PERF: Allow Index.to_frame to return RangeIndex columns (#58018)

doc/source/whatsnew/v3.0.0.rst

@@ -299,6 +299,7 @@ Performance improvements
 - Performance improvement in :meth:`DataFrameGroupBy.ffill`, :meth:`DataFrameGroupBy.bfill`, :meth:`SeriesGroupBy.ffill`, and :meth:`SeriesGroupBy.bfill` (:issue:`56902`)
 - 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:`Index.to_frame` returning a :class:`RangeIndex` columns of a :class:`Index` when possible. (:issue:`58018`)
 - Performance improvement in :meth:`MultiIndex.equals` for equal length indexes (:issue:`56990`)
 - 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`)

pandas/core/indexes/base.py

@@ -1374,16 +1374,19 @@ def _format_attrs(self) -> list[tuple[str_t, str_t | int | bool | None]]:
         return attrs
 
     @final
-    def _get_level_names(self) -> Hashable | Sequence[Hashable]:
+    def _get_level_names(self) -> range | Sequence[Hashable]:
         """
         Return a name or list of names with None replaced by the level number.
         """
         if self._is_multi:
-            return [
-                level if name is None else name for level, name in enumerate(self.names)
-            ]
+            return maybe_sequence_to_range(
+                [
+                    level if name is None else name
+                    for level, name in enumerate(self.names)
+                ]
+            )
         else:
-            return 0 if self.name is None else self.name
+            return range(1) if self.name is None else [self.name]
 
     @final
     def _mpl_repr(self) -> np.ndarray:
@@ -1630,8 +1633,11 @@ def to_frame(
         from pandas import DataFrame
 
         if name is lib.no_default:
-            name = self._get_level_names()
-        result = DataFrame({name: self}, copy=False)
+            result_name = self._get_level_names()
+        else:
+            result_name = Index([name])  # type: ignore[assignment]
+        result = DataFrame(self, copy=False)
+        result.columns = result_name
 
         if index:
             result.index = self

pandas/tests/indexes/multi/test_conversion.py

@@ -5,6 +5,7 @@
 from pandas import (
     DataFrame,
     MultiIndex,
+    RangeIndex,
 )
 import pandas._testing as tm
 
@@ -148,6 +149,13 @@ def test_to_frame_duplicate_labels():
     tm.assert_frame_equal(result, expected)
 
 
+def test_to_frame_column_rangeindex():
+    mi = MultiIndex.from_arrays([[1, 2], ["a", "b"]])
+    result = mi.to_frame().columns
+    expected = RangeIndex(2)
+    tm.assert_index_equal(result, expected, exact=True)
+
+
 def test_to_flat_index(idx):
     expected = pd.Index(
         (

pandas/tests/indexes/test_common.py

@@ -508,3 +508,17 @@ def test_compare_read_only_array():
     idx = pd.Index(arr)
     result = idx > 69
     assert result.dtype == bool
+
+
+def test_to_frame_column_rangeindex():
+    idx = pd.Index([1])
+    result = idx.to_frame().columns
+    expected = RangeIndex(1)
+    tm.assert_index_equal(result, expected, exact=True)
+
+
+def test_to_frame_name_tuple_multiindex():
+    idx = pd.Index([1])
+    result = idx.to_frame(name=(1, 2))
+    expected = pd.DataFrame([1], columns=MultiIndex.from_arrays([[1], [2]]), index=idx)
+    tm.assert_frame_equal(result, expected)