PERF: RangeIndex.insert (#43988)

Author: jbrockmendel

pandas/core/indexes/range.py

@@ -713,6 +713,20 @@ def symmetric_difference(self, other, result_name: Hashable = None, sort=None):
 
     # --------------------------------------------------------------------
 
+    def insert(self, loc: int, item) -> Index:
+        if len(self) and (is_integer(item) or is_float(item)):
+            # We can retain RangeIndex is inserting at the beginning or end
+            rng = self._range
+            if loc == 0 and item == self[0] - self.step:
+                new_rng = range(rng.start - rng.step, rng.stop, rng.step)
+                return type(self)._simple_new(new_rng, name=self.name)
+
+            elif loc == len(self) and item == self[-1] + self.step:
+                new_rng = range(rng.start, rng.stop + rng.step, rng.step)
+                return type(self)._simple_new(new_rng, name=self.name)
+
+        return super().insert(loc, item)
+
     def _concat(self, indexes: list[Index], name: Hashable) -> Index:
         """
         Overriding parent method for the case of all RangeIndex instances.

pandas/tests/indexes/common.py

@@ -821,7 +821,7 @@ def test_insert_non_na(self, simple_index):
             cls = Int64Index
 
         expected = cls([index[0]] + list(index), dtype=index.dtype)
-        tm.assert_index_equal(result, expected)
+        tm.assert_index_equal(result, expected, exact=True)
 
     def test_insert_na(self, nulls_fixture, simple_index):
         # GH 18295 (test missing)