Update for comments

Author: TomAugspurger

doc/source/whatsnew/v0.21.0.txt

@@ -278,8 +278,11 @@ The configuration option ``pd.options.mode.use_inf_as_null`` is deprecated, and
 MultiIndex Constructor with a Single Level
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Previous versions would automatically squeeze a ``MultiIndex`` with length-one
-``levels`` down to an ``Index``:
+The ``MultiIndex`` constructors no longer squeeze a MultiIndex with all
+length-one levels down to a regular ``Index``. This affects all the
+``MultiIndex`` constructors. (:issue:`17178`)
+
+Previous behavior:
 
 .. code-block:: ipython
 

pandas/core/indexes/base.py

@@ -4037,6 +4037,10 @@ def _ensure_index_from_sequences(sequences, names=None):
     MultiIndex(levels=[['a'], ['a', 'b']],
                labels=[[0, 0], [0, 1]],
                names=['L1', 'L2'])
+
+    See Also
+    --------
+    _ensure_index
     """
     from .multi import MultiIndex
 
@@ -4073,6 +4077,10 @@ def _ensure_index(index_like, copy=False):
     >>> _ensure_index([['a', 'a'], ['b', 'c']])
     MultiIndex(levels=[['a'], ['b', 'c']],
                labels=[[0, 0], [0, 1]])
+
+    See Also
+    --------
+    _ensure_index_from_sequences
     """
     if isinstance(index_like, Index):
         if copy:

pandas/core/reshape/reshape.py

@@ -450,12 +450,12 @@ def _slow_pivot(index, columns, values):
 
 def unstack(obj, level, fill_value=None):
     if isinstance(level, (tuple, list)):
-        if len(level) == 1:
-            # unstack_multiple only handles MultiIndexes,
+        if len(level) != 1:
+            # _unstack_multiple only handles MultiIndexes,
             # and isn't needed for a single level
-            level = level[0]
-        else:
             return _unstack_multiple(obj, level)
+        else:
+            level = level[0]
 
     if isinstance(obj, DataFrame):
         if isinstance(obj.index, MultiIndex):