From 011c8d2266d2ef20ff3cd385504efdd03bf0ebdc Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Mon, 2 Nov 2020 13:11:47 -0500
Subject: [PATCH 01/16] TST Adds failing test

---
 sklearn/tests/test_config.py | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index eec349861258c..6d7a5a16b4684 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -1,5 +1,9 @@
+import time
+from joblib import Parallel
+
 from sklearn import get_config, set_config, config_context
 from sklearn.utils._testing import assert_raises
+from sklearn.utils.fixes import delayed
 
 
 def test_config_context():
@@ -72,3 +76,20 @@ def test_set_config():
 
     # No unknown arguments
     assert_raises(TypeError, set_config, do_something_else=True)
+
+
+def test_config_threadsafe():
+    """Test that the global config is threadsafe."""
+
+    def set_assume_finite(assume_finite, sleep_dur):
+        with config_context(assume_finite=assume_finite):
+            time.sleep(sleep_dur)
+            return get_config()['assume_finite']
+
+    booleans = [False, True]
+    sleep_seconds = [0.5, 1]
+    items = Parallel(backend='threading', n_jobs=2)(
+        delayed(set_assume_finite)(value, sleep_dur)
+        for value, sleep_dur in zip(booleans, sleep_seconds))
+
+    assert items == [False, True]

From 59b4a8e2299d22b3487a97a0be860ddb3b798127 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Mon, 2 Nov 2020 13:58:16 -0500
Subject: [PATCH 02/16] ENH Makes copies of global configuration per thread

---
 sklearn/_config.py           | 17 ++++++++++++-----
 sklearn/tests/test_config.py | 21 +++++++++++++--------
 2 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index feb5e86287c38..b54c27bdf8b07 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -2,6 +2,7 @@
 """
 import os
 from contextlib import contextmanager as contextmanager
+import threading
 
 _global_config = {
     'assume_finite': bool(os.environ.get('SKLEARN_ASSUME_FINITE', False)),
@@ -9,6 +10,7 @@
     'print_changed_only': True,
     'display': 'text',
 }
+_threadlocal = threading.local()
 
 
 def get_config():
@@ -24,7 +26,9 @@ def get_config():
     config_context : Context manager for global scikit-learn configuration.
     set_config : Set global scikit-learn configuration.
     """
-    return _global_config.copy()
+    if not hasattr(_threadlocal, 'global_config'):
+        _threadlocal.global_config = _global_config.copy()
+    return _threadlocal.global_config.copy()
 
 
 def set_config(assume_finite=None, working_memory=None,
@@ -72,14 +76,17 @@ def set_config(assume_finite=None, working_memory=None,
     config_context : Context manager for global scikit-learn configuration.
     get_config : Retrieve current values of the global configuration.
     """
+    if not hasattr(_threadlocal, 'global_config'):
+        _threadlocal.global_config = _global_config.copy()
+
     if assume_finite is not None:
-        _global_config['assume_finite'] = assume_finite
+        _threadlocal.global_config['assume_finite'] = assume_finite
     if working_memory is not None:
-        _global_config['working_memory'] = working_memory
+        _threadlocal.global_config['working_memory'] = working_memory
     if print_changed_only is not None:
-        _global_config['print_changed_only'] = print_changed_only
+        _threadlocal.global_config['print_changed_only'] = print_changed_only
     if display is not None:
-        _global_config['display'] = display
+        _threadlocal.global_config['display'] = display
 
 
 @contextmanager
diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index 6d7a5a16b4684..5843f07048a24 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -1,5 +1,7 @@
 import time
+
 from joblib import Parallel
+import pytest
 
 from sklearn import get_config, set_config, config_context
 from sklearn.utils._testing import assert_raises
@@ -78,17 +80,20 @@ def test_set_config():
     assert_raises(TypeError, set_config, do_something_else=True)
 
 
-def test_config_threadsafe():
-    """Test that the global config is threadsafe."""
+def set_assume_finite(assume_finite, sleep_dur):
+    with config_context(assume_finite=assume_finite):
+        time.sleep(sleep_dur)
+        return get_config()['assume_finite']
 
