test reduce dimensions error.

Author: dcherian

xarray/tests/test_dataarray.py

@@ -2582,6 +2582,15 @@ def change_metadata(x):
         expected = change_metadata(expected)
         assert_equal(expected, actual)
 
+    def test_groupby_reduce_dimension_error(self):
+        array = self.make_groupby_example_array()
+        grouped = array.groupby("y")
+        with raises_regex(ValueError, "cannot reduce over dimension 'y'"):
+            grouped.mean()
+
+        grouped = array.groupby("y", squeeze=False)
+        assert_identical(array, grouped.mean())
+
     def test_groupby_math(self):
         array = self.make_groupby_example_array()
         for squeeze in [True, False]: