TST/API: Forbid str-accessor for 1-level MultiIndex (pandas-dev#26608)

Author: h-vetinari

doc/source/whatsnew/v0.25.0.rst

@@ -434,6 +434,7 @@ Other API Changes
 - The ``arg`` argument in :meth:`pandas.core.groupby.DataFrameGroupBy.agg` has been renamed to ``func`` (:issue:`26089`)
 - The ``arg`` argument in :meth:`pandas.core.window._Window.aggregate` has been renamed to ``func`` (:issue:`26372`)
 - Most Pandas classes had a ``__bytes__`` method, which was used for getting a python2-style bytestring representation of the object. This method has been removed as a part of dropping Python2 (:issue:`26447`)
+- The `.str`-accessor has been disabled for 1-level :class:`MultiIndex`, use :meth:`MultiIndex.to_flat_index` if necessary (:issue:`23679`)
 - Removed support of gtk package for clipboards (:issue:`26563`)
 
 .. _whatsnew_0250.deprecations:

pandas/tests/test_strings.py

@@ -169,6 +169,14 @@ def test_api(self):
         assert Series.str is strings.StringMethods
         assert isinstance(Series(['']).str, strings.StringMethods)
 
+    def test_api_mi_raises(self):
+        # GH 23679
+        mi = MultiIndex.from_arrays([['a', 'b', 'c']])
+        with pytest.raises(AttributeError, match='Can only use .str accessor '
+                           'with Index, not MultiIndex'):
+            mi.str
+        assert not hasattr(mi, 'str')
+
     @pytest.mark.parametrize('dtype', [object, 'category'])
     @pytest.mark.parametrize('box', [Series, Index])
     def test_api_per_dtype(self, box, dtype, any_skipna_inferred_dtype):