Fix.

Author: ueshin

databricks/koalas/frame.py

@@ -6774,7 +6774,10 @@ def melt(self, id_vars=None, value_vars=None, var_name=None,
         elif isinstance(id_vars, str):
             id_vars = [(id_vars,)]
         elif isinstance(id_vars, tuple):
-            id_vars = [id_vars]
+            if self._internal.column_index_level == 1:
+                id_vars = [idv if isinstance(idv, tuple) else (idv,) for idv in id_vars]
+            else:
+                raise ValueError('id_vars must be a list of tuples when columns are a MultiIndex')
         else:
             id_vars = [idv if isinstance(idv, tuple) else (idv,) for idv in id_vars]
 

databricks/koalas/tests/test_dataframe.py

@@ -1770,6 +1770,9 @@ def test_melt(self):
         self.assert_eq(kdf.melt(id_vars=['A', 'B']).sort_values(['variable', 'value'])
                        .reset_index(drop=True),
                        pdf.melt(id_vars=['A', 'B']).sort_values(['variable', 'value']))
+        self.assert_eq(kdf.melt(id_vars=('A', 'B')).sort_values(['variable', 'value'])
+                       .reset_index(drop=True),
+                       pdf.melt(id_vars=('A', 'B')).sort_values(['variable', 'value']))
         self.assert_eq(kdf.melt(id_vars=['A'], value_vars=['C']).sort_values(['variable', 'value'])
                        .reset_index(drop=True),
                        pdf.melt(id_vars=['A'], value_vars=['C']).sort_values(['variable', 'value']))
@@ -1811,6 +1814,8 @@ def test_melt(self):
                        .reset_index(drop=True),
                        pdf.melt().sort_values(['v0', 'v1', 'value']))
 
+        self.assertRaises(ValueError, lambda: kdf.melt(id_vars=('X', 'A')))
+
     def test_all(self):
         pdf = pd.DataFrame({
             'col1': [False, False, False],