PERF: RangeIndex.__getitem__ with integers return RangeIndex (#57770)

* PERF: RangeIndex.take with 1 value return RangeIndex

* add issue number

* Move to _shallow_copy, support empty join as well

* Fix self.name

* FIx error message

* Fix hdf test

* PERF: RangeIndex.__getitem__ with integers return RangeIndex

* PERF: RangeIndex.__getitem__ with integers return RangeIndex

* Handle ellipse

* Catch ValueError

Author: mroeschke

doc/source/whatsnew/v3.0.0.rst

@@ -272,11 +272,11 @@ Performance improvements
 - Performance improvement in :meth:`Index.join` by propagating cached attributes in cases where the result matches one of the inputs (:issue:`57023`)
 - Performance improvement in :meth:`Index.take` when ``indices`` is a full range indexer from zero to length of index (:issue:`56806`)
 - Performance improvement in :meth:`MultiIndex.equals` for equal length indexes (:issue:`56990`)
-- Performance improvement in :meth:`RangeIndex.__getitem__` with a boolean mask returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57588`)
+- Performance improvement in :meth:`RangeIndex.__getitem__` with a boolean mask or integers returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57588`)
 - Performance improvement in :meth:`RangeIndex.append` when appending the same index (:issue:`57252`)
-- Performance improvement in :meth:`RangeIndex.join` returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57651`)
-- Performance improvement in :meth:`RangeIndex.reindex` returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57647`)
-- Performance improvement in :meth:`RangeIndex.take` returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57445`)
+- Performance improvement in :meth:`RangeIndex.join` returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57651`, :issue:`57752`)
+- Performance improvement in :meth:`RangeIndex.reindex` returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57647`, :issue:`57752`)
+- Performance improvement in :meth:`RangeIndex.take` returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57445`, :issue:`57752`)
 - Performance improvement in ``DataFrameGroupBy.__len__`` and ``SeriesGroupBy.__len__`` (:issue:`57595`)
 - Performance improvement in indexing operations for string dtypes (:issue:`56997`)
 

pandas/core/indexes/base.py

@@ -4235,7 +4235,6 @@ def join(
 
         return self._join_via_get_indexer(other, how, sort)
 
-    @final
     def _join_empty(
         self, other: Index, how: JoinHow, sort: bool
     ) -> tuple[Index, npt.NDArray[np.intp] | None, npt.NDArray[np.intp] | None]:

pandas/core/indexes/range.py

@@ -472,18 +472,31 @@ def _shallow_copy(self, values, name: Hashable = no_default):
 
         if values.dtype.kind == "f":
             return Index(values, name=name, dtype=np.float64)
-        if values.dtype.kind == "i" and values.ndim == 1 and len(values) > 1:
+        if values.dtype.kind == "i" and values.ndim == 1:
             # GH 46675 & 43885: If values is equally spaced, return a
             # more memory-compact RangeIndex instead of Index with 64-bit dtype
+            if len(values) == 0:
+                return type(self)._simple_new(_empty_range, name=name)
+            elif len(values) == 1:
+                start = values[0]
+                new_range = range(start, start + self.step, self.step)
+                return type(self)._simple_new(new_range, name=name)
             diff = values[1] - values[0]
             if not missing.isna(diff) and diff != 0:
-                maybe_range_indexer, remainder = np.divmod(values - values[0], diff)
-                if (
-                    lib.is_range_indexer(maybe_range_indexer, len(maybe_range_indexer))
-                    and not remainder.any()
-                ):
+                if len(values) == 2:
+                    # Can skip is_range_indexer check
                     new_range = range(values[0], values[-1] + diff, diff)
                     return type(self)._simple_new(new_range, name=name)
+                else:
+                    maybe_range_indexer, remainder = np.divmod(values - values[0], diff)
+                    if (
+                        lib.is_range_indexer(
+                            maybe_range_indexer, len(maybe_range_indexer)
+                        )
+                        and not remainder.any()
+                    ):
+                        new_range = range(values[0], values[-1] + diff, diff)
+                        return type(self)._simple_new(new_range, name=name)
         return self._constructor._simple_new(values, name=name)
 
     def _view(self) -> Self:
@@ -894,12 +907,19 @@ def symmetric_difference(
             result = result.rename(result_name)
         return result
 
