = self._obj.differentiate(coord, *xr_args, **xr_kwargs)
+        if isinstance(self._obj, DataArray):
+            try:
+                result.attrs["units"] = "{:s} / ({:s})".format(
+                    self._obj.attrs["units"], self._obj[coord].attrs["units"]
+                )
+            except KeyError:
+                pass
+        else:
+            try:
+                coord_units = self._obj[coord].attrs["units"]
+            except KeyError:
+                pass
+            else:
+                for name in result.data_vars:
+                    try:
+                        result[name].attrs["units"] = "{:s} / ({:s})".format(
+                            self._obj[name].attrs["units"], coord_units
+                        )
+                    except KeyError:
+                        pass
         if positive_upward:
             coord = self._obj[coord]
             attrs = coord.attrs

cf_xarray/tests/test_accessor.py

@@ -1472,12 +1472,22 @@ def test_rename_tuple():
 
 @pytest.mark.parametrize("ds", datasets)
 def test_differentiate(ds):
+    from ..units import units
     # Add data_var and coord to test _get_coords
     ds["lon_var"] = ds["lon"]
     ds = ds.assign_coords(lon_coord=ds["lon"])
 
+    result = ds.cf.differentiate("lon")
+    lon_units = units.Unit(ds["lon"].attrs["units"])
+    for name in ds.data_vars:
+        expected_units = (
+            units.Unit(ds[name].attrs["units"]) / lon_units
+        )
+        assert units.Unit(result[name].attrs["units"]) == expected_units
+        del result[name].attrs["units"]
+
     # Coordinate
-    assert_identical(ds.differentiate("lon"), ds.cf.differentiate("lon"))
+    assert_identical(ds.differentiate("lon"), result)
 
     # Multiple coords (test error raised by _single)
     with pytest.raises(KeyError, match=".*I expected only one."):