Use tm.assert_frame_equal and smoke test from GH issue

Author: fleimgruber

pandas/core/indexing.py

@@ -2732,8 +2732,8 @@ def _non_reducing_slice(slice_):
         slice_ = IndexSlice[:, slice_]
 
     def pred(part):
-        # true when slice does *not* reduce
-        # False when part is a tuple, i.e. MultiIndex slice
+        # true when slice does *not* reduce False when part is a tuple,
+        # i.e. MultiIndex slice
         if isinstance(part, tuple):
             return False
         return isinstance(part, slice) or is_list_like(part)

pandas/tests/indexing/test_indexing.py

@@ -908,13 +908,14 @@ def test_non_reducing_slice_on_multiindex(self):
             ('b', 'c'): [3, 2],
             ('b', 'd'): [4, 1]
         }
-
         df = pd.DataFrame(dic, index=[0, 1])
         idx = pd.IndexSlice
-
         slice_ = idx[:, idx['b', 'd']]
         tslice_ = _non_reducing_slice(slice_)
-        assert isinstance(df.loc[tslice_], DataFrame)
+
+        result = df.loc[tslice_]
+        expected = pd.DataFrame({('b', 'd'): [4, 1]})
+        tm.assert_frame_equal(result, expected)
 
     def test_list_slice(self):
         # like dataframe getitem

pandas/tests/io/formats/test_style.py

@@ -274,6 +274,31 @@ def f(x):
                         col in self.df.loc[slice_].columns}
             assert result == expected
 
+    def test_applymap_subset_multiindex(self):
+        # https://github.com/pandas-dev/pandas/issues/19861
+        def color_negative_red(val):
+            """
+            Takes a scalar and returns a string with
+            the css property `'color: red'` for negative
+            strings, black otherwise.
+            """
+            color = 'red' if val < 0 else 'black'
+            return 'color: %s' % color
+
+        dic = {
+            ('a', 'd'): [-1.12, 2.11],
+            ('a', 'c'): [2.78, -2.88],
+            ('b', 'c'): [-3.99, 3.77],
+            ('b', 'd'): [4.21, -1.22],
+        }
+
+        idx = pd.IndexSlice
+        df = pd.DataFrame(dic, index=[0, 1])
+
+        (df.style
+         .applymap(color_negative_red, subset=idx[:, idx['b', 'd']])
+         .render())
+
     def test_where_with_one_style(self):
         # GH 17474
         def f(x):