-    def set_assume_finite(assume_finite, sleep_dur):
-        with config_context(assume_finite=assume_finite):
-            time.sleep(sleep_dur)
-            return get_config()['assume_finite']
+
+@pytest.mark.parametrize("backend",
+                         ["loky", "multiprocessing", "threading"])
+def test_config_threadsafe(backend):
+    """Test that the global config is threadsafe."""
 
     booleans = [False, True]
-    sleep_seconds = [0.5, 1]
-    items = Parallel(backend='threading', n_jobs=2)(
+    sleep_seconds = [0.1, 0.2]
+    items = Parallel(backend=backend, n_jobs=2)(
         delayed(set_assume_finite)(value, sleep_dur)
         for value, sleep_dur in zip(booleans, sleep_seconds))
 

From f6a135786cf8b5af849dd59c664a343ae4a9e328 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Mon, 2 Nov 2020 14:30:14 -0500
Subject: [PATCH 03/16] TST Skip loky tests for joblib version < 0.12

---
 sklearn/tests/test_config.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index 5843f07048a24..a49caae5b4082 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -1,11 +1,13 @@
 import time
 
 from joblib import Parallel
+import joblib
 import pytest
 
 from sklearn import get_config, set_config, config_context
 from sklearn.utils._testing import assert_raises
 from sklearn.utils.fixes import delayed
+from sklearn.utils.fixes import parse_version
 
 
 def test_config_context():
@@ -91,8 +93,13 @@ def set_assume_finite(assume_finite, sleep_dur):
 def test_config_threadsafe(backend):
     """Test that the global config is threadsafe."""
 
+    if (parse_version(joblib.__version__) < parse_version('0.12')
+            and backend == 'loky'):
+        pytest.skip('loky backend does not exist in joblib <0.12')
+
     booleans = [False, True]
     sleep_seconds = [0.1, 0.2]
+
     items = Parallel(backend=backend, n_jobs=2)(
         delayed(set_assume_finite)(value, sleep_dur)
         for value, sleep_dur in zip(booleans, sleep_seconds))

From 99a4ce16f29ee05d6094db1b8fe4ec4242d2865e Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Thu, 26 Nov 2020 13:30:06 -0500
Subject: [PATCH 04/16] ENH Small refactor

---
 sklearn/_config.py           | 12 ++++++++----
 sklearn/tests/test_config.py |  2 +-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index b54c27bdf8b07..68242080b4080 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -13,6 +13,12 @@
 _threadlocal = threading.local()
 
 
+def _set_threadlocal():
+    """Set the configuration that is local to the thread."""
+    if not hasattr(_threadlocal, 'global_config'):
+        _threadlocal.global_config = _global_config.copy()
+
+
 def get_config():
     """Retrieve current values for configuration set by :func:`set_config`
 
@@ -26,8 +32,7 @@ def get_config():
     config_context : Context manager for global scikit-learn configuration.
     set_config : Set global scikit-learn configuration.
     """
-    if not hasattr(_threadlocal, 'global_config'):
-        _threadlocal.global_config = _global_config.copy()
+    _set_threadlocal()
     return _threadlocal.global_config.copy()
 
 
@@ -76,8 +81,7 @@ def set_config(assume_finite=None, working_memory=None,
     config_context : Context manager for global scikit-learn configuration.
     get_config : Retrieve current values of the global configuration.
     """
-    if not hasattr(_threadlocal, 'global_config'):
-        _threadlocal.global_config = _global_config.copy()
+    _set_threadlocal()
 
     if assume_finite is not None:
         _threadlocal.global_config['assume_finite'] = assume_finite
diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index a49caae5b4082..9b040e2376ff4 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -95,7 +95,7 @@ def test_config_threadsafe(backend):
 
     if (parse_version(joblib.__version__) < parse_version('0.12')
             and backend == 'loky'):
-        pytest.skip('loky backend does not exist in joblib <0.12')
+        pytest.skip('loky backend does not exist in joblib <0.12')  # noqa
 
     booleans = [False, True]
     sleep_seconds = [0.1, 0.2]

