Added additional Exception when user is trying to destagger unstaggered variable

tests/test_destagger.py

@@ -46,12 +46,23 @@ def test_rename_staggered_coordinate(input_name, stagger_dim, unstag_dim_name, o
 
 
 def test_destag_variable_dataarray():
-    with pytest.raises(ValueError):
-        _destag_variable(
-            xr.DataArray(
-                np.zeros((2, 2)), dims=('x_stag', 'y'), coords={'x_stag': [0, 1], 'y': [0, 1]}
-            )
+    with pytest.raises(ValueError) as exc_info:
+        mock_da = xr.DataArray(
+            np.zeros((2, 2)), dims=('x_stag', 'y'), coords={'x_stag': [0, 1], 'y': [0, 1]}
         )
+        _destag_variable(mock_da)
+    assert (
+        exc_info.value.args[0] == f'Parameter datavar must be xarray.Variable, not {type(mock_da)}'
+    )
+
+
+def test_destag_variable_unstaggered():
+    with pytest.raises(ValueError) as exc_info:
+        _destag_variable(xr.Variable(('x', 'y'), np.zeros((2, 2))))
+    assert (
+        exc_info.value.args[0]
+        == 'No dimension available to destagger. This variable does not appear to be staggered.'
+    )
 
 
 def test_destag_variable_missing_dim():

xwrf/destagger.py

@@ -50,6 +50,11 @@ def _destag_variable(datavar, stagger_dim=None, unstag_dim_name=None):
                 'Expected a single destagger dimensions. Found multiple destagger dimensions: '
                 f'{stagger_dim}'
             )
+        elif len(stagger_dim) == 0:
+            raise ValueError(
+                'No dimension available to destagger. This variable does not appear to be '
+                'staggered.'
+            )
 
         # we need a string, not a list containing a string
         stagger_dim = stagger_dim[0]