BUG: Fixed handling of non-list value_vars in melt

doc/source/whatsnew/v0.20.0.txt

@@ -520,4 +520,5 @@ Bug Fixes
 
 - Bug in ``DataFrame.boxplot`` where ``fontsize`` was not applied to the tick labels on both axes (:issue:`15108`)
 - Bug in ``Series.replace`` and ``DataFrame.replace`` which failed on empty replacement dicts (:issue:`15289`)
+- Bug in ``pd.melt`` where passing a tuple value for ``value_vars`` caused a ``TypeError`` (:issue:`15348`)
 - Bug in ``.eval()`` which caused multiline evals to fail with local variables not on the first line (:issue:`15342`)

pandas/core/reshape.py

@@ -761,16 +761,26 @@ def melt(frame, id_vars=None, value_vars=None, var_name=None,
     """
     # TODO: what about the existing index?
     if id_vars is not None:
-        if not isinstance(id_vars, (tuple, list, np.ndarray)):
+        if not is_list_like(id_vars):
             id_vars = [id_vars]
+        elif (isinstance(frame.columns, MultiIndex)
+              and not isinstance(id_vars, list)):
+            raise TypeError('id_vars must be a list of tuples when columns'
+                            ' are a MultiIndex')
         else:
             id_vars = list(id_vars)
     else:
         id_vars = []
 
     if value_vars is not None:
-        if not isinstance(value_vars, (tuple, list, np.ndarray)):
+        if not is_list_like(value_vars):
             value_vars = [value_vars]
+        elif (isinstance(frame.columns, MultiIndex)
+              and not isinstance(value_vars, list)):
+            raise TypeError('value_vars must be a list of tuples when'
+                            ' columns are a MultiIndex')
+        else:
+            value_vars = list(value_vars)
         frame = frame.loc[:, id_vars + value_vars]
     else:
         frame = frame.copy()

pandas/tests/test_reshape.py

@@ -56,6 +56,38 @@ def test_value_vars(self):
                               columns=['id1', 'id2', 'variable', 'value'])
         tm.assert_frame_equal(result4, expected4)
 
+    def test_value_vars_types(self):
+        # GH 15348
+        expected = DataFrame({'id1': self.df['id1'].tolist() * 2,
+                              'id2': self.df['id2'].tolist() * 2,
+                              'variable': ['A'] * 10 + ['B'] * 10,
+                              'value': (self.df['A'].tolist() +
+                                        self.df['B'].tolist())},
+                             columns=['id1', 'id2', 'variable', 'value'])
+
+        for type_ in (tuple, list, np.array):
+            result = melt(self.df, id_vars=['id1', 'id2'],
+                          value_vars=type_(('A', 'B')))
+            tm.assert_frame_equal(result, expected)
+
+    def test_id_and_value_vars_types_with_multiindex(self):
+        expected = DataFrame({
+            ('A', 'a'): self.df1[('A', 'a')],
+            'CAP': ['B'] * len(self.df1),
+            'low': ['b'] * len(self.df1),
+            'value': self.df1[('B', 'b')],
+        }, columns=[('A', 'a'), 'CAP', 'low', 'value'])
+
+        for id_vars in ([('A', 'a')], ('A', 'a')):
+            for value_vars in ([('B', 'b')], ('B', 'b')):
+                if isinstance(id_vars, list) and isinstance(value_vars, list):
+                    result = melt(self.df1, id_vars=id_vars,
+                                  value_vars=value_vars)
+                    tm.assert_frame_equal(result, expected)
+                else:
+                    with self.assertRaisesRegex(TypeError, r'MultiIndex'):
+                        melt(self.df1, id_vars=id_vars, value_vars=value_vars)
+
     def test_custom_var_name(self):
         result5 = melt(self.df, var_name=self.var_name)
         self.assertEqual(result5.columns.tolist(), ['var', 'value'])