whatsnew and changes requested

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

@@ -2732,11 +2732,10 @@ def _non_reducing_slice(slice_):
         slice_ = IndexSlice[:, slice_]
 
     def pred(part):
-        # true when slice does *not* reduce False when part is a tuple,
+        # 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)
+        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):