BUG: Series.__setitem__[listlike_of_integers] with IntervalIndex

doc/source/whatsnew/v1.1.0.rst

@@ -525,6 +525,7 @@ Indexing
 - Bug in `Series.__getitem__` with an integer key and a :class:`MultiIndex` with leading integer level failing to raise ``KeyError`` if the key is not present in the first level (:issue:`33355`)
 - Bug in :meth:`DataFrame.iloc` when slicing a single column-:class:`DataFrame`` with ``ExtensionDtype`` (e.g. ``df.iloc[:, :1]``) returning an invalid result (:issue:`32957`)
 - Bug in :meth:`DatetimeIndex.insert` and :meth:`TimedeltaIndex.insert` causing index ``freq`` to be lost when setting an element into an empty :class:`Series` (:issue:33573`)
+- Bug in :meth:`Series.__setitem__` with an :class:`IntervalIndex` and a list-like key of integers (:issue:`33473`)
 
 Missing
 ^^^^^^^

pandas/core/indexes/base.py

@@ -4604,9 +4604,9 @@ def _check_indexing_error(self, key):
 
     def _should_fallback_to_positional(self) -> bool:
         """
-        If an integer key is not found, should we fall back to positional indexing?
+        Should an integer key be treated as positional?
         """
-        if len(self) > 0 and (self.holds_integer() or self.is_boolean()):
+        if self.holds_integer() or self.is_boolean():
             return False
         return True
 

pandas/core/indexes/interval.py

@@ -514,7 +514,7 @@ def is_overlapping(self) -> bool:
         # GH 23309
         return self._engine.is_overlapping
 
-    def _should_fallback_to_positional(self):
+    def _should_fallback_to_positional(self) -> bool:
         # integer lookups in Series.__getitem__ are unambiguously
         #  positional in this case
         return self.dtype.subtype.kind in ["m", "M"]

pandas/core/indexes/multi.py

@@ -2342,10 +2342,8 @@ def _check_indexing_error(self, key):
 
     def _should_fallback_to_positional(self) -> bool:
         """
-        If an integer key is not found, should we fall back to positional indexing?
+        Should integer key(s) be treated as positional?
         """
-        if not self.nlevels:
-            return False
         # GH#33355
         return self.levels[0]._should_fallback_to_positional()
 

pandas/core/indexes/numeric.py

@@ -376,7 +376,7 @@ def astype(self, dtype, copy=True):
     # Indexing Methods
 
     @doc(Index._should_fallback_to_positional)
-    def _should_fallback_to_positional(self):
+    def _should_fallback_to_positional(self) -> bool:
         return False
 
     @doc(Index._convert_slice_indexer)

pandas/core/series.py

@@ -79,7 +79,6 @@
 from pandas.core.indexes.api import (
     Float64Index,
     Index,
-    IntervalIndex,
     InvalidIndexError,
     MultiIndex,
     ensure_index,
@@ -945,9 +944,7 @@ def _get_with(self, key):
         if key_type == "integer":
             # We need to decide whether to treat this as a positional indexer
             #  (i.e. self.iloc) or label-based (i.e. self.loc)
-            if self.index.is_integer() or self.index.is_floating():
-                return self.loc[key]
-            elif isinstance(self.index, IntervalIndex):
+            if not self.index._should_fallback_to_positional():
                 return self.loc[key]
             else:
                 return self.iloc[key]
@@ -1070,7 +1067,7 @@ def _set_with(self, key, value):
             # Note: key_type == "boolean" should not occur because that
             #  should be caught by the is_bool_indexer check in __setitem__
             if key_type == "integer":
-                if self.index.inferred_type == "integer":
+                if not self.index._should_fallback_to_positional():
                     self._set_labels(key, value)
                 else:
                     self._set_values(key, value)

pandas/tests/series/indexing/test_getitem.py

@@ -90,6 +90,18 @@ def test_getitem_intlist_intindex_periodvalues(self):
         tm.assert_series_equal(result, exp)
         assert result.dtype == "Period[D]"
 
+    @pytest.mark.parametrize("box", [list, np.array, pd.Index])
+    def test_getitem_intlist_intervalindex_non_int(self, box):
+        # GH#33404 fall back to positional since ints are unambiguous
+        dti = date_range("2000-01-03", periods=3)
+        ii = pd.IntervalIndex.from_breaks(dti)
+        ser = Series(range(len(ii)), index=ii)
+
+        expected = ser.iloc[:1]
+        key = box([0])
+        result = ser[key]
+        tm.assert_series_equal(result, expected)
+
 
 def test_getitem_generator(string_series):
     gen = (x > 0 for x in string_series)

pandas/tests/series/indexing/test_indexing.py

@@ -159,9 +159,9 @@ def test_getitem_out_of_bounds(datetime_series):
         datetime_series[len(datetime_series)]
 
     # GH #917
-    msg = r"index -\d+ is out of bounds for axis 0 with size \d+"
+    # With a RangeIndex, an int key gives a KeyError
     s = Series([], dtype=object)
-    with pytest.raises(IndexError, match=msg):
+    with pytest.raises(KeyError, match="-1"):
         s[-1]