REF: Avoid new object creation when reverse slicing when possible (#57902)

* REF: Avoid new objects when reverse slicing when possible

* Adjust test

* Remove astypes

* Fix typing

Author: mroeschke

pandas/core/arrays/datetimelike.py

@@ -2371,11 +2371,12 @@ def factorize(
     ):
         if self.freq is not None:
             # We must be unique, so can short-circuit (and retain freq)
-            codes = np.arange(len(self), dtype=np.intp)
-            uniques = self.copy()  # TODO: copy or view?
             if sort and self.freq.n < 0:
-                codes = codes[::-1]
-                uniques = uniques[::-1]
+                codes = np.arange(len(self) - 1, -1, -1, dtype=np.intp)
+                uniques = self[::-1]
+            else:
+                codes = np.arange(len(self), dtype=np.intp)
+                uniques = self.copy()  # TODO: copy or view?
             return codes, uniques
 
         if sort:

pandas/core/indexes/base.py

@@ -2116,7 +2116,7 @@ def droplevel(self, level: IndexLabel = 0):
         if not isinstance(level, (tuple, list)):
             level = [level]
 
-        levnums = sorted(self._get_level_number(lev) for lev in level)[::-1]
+        levnums = sorted((self._get_level_number(lev) for lev in level), reverse=True)
 
         return self._drop_level_numbers(levnums)
 

pandas/core/indexes/multi.py

@@ -3589,7 +3589,7 @@ def _reorder_indexer(
                 new_order = key_order_map[self.codes[i][indexer]]
             elif isinstance(k, slice) and k.step is not None and k.step < 0:
                 # flip order for negative step
-                new_order = np.arange(n)[::-1][indexer]
+                new_order = np.arange(n - 1, -1, -1)[indexer]
             elif isinstance(k, slice) and k.start is None and k.stop is None:
                 # slice(None) should not determine order GH#31330
                 new_order = np.ones((n,), dtype=np.intp)[indexer]

pandas/core/indexes/range.py

@@ -64,6 +64,12 @@
 _dtype_int64 = np.dtype(np.int64)
 
 
+def min_fitting_element(start: int, step: int, lower_limit: int) -> int:
+    """Returns the smallest element greater than or equal to the limit"""
+    no_steps = -(-(lower_limit - start) // abs(step))
+    return start + abs(step) * no_steps
+
+
 class RangeIndex(Index):
     """
     Immutable Index implementing a monotonic integer range.
@@ -570,25 +576,30 @@ def argsort(self, *args, **kwargs) -> npt.NDArray[np.intp]:
         kwargs.pop("kind", None)  # e.g. "mergesort" is irrelevant
         nv.validate_argsort(args, kwargs)
 
+        start, stop, step = None, None, None
         if self._range.step > 0:
-            result = np.arange(len(self), dtype=np.intp)
+            if ascending:
+                start = len(self)
+            else:
+                start, stop, step = len(self) - 1, -1, -1
+        elif ascending:
+            start, stop, step = len(self) - 1, -1, -1
         else:
-            result = np.arange(len(self) - 1, -1, -1, dtype=np.intp)
+            start = len(self)
 
-        if not ascending:
-            result = result[::-1]
-        return result
+        return np.arange(start, stop, step, dtype=np.intp)
 
     def factorize(
         self,
         sort: bool = False,
         use_na_sentinel: bool = True,
     ) -> tuple[npt.NDArray[np.intp], RangeIndex]:
-        codes = np.arange(len(self), dtype=np.intp)
-        uniques = self
         if sort and self.step < 0:
-            codes = codes[::-1]
-            uniques = uniques[::-1]
+            codes = np.arange(len(self) - 1, -1, -1, dtype=np.intp)
+            uniques = self[::-1]
+        else:
+            codes = np.arange(len(self), dtype=np.intp)
+            uniques = self
         return codes, uniques
 
     def equals(self, other: object) -> bool:
@@ -699,26 +710,15 @@ def _intersection(self, other: Index, sort: bool = False):
         # intersection disregarding the lower bounds
         tmp_start = first.start + (second.start - first.start) * first.step // gcd * s
         new_step = first.step * second.step // gcd
-        new_range = range(tmp_start, int_high, new_step)
-        new_index = self._simple_new(new_range)
 