+    def _join_empty(
+        self, other: Index, how: JoinHow, sort: bool
+    ) -> tuple[Index, npt.NDArray[np.intp] | None, npt.NDArray[np.intp] | None]:
+        if other.dtype.kind == "i":
+            other = self._shallow_copy(other._values, name=other.name)
+        return super()._join_empty(other, how=how, sort=sort)
+
     def _join_monotonic(
         self, other: Index, how: JoinHow = "left"
     ) -> tuple[Index, npt.NDArray[np.intp] | None, npt.NDArray[np.intp] | None]:
         # This currently only gets called for the monotonic increasing case
         if not isinstance(other, type(self)):
-            maybe_ri = self._shallow_copy(other._values)
+            maybe_ri = self._shallow_copy(other._values, name=other.name)
             if not isinstance(maybe_ri, type(self)):
                 return super()._join_monotonic(other, how=how)
             other = maybe_ri
@@ -1075,6 +1095,8 @@ def __getitem__(self, key):
         """
         Conserve RangeIndex type for scalar and slice keys.
         """
+        if key is Ellipsis:
+            key = slice(None)
         if isinstance(key, slice):
             return self._getitem_slice(key)
         elif is_integer(key):
@@ -1094,17 +1116,20 @@ def __getitem__(self, key):
             )
         elif com.is_bool_indexer(key):
             if isinstance(getattr(key, "dtype", None), ExtensionDtype):
-                np_key = key.to_numpy(dtype=bool, na_value=False)
+                key = key.to_numpy(dtype=bool, na_value=False)
             else:
-                np_key = np.asarray(key, dtype=bool)
-            check_array_indexer(self._range, np_key)  # type: ignore[arg-type]
+                key = np.asarray(key, dtype=bool)
+            check_array_indexer(self._range, key)  # type: ignore[arg-type]
             # Short circuit potential _shallow_copy check
-            if np_key.all():
+            if key.all():
                 return self._simple_new(self._range, name=self.name)
-            elif not np_key.any():
+            elif not key.any():
                 return self._simple_new(_empty_range, name=self.name)
-            return self.take(np.flatnonzero(np_key))
-        return super().__getitem__(key)
+            key = np.flatnonzero(key)
+        try:
+            return self.take(key)
+        except (TypeError, ValueError):
+            return super().__getitem__(key)
 
     def _getitem_slice(self, slobj: slice) -> Self:
         """

pandas/tests/indexes/ranges/test_join.py

@@ -207,9 +207,15 @@ def test_join_self(self, join_type):
             [-1, -1, 0, 1],
             "outer",
         ],
+        [RangeIndex(2), RangeIndex(0), RangeIndex(2), None, [-1, -1], "left"],
+        [RangeIndex(2), RangeIndex(0), RangeIndex(0), [], None, "right"],
+        [RangeIndex(2), RangeIndex(0), RangeIndex(0), [], None, "inner"],
+        [RangeIndex(2), RangeIndex(0), RangeIndex(2), None, [-1, -1], "outer"],
     ],
 )
-@pytest.mark.parametrize("right_type", [RangeIndex, lambda x: Index(list(x))])
+@pytest.mark.parametrize(
+    "right_type", [RangeIndex, lambda x: Index(list(x), dtype=x.dtype)]
+)
 def test_join_preserves_rangeindex(
     left, right, expected, expected_lidx, expected_ridx, how, right_type
 ):

pandas/tests/indexes/ranges/test_range.py

@@ -608,6 +608,26 @@ def test_range_index_rsub_by_const(self):
         tm.assert_index_equal(result, expected)
 