From 966498d956dae0253a91049d340a451d8bd3da9a Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Thu, 26 Nov 2020 17:28:06 -0500
Subject: [PATCH 05/16] DOC Update docstring of test

---
 sklearn/tests/test_config.py | 33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index 9b040e2376ff4..86e376e10eb2a 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -1,4 +1,5 @@
 import time
+from concurrent.futures import ThreadPoolExecutor
 
 from joblib import Parallel
 import joblib
@@ -83,6 +84,7 @@ def test_set_config():
 
 
 def set_assume_finite(assume_finite, sleep_dur):
+    """Return the value of assume_finite after waiting `sleep_dur`."""
     with config_context(assume_finite=assume_finite):
         time.sleep(sleep_dur)
         return get_config()['assume_finite']
@@ -90,18 +92,39 @@ def set_assume_finite(assume_finite, sleep_dur):
 
 @pytest.mark.parametrize("backend",
                          ["loky", "multiprocessing", "threading"])
-def test_config_threadsafe(backend):
-    """Test that the global config is threadsafe."""
+def test_config_threadsafe_joblib(backend):
+    """Test that the global config is threadsafe with all joblib backends.
+    Two jobs are spawned and sets assume_finite to two different values.
+    When the job with a duration 0.1s completes, the assume_finite value
+    should be the same as the value passed to the function. In other words,
+    it is not influenced by the other job setting assume_finite to True.
+    """
 
     if (parse_version(joblib.__version__) < parse_version('0.12')
             and backend == 'loky'):
         pytest.skip('loky backend does not exist in joblib <0.12')  # noqa
 
-    booleans = [False, True]
+    assume_finite_bools = [False, True]
     sleep_seconds = [0.1, 0.2]
 
     items = Parallel(backend=backend, n_jobs=2)(
-        delayed(set_assume_finite)(value, sleep_dur)
-        for value, sleep_dur in zip(booleans, sleep_seconds))
+        delayed(set_assume_finite)(assume_finite, sleep_dur)
+        for assume_finite, sleep_dur
+        in zip(assume_finite_bools, sleep_seconds))
+
+    assert items == [False, True]
+
+
+def test_config_threadsafe():
+    """Uses threads directly to test that the global config does not change
+    between threads. Same test as `test_config_threadsafe_joblib` by with
+    `ThreadPoolExecutor`."""
+
+    assume_finite_bools = [False, True]
+    sleep_seconds = [0.1, 0.2]
+
+    with ThreadPoolExecutor(max_workers=2) as e:
+        items = [output for output in
+                 e.map(set_assume_finite, assume_finite_bools, sleep_seconds)]
 
     assert items == [False, True]

From df291d9861baa6ddd9661a5dfcfac03b7f4de85a Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Thu, 26 Nov 2020 17:31:53 -0500
Subject: [PATCH 06/16] DOC Update names

---
 sklearn/tests/test_config.py | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index 86e376e10eb2a..b611808b7ec4d 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -83,10 +83,10 @@ def test_set_config():
     assert_raises(TypeError, set_config, do_something_else=True)
 
 
-def set_assume_finite(assume_finite, sleep_dur):
-    """Return the value of assume_finite after waiting `sleep_dur`."""
+def set_assume_finite(assume_finite, sleep_duration):
+    """Return the value of assume_finite after waiting `sleep_duration`."""
     with config_context(assume_finite=assume_finite):
-        time.sleep(sleep_dur)
+        time.sleep(sleep_duration)
         return get_config()['assume_finite']
 
 
@@ -104,13 +104,13 @@ def test_config_threadsafe_joblib(backend):
             and backend == 'loky'):
         pytest.skip('loky backend does not exist in joblib <0.12')  # noqa
 
-    assume_finite_bools = [False, True]
-    sleep_seconds = [0.1, 0.2]
+    assume_finites = [False, True]
+    sleep_durations = [0.1, 0.2]
 
     items = Parallel(backend=backend, n_jobs=2)(
         delayed(set_assume_finite)(assume_finite, sleep_dur)
         for assume_finite, sleep_dur
-        in zip(assume_finite_bools, sleep_seconds))
+        in zip(assume_finites, sleep_durations))
 
     assert items == [False, True]
 
@@ -120,11 +120,11 @@ def test_config_threadsafe():
     between threads. Same test as `test_config_threadsafe_joblib` by with
     `ThreadPoolExecutor`."""
 
