From 7d51e13676326b19cac400dc715793c576944944 Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Thu, 8 Dec 2022 16:23:14 +0100
Subject: [PATCH 1/2] Backport PR #49771: PERF: Use fastpath for accessing
 option value in internals

---
 pandas/core/internals/managers.py        |  7 ++++--
 pandas/tests/copy_view/test_internals.py | 30 ++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index f55fcead61fae..9641cdc9999ed 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -15,7 +15,7 @@
 
 import numpy as np
 
-from pandas._config import get_option
+from pandas._config.config import _global_config
 
 from pandas._libs import (
     algos as libalgos,
@@ -2424,5 +2424,8 @@ def _preprocess_slice_or_indexer(
         return "fancy", indexer, len(indexer)
 
 
+_mode_options = _global_config["mode"]
+
+
 def _using_copy_on_write():
-    return get_option("mode.copy_on_write")
+    return _mode_options["copy_on_write"]
diff --git a/pandas/tests/copy_view/test_internals.py b/pandas/tests/copy_view/test_internals.py
index edfa7f843f17f..7a2965f2e1c61 100644
--- a/pandas/tests/copy_view/test_internals.py
+++ b/pandas/tests/copy_view/test_internals.py
@@ -3,6 +3,7 @@
 
 import pandas.util._test_decorators as td
 
+import pandas as pd
 from pandas import DataFrame
 from pandas.tests.copy_view.util import get_array
 
@@ -62,3 +63,32 @@ def test_clear_parent(using_copy_on_write):
     # when losing the last reference, also the parent should be reset
     subset["b"] = 0
     assert subset._mgr.parent is None
+
+
+@td.skip_array_manager_invalid_test
+def test_switch_options():
+    # ensure we can switch the value of the option within one session
+    # (assuming data is constructed after switching)
+
+    # using the option_context to ensure we set back to global option value
+    # after running the test
+    with pd.option_context("mode.copy_on_write", False):
+        df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+        subset = df[:]
+        subset.iloc[0, 0] = 0
+        # df updated with CoW disabled
+        assert df.iloc[0, 0] == 0
+
+        pd.options.mode.copy_on_write = True
+        df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+        subset = df[:]
+        subset.iloc[0, 0] = 0
+        # df not updated with CoW enabled
+        assert df.iloc[0, 0] == 1
+
+        pd.options.mode.copy_on_write = False
+        df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+        subset = df[:]
+        subset.iloc[0, 0] = 0
+        # df updated with CoW disabled
+        assert df.iloc[0, 0] == 0

From a9920df1463e2cb4f2f8a7a2e0a31301a4a35441 Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Fri, 13 Jan 2023 10:08:02 +0100
Subject: [PATCH 2/2] don't run options tests in parallel

---
 pandas/tests/copy_view/test_internals.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pandas/tests/copy_view/test_internals.py b/pandas/tests/copy_view/test_internals.py
index 7a2965f2e1c61..1938a1c58fe3d 100644
--- a/pandas/tests/copy_view/test_internals.py
+++ b/pandas/tests/copy_view/test_internals.py
@@ -65,6 +65,7 @@ def test_clear_parent(using_copy_on_write):
     assert subset._mgr.parent is None
 
 
+@pytest.mark.single_cpu
 @td.skip_array_manager_invalid_test
 def test_switch_options():
     # ensure we can switch the value of the option within one session