 
+def test_reindex_1_value_returns_rangeindex():
+    ri = RangeIndex(0, 10, 2, name="foo")
+    result, result_indexer = ri.reindex([2])
+    expected = RangeIndex(2, 4, 2, name="foo")
+    tm.assert_index_equal(result, expected, exact=True)
+
+    expected_indexer = np.array([1], dtype=np.intp)
+    tm.assert_numpy_array_equal(result_indexer, expected_indexer)
+
+
+def test_reindex_empty_returns_rangeindex():
+    ri = RangeIndex(0, 10, 2, name="foo")
+    result, result_indexer = ri.reindex([])
+    expected = RangeIndex(0, 0, 2, name="foo")
+    tm.assert_index_equal(result, expected, exact=True)
+
+    expected_indexer = np.array([], dtype=np.intp)
+    tm.assert_numpy_array_equal(result_indexer, expected_indexer)
+
+
 def test_append_non_rangeindex_return_rangeindex():
     ri = RangeIndex(1)
     result = ri.append(Index([1]))
@@ -653,6 +673,21 @@ def test_take_return_rangeindex():
     tm.assert_index_equal(result, expected, exact=True)
 
 
+@pytest.mark.parametrize(
+    "rng, exp_rng",
+    [
+        [range(5), range(3, 4)],
+        [range(0, -10, -2), range(-6, -8, -2)],
+        [range(0, 10, 2), range(6, 8, 2)],
+    ],
+)
+def test_take_1_value_returns_rangeindex(rng, exp_rng):
+    ri = RangeIndex(rng, name="foo")
+    result = ri.take([3])
+    expected = RangeIndex(exp_rng, name="foo")
+    tm.assert_index_equal(result, expected, exact=True)
+
+
 def test_append_one_nonempty_preserve_step():
     expected = RangeIndex(0, -1, -1)
     result = RangeIndex(0).append([expected])
@@ -695,3 +730,25 @@ def test_getitem_boolmask_wrong_length():
     ri = RangeIndex(4, name="foo")
     with pytest.raises(IndexError, match="Boolean index has wrong length"):
         ri[[True]]
+
+
+def test_getitem_integers_return_rangeindex():
+    result = RangeIndex(0, 10, 2, name="foo")[[0, -1]]
+    expected = RangeIndex(start=0, stop=16, step=8, name="foo")
+    tm.assert_index_equal(result, expected, exact=True)
+
+    result = RangeIndex(0, 10, 2, name="foo")[[3]]
+    expected = RangeIndex(start=6, stop=8, step=2, name="foo")
+    tm.assert_index_equal(result, expected, exact=True)
+
+
+def test_getitem_empty_return_rangeindex():
+    result = RangeIndex(0, 10, 2, name="foo")[[]]
+    expected = RangeIndex(start=0, stop=0, step=1, name="foo")
+    tm.assert_index_equal(result, expected, exact=True)
+
+
+def test_getitem_integers_return_index():
+    result = RangeIndex(0, 10, 2, name="foo")[[0, 1, -1]]
+    expected = Index([0, 2, 8], dtype="int64", name="foo")
+    tm.assert_index_equal(result, expected)

pandas/tests/indexing/test_loc.py

@@ -509,7 +509,7 @@ def test_loc_getitem_list_with_fail(self):
 
         s.loc[[2]]
 
-        msg = f"\"None of [Index([3], dtype='{np.dtype(int)}')] are in the [index]"
+        msg = "None of [RangeIndex(start=3, stop=4, step=1)] are in the [index]"
         with pytest.raises(KeyError, match=re.escape(msg)):
             s.loc[[3]]
 

pandas/tests/io/pytables/test_append.py

@@ -968,6 +968,8 @@ def test_append_to_multiple_min_itemsize(setup_path):
         }
     )
     expected = df.iloc[[0]]
+    # Reading/writing RangeIndex info is not supported yet
+    expected.index = Index(list(range(len(expected.index))))
 
     with ensure_clean_store(setup_path) as store:
         store.append_to_multiple(