add test for coverage

Author: itholic

databricks/koalas/tests/test_groupby.py

@@ -71,10 +71,12 @@ def test_groupby(self):
         # we can't use column name/names as a parameter `by` for `SeriesGroupBy`.
         self.assertRaises(KeyError, lambda: kdf.a.groupby(by='a'))
         self.assertRaises(KeyError, lambda: kdf.a.groupby(by=['a', 'b']))
+        self.assertRaises(KeyError, lambda: kdf.a.groupby(by=('a', 'b')))
 
         # we can't use DataFrame as a parameter `by` for `DataFrameGroupBy`/`SeriesGroupBy`.
         self.assertRaises(ValueError, lambda: kdf.groupby(kdf))
         self.assertRaises(ValueError, lambda: kdf.a.groupby(kdf))
+        self.assertRaises(ValueError, lambda: kdf.a.groupby((kdf,)))
 
     def test_groupby_multiindex_columns(self):
         pdf = pd.DataFrame({('x', 'a'): [1, 2, 6, 4, 4, 6, 4, 3, 7],