         # adjust index to limiting interval
-        new_start = new_index._min_fitting_element(int_low)
-        new_range = range(new_start, new_index.stop, new_index.step)
-        new_index = self._simple_new(new_range)
+        new_start = min_fitting_element(tmp_start, new_step, int_low)
+        new_range = range(new_start, int_high, new_step)
 
-        if (self.step < 0 and other.step < 0) is not (new_index.step < 0):
-            new_index = new_index[::-1]
+        if (self.step < 0 and other.step < 0) is not (new_range.step < 0):
+            new_range = new_range[::-1]
 
-        if sort is None:
-            new_index = new_index.sort_values()
-
-        return new_index
-
-    def _min_fitting_element(self, lower_limit: int) -> int:
-        """Returns the smallest element greater than or equal to the limit"""
-        no_steps = -(-(lower_limit - self.start) // abs(self.step))
-        return self.start + abs(self.step) * no_steps
+        return self._simple_new(new_range)
 
     def _extended_gcd(self, a: int, b: int) -> tuple[int, int, int]:
         """
@@ -904,9 +904,9 @@ def _difference(self, other, sort=None):
                 # e.g. range(10) and range(0, 10, 3)
                 return super()._difference(other, sort=sort)
 
-        new_index = type(self)._simple_new(new_rng, name=res_name)
         if first is not self._range:
-            new_index = new_index[::-1]
+            new_rng = new_rng[::-1]
+        new_index = type(self)._simple_new(new_rng, name=res_name)
 
         return new_index
 

pandas/core/indexing.py

@@ -1145,7 +1145,7 @@ def _contains_slice(x: object) -> bool:
         # GH#41369 Loop in reverse order ensures indexing along columns before rows
         # which selects only necessary blocks which avoids dtype conversion if possible
         axis = len(tup) - 1
-        for key in tup[::-1]:
+        for key in reversed(tup):
             if com.is_null_slice(key):
                 axis -= 1
                 continue

pandas/core/internals/managers.py

@@ -1549,9 +1549,9 @@ def _insert_update_blklocs_and_blknos(self, loc) -> None:
             self._blklocs = np.append(self._blklocs, 0)
             self._blknos = np.append(self._blknos, len(self.blocks))
         elif loc == 0:
-            # np.append is a lot faster, let's use it if we can.
-            self._blklocs = np.append(self._blklocs[::-1], 0)[::-1]
-            self._blknos = np.append(self._blknos[::-1], len(self.blocks))[::-1]
+            # As of numpy 1.26.4, np.concatenate faster than np.append
+            self._blklocs = np.concatenate([[0], self._blklocs])
+            self._blknos = np.concatenate([[len(self.blocks)], self._blknos])
         else:
             new_blklocs, new_blknos = libinternals.update_blklocs_and_blknos(
                 self.blklocs, self.blknos, loc, len(self.blocks)

pandas/core/reshape/reshape.py

@@ -910,9 +910,10 @@ def stack_v3(frame: DataFrame, level: list[int]) -> Series | DataFrame:
         raise ValueError("Columns with duplicate values are not supported in stack")
 