-    assume_finite_bools = [False, True]
-    sleep_seconds = [0.1, 0.2]
+    assume_finites = [False, True]
+    sleep_durations = [0.1, 0.2]
 
     with ThreadPoolExecutor(max_workers=2) as e:
         items = [output for output in
-                 e.map(set_assume_finite, assume_finite_bools, sleep_seconds)]
+                 e.map(set_assume_finite, assume_finites, sleep_durations)]
 
     assert items == [False, True]

From ba93b54fb729e4919d7b484cbb15938c8e1b9542 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Fri, 9 Apr 2021 09:29:39 -0400
Subject: [PATCH 07/16] DOC Updates changelog and docstring about
 threadsafeness

---
 doc/whats_new/v1.0.rst | 5 +++++
 sklearn/_config.py     | 3 +--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/doc/whats_new/v1.0.rst b/doc/whats_new/v1.0.rst
index 602d4b1246878..e254aadc23f4e 100644
--- a/doc/whats_new/v1.0.rst
+++ b/doc/whats_new/v1.0.rst
@@ -76,6 +76,11 @@ Changelog
   - For :class:`tree.ExtraTreeRegressor`, `criterion="mse"` is deprecated,
     use `"squared_error"` instead which is now the default.
 
+:mod:`sklearn.base`
+...................
+
+- |Fix| :func:`config_context` is now threadsafe. :pr:`18736` by `Thomas Fan`_.
+
 :mod:`sklearn.calibration`
 ..........................
 
diff --git a/sklearn/_config.py b/sklearn/_config.py
index 68242080b4080..eb613dc29909e 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -131,8 +131,7 @@ def config_context(**new_config):
     Notes
     -----
     All settings, not just those presently modified, will be returned to
-    their previous values when the context manager is exited. This is not
-    thread-safe.
+    their previous values when the context manager is exited.
 
     Examples
     --------

From ea516c44c899e008c0f79abbd1bba40a09f3835f Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Fri, 9 Apr 2021 09:33:04 -0400
Subject: [PATCH 08/16] DOC Update test's docstring

---
 sklearn/tests/test_config.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index b611808b7ec4d..580cae0c31842 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -117,7 +117,7 @@ def test_config_threadsafe_joblib(backend):
 
 def test_config_threadsafe():
     """Uses threads directly to test that the global config does not change
-    between threads. Same test as `test_config_threadsafe_joblib` by with
+    between threads. Same test as `test_config_threadsafe_joblib` but with
     `ThreadPoolExecutor`."""
 
     assume_finites = [False, True]

From 6fe06197004124472490dd5d3eddfe2eb1a46b00 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Fri, 9 Apr 2021 09:36:55 -0400
Subject: [PATCH 09/16] ENH Cleaner code

---
 sklearn/_config.py | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index eb613dc29909e..9fba4d32d7748 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -13,10 +13,11 @@
 _threadlocal = threading.local()
 
 
-def _set_threadlocal():
+def _get_threadlocal():
     """Set the configuration that is local to the thread."""
     if not hasattr(_threadlocal, 'global_config'):
         _threadlocal.global_config = _global_config.copy()
+    return _threadlocal.global_config
 
 
 def get_config():
@@ -32,8 +33,7 @@ def get_config():
     config_context : Context manager for global scikit-learn configuration.
     set_config : Set global scikit-learn configuration.
     """
-    _set_threadlocal()
-    return _threadlocal.global_config.copy()
+    return _get_threadlocal()
 
 
 def set_config(assume_finite=None, working_memory=None,
@@ -81,16 +81,16 @@ def set_config(assume_finite=None, working_memory=None,
     config_context : Context manager for global scikit-learn configuration.
     get_config : Retrieve current values of the global configuration.
     """
-    _set_threadlocal()
+    local_config = _get_threadlocal()
 
