BUG: Index.__getitem__ with 0d ndarray key

doc/source/whatsnew/v3.0.0.rst

@@ -945,6 +945,7 @@ Indexing
 - Bug in :meth:`Series.__setitem__` when assigning boolean series with boolean indexer will raise ``LossySetitemError`` (:issue:`57338`)
 - Bug in printing :attr:`Index.names` and :attr:`MultiIndex.levels` would not escape single quotes (:issue:`60190`)
 - Bug in reindexing of :class:`DataFrame` with :class:`PeriodDtype` columns in case of consolidated block (:issue:`60980`, :issue:`60273`)
+- Bug in :meth:`Index.__getitem__` incorrectly raising with a 0-dim ``np.ndarray`` key (:issue:`55601`)
 
 Missing
 ^^^^^^^

pandas/core/indexes/base.py

@@ -5219,6 +5219,7 @@ def __getitem__(self, key):
         """
         getitem = self._data.__getitem__
 
+        key = lib.item_from_zerodim(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)

pandas/core/indexes/multi.py

@@ -2235,6 +2235,7 @@ def __reduce__(self):
     # --------------------------------------------------------------------
 
     def __getitem__(self, key):
+        key = lib.item_from_zerodim(key)
         if is_scalar(key):
             key = com.cast_scalar_indexer(key)
 

pandas/core/indexes/range.py

@@ -1175,6 +1175,7 @@ def __getitem__(self, key):
         """
         Conserve RangeIndex type for scalar and slice keys.
         """
+        key = lib.item_from_zerodim(key)
         if key is Ellipsis:
             key = slice(None)
         if isinstance(key, slice):

pandas/tests/indexes/test_any_index.py

@@ -117,6 +117,15 @@ def test_pickle_preserves_name(self, index):
 
 
 class TestIndexing:
+    def test_getitem_0d_ndarray(self, index):
+        # GH#55601
+        if len(index) == 0:
+            pytest.skip(reason="Test assumes non-empty index")
+        key = np.array(0)
+        result = index[key]
+
+        assert result == index[0]
+
     def test_get_loc_listlike_raises_invalid_index_error(self, index):
         # and never TypeError
         key = np.array([0, 1], dtype=np.intp)