DEPR: Index.__getitem__[bool] GH#44051

doc/source/whatsnew/v1.4.0.rst

@@ -540,6 +540,7 @@ Other Deprecations
 - Deprecated parameter ``names`` in :meth:`Index.copy` (:issue:`44916`)
 - A deprecation warning is now shown for :meth:`DataFrame.to_latex` indicating the arguments signature may change and emulate more the arguments to :meth:`.Styler.to_latex` in future versions (:issue:`44411`)
 - Deprecated :meth:`Categorical.replace`, use :meth:`Series.replace` instead (:issue:`44929`)
+- Deprecated :meth:`Index.__getitem__` with a bool key; use ``index.values[key]`` to get the old behavior (:issue:`44051`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/indexes/base.py

@@ -4940,7 +4940,8 @@ def __getitem__(self, key):
         """
         getitem = self._data.__getitem__
 
-        if is_scalar(key):
+        if is_integer(key) or is_float(key):
+            # GH#44051 exclude bool, which would return a 2d ndarray
             key = com.cast_scalar_indexer(key, warn_float=True)
             return getitem(key)
 

pandas/tests/indexes/common.py

@@ -670,6 +670,21 @@ def test_getitem_2d_deprecated(self, simple_index):
 
         assert isinstance(res, np.ndarray), type(res)
 
+        if not isinstance(idx, RangeIndex):
+            # GH#44051 RangeIndex already raises
+            with tm.assert_produces_warning(FutureWarning, match=msg):
+                res = idx[True]
+            assert isinstance(res, np.ndarray), type(res)
+            with tm.assert_produces_warning(FutureWarning, match=msg):
+                res = idx[False]
+            assert isinstance(res, np.ndarray), type(res)
+        else:
+            msg = "only integers, slices"
+            with pytest.raises(IndexError, match=msg):
+                idx[True]
+            with pytest.raises(IndexError, match=msg):
+                idx[False]
+
     def test_copy_shares_cache(self, simple_index):
         # GH32898, GH36840
         idx = simple_index

pandas/tests/indexes/interval/test_base.py

@@ -63,3 +63,9 @@ def test_getitem_2d_deprecated(self, simple_index):
         with pytest.raises(ValueError, match="multi-dimensional indexing not allowed"):
             with tm.assert_produces_warning(FutureWarning):
                 idx[:, None]
+        with pytest.raises(ValueError, match="multi-dimensional indexing not allowed"):
+            # GH#44051
+            idx[True]
+        with pytest.raises(ValueError, match="multi-dimensional indexing not allowed"):
+            # GH#44051
+            idx[False]