Include MultiIndex slice in non-reducing slices (pandas-dev#19881)

Author: fleimgruber

doc/source/whatsnew/v0.24.0.rst

@@ -1538,6 +1538,7 @@ Missing
 MultiIndex
 ^^^^^^^^^^
 
+- Bug in :func:`io.formats.style.Styler.applymap` where ``subset=`` with :class:`MultiIndex` slice would reduce to :class:`Series` (:issue:`19861`)
 - Removed compatibility for :class:`MultiIndex` pickles prior to version 0.8.0; compatibility with :class:`MultiIndex` pickles from version 0.13 forward is maintained (:issue:`21654`)
 - :meth:`MultiIndex.get_loc_level` (and as a consequence, ``.loc`` on a ``Series`` or ``DataFrame`` with a :class:`MultiIndex` index) will now raise a ``KeyError``, rather than returning an empty ``slice``, if asked a label which is present in the ``levels`` but is unused (:issue:`22221`)
 - :class:`MultiIndex` has gained the :meth:`MultiIndex.from_frame`, it allows constructing a :class:`MultiIndex` object from a :class:`DataFrame` (:issue:`22420`)

pandas/core/indexing.py

@@ -2732,8 +2732,10 @@ def _non_reducing_slice(slice_):
         slice_ = IndexSlice[:, slice_]
 
     def pred(part):
-        # true when slice does *not* reduce
-        return isinstance(part, slice) or is_list_like(part)
+        # true when slice does *not* reduce, False when part is a tuple,
+        # i.e. MultiIndex slice
+        return ((isinstance(part, slice) or is_list_like(part))
+                and not isinstance(part, tuple))
 
     if not is_list_like(slice_):
         if not isinstance(slice_, slice):

pandas/tests/indexing/test_indexing.py

@@ -812,6 +812,23 @@ def test_non_reducing_slice(self):
             tslice_ = _non_reducing_slice(slice_)
             assert isinstance(df.loc[tslice_], DataFrame)
 
+    def test_non_reducing_slice_on_multiindex(self):
+        # GH 19861
+        dic = {
+            ('a', 'd'): [1, 4],
+            ('a', 'c'): [2, 3],
+            ('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_)
+
+        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
         slices = [['A'], Series(['A']), np.array(['A'])]

pandas/tests/io/formats/test_style.py

@@ -274,6 +274,32 @@ def f(x):
                         col in self.df.loc[slice_].columns}
             assert result == expected
 
+    def test_applymap_subset_multiindex(self):
+        # GH 19861
+        # Smoke test for applymap
+        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):