     # If we need to drop `level` from columns, it needs to be in descending order
+    set_levels = set(level)
     drop_levnums = sorted(level, reverse=True)
     stack_cols = frame.columns._drop_level_numbers(
-        [k for k in range(frame.columns.nlevels) if k not in level][::-1]
+        [k for k in range(frame.columns.nlevels - 1, -1, -1) if k not in set_levels]
     )
     if len(level) > 1:
         # Arrange columns in the order we want to take them, e.g. level=[2, 0, 1]
@@ -936,7 +937,7 @@ def stack_v3(frame: DataFrame, level: list[int]) -> Series | DataFrame:
                 idx = (idx,)
             gen = iter(idx)
             column_indexer = tuple(
-                next(gen) if k in level else slice(None)
+                next(gen) if k in set_levels else slice(None)
                 for k in range(frame.columns.nlevels)
             )
             data = frame.loc[:, column_indexer]

pandas/core/series.py

@@ -5510,9 +5510,9 @@ def case_when(
             replacements = updated_replacements
             default = default.astype(common_dtype)
 
-        counter = reversed(range(len(conditions)))
+        counter = range(len(conditions) - 1, -1, -1)
         for position, condition, replacement in zip(
-            counter, conditions[::-1], replacements[::-1]
+            counter, reversed(conditions), reversed(replacements)
         ):
             try:
                 default = default.mask(

pandas/core/sorting.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import itertools
 from typing import (
     TYPE_CHECKING,
     Callable,
@@ -334,13 +335,15 @@ def lexsort_indexer(
         raise ValueError(f"invalid na_position: {na_position}")
 
     if isinstance(orders, bool):
-        orders = [orders] * len(keys)
+        orders = itertools.repeat(orders, len(keys))
     elif orders is None:
-        orders = [True] * len(keys)
+        orders = itertools.repeat(True, len(keys))
+    else:
+        orders = reversed(orders)
 
     labels = []
 
-    for k, order in zip(keys, orders):
+    for k, order in zip(reversed(keys), orders):
         k = ensure_key_mapped(k, key)
         if codes_given:
             codes = cast(np.ndarray, k)
@@ -361,7 +364,7 @@ def lexsort_indexer(
 
         labels.append(codes)
 
-    return np.lexsort(labels[::-1])
+    return np.lexsort(labels)
 
 
 def nargsort(

pandas/tests/indexes/ranges/test_range.py

@@ -9,6 +9,7 @@
     RangeIndex,
 )
 import pandas._testing as tm
+from pandas.core.indexes.range import min_fitting_element
 
 
 class TestRangeIndex:
@@ -419,21 +420,21 @@ def test_extended_gcd(self, simple_index):
         assert 2 == result[0]
 
     def test_min_fitting_element(self):
-        result = RangeIndex(0, 20, 2)._min_fitting_element(1)
+        result = min_fitting_element(0, 2, 1)
         assert 2 == result
 
-        result = RangeIndex(1, 6)._min_fitting_element(1)
+        result = min_fitting_element(1, 1, 1)
         assert 1 == result
 
-        result = RangeIndex(18, -2, -2)._min_fitting_element(1)
+        result = min_fitting_element(18, -2, 1)
         assert 2 == result
 
-        result = RangeIndex(5, 0, -1)._min_fitting_element(1)
+        result = min_fitting_element(5, -1, 1)
         assert 1 == result
 
         big_num = 500000000000000000000000
 
-        result = RangeIndex(5, big_num * 2, 1)._min_fitting_element(big_num)
+        result = min_fitting_element(5, 1, big_num)
         assert big_num == result
 
     def test_slice_specialised(self, simple_index):

pandas/tests/indexes/ranges/test_setops.py

@@ -93,12 +93,12 @@ def test_intersection(self, sort):
         # GH 17296: intersect two decreasing RangeIndexes
         first = RangeIndex(10, -2, -2)
         other = RangeIndex(5, -4, -1)
-        expected = first.astype(int).intersection(other.astype(int), sort=sort)
-        result = first.intersection(other, sort=sort).astype(int)
+        expected = RangeIndex(start=4, stop=-2, step=-2)
+        result = first.intersection(other, sort=sort)
         tm.assert_index_equal(result, expected)
 
         # reversed
-        result = other.intersection(first, sort=sort).astype(int)
+        result = other.intersection(first, sort=sort)
         tm.assert_index_equal(result, expected)
 
         index = RangeIndex(5, name="foo")