     if assume_finite is not None:
-        _threadlocal.global_config['assume_finite'] = assume_finite
+        local_config['assume_finite'] = assume_finite
     if working_memory is not None:
-        _threadlocal.global_config['working_memory'] = working_memory
+        local_config['working_memory'] = working_memory
     if print_changed_only is not None:
-        _threadlocal.global_config['print_changed_only'] = print_changed_only
+        local_config['print_changed_only'] = print_changed_only
     if display is not None:
-        _threadlocal.global_config['display'] = display
+        local_config['display'] = display
 
 
 @contextmanager

From 91d18c116d71cf74b2cfb8e36eee22eb61b8ec14 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Fri, 9 Apr 2021 09:37:36 -0400
Subject: [PATCH 10/16] DOC Better docstring

---
 sklearn/_config.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index 9fba4d32d7748..56ab18482a07e 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -14,7 +14,8 @@
 
 
 def _get_threadlocal():
-    """Set the configuration that is local to the thread."""
+    """Get the configuration that is local to the thread. If the configuration
+    does not exist, copy the default global configuration."""
     if not hasattr(_threadlocal, 'global_config'):
         _threadlocal.global_config = _global_config.copy()
     return _threadlocal.global_config

From 276c720cd30a8bfb6717326422097fc2c6ff3836 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Sat, 10 Apr 2021 08:20:39 -0400
Subject: [PATCH 11/16] WIP Fixes merge error

---
 sklearn/tests/test_config.py | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/sklearn/tests/test_config.py b/sklearn/tests/test_config.py
index 23f341a1036aa..6d458088a37a8 100644
--- a/sklearn/tests/test_config.py
+++ b/sklearn/tests/test_config.py
@@ -6,7 +6,6 @@
 import pytest
 
 from sklearn import get_config, set_config, config_context
-from sklearn.utils._testing import assert_raises
 from sklearn.utils.fixes import delayed
 from sklearn.utils.fixes import parse_version
 
@@ -83,8 +82,8 @@ def test_set_config():
     assert get_config()['assume_finite'] is False
 
     # No unknown arguments
-<<<<<<< HEAD
-    assert_raises(TypeError, set_config, do_something_else=True)
+    with pytest.raises(TypeError):
+        set_config(do_something_else=True)
 
 
 def set_assume_finite(assume_finite, sleep_duration):
@@ -132,7 +131,3 @@ def test_config_threadsafe():
                  e.map(set_assume_finite, assume_finites, sleep_durations)]
 
     assert items == [False, True]
-=======
-    with pytest.raises(TypeError):
-        set_config(do_something_else=True)
->>>>>>> upstream/main

From 1b28965376158f13da51816cbae49e544312edf6 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Sat, 10 Apr 2021 08:24:18 -0400
Subject: [PATCH 12/16] CLN Less lines of code

---
 sklearn/_config.py | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index 56ab18482a07e..3530d2b7beb41 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -13,14 +13,6 @@
 _threadlocal = threading.local()
 
 
-def _get_threadlocal():
-    """Get the configuration that is local to the thread. If the configuration
-    does not exist, copy the default global configuration."""
-    if not hasattr(_threadlocal, 'global_config'):
-        _threadlocal.global_config = _global_config.copy()
-    return _threadlocal.global_config
-
-
 def get_config():
     """Retrieve current values for configuration set by :func:`set_config`
 
@@ -34,7 +26,11 @@ def get_config():
     config_context : Context manager for global scikit-learn configuration.
     set_config : Set global scikit-learn configuration.
     """
-    return _get_threadlocal()
+    # Get the configuration that is local to the thread. If the configuration
+    # does not exist, copy the default global configuration.
+    if not hasattr(_threadlocal, 'global_config'):
+        _threadlocal.global_config = _global_config.copy()
+    return _threadlocal.global_config
 
 
 def set_config(assume_finite=None, working_memory=None,
@@ -82,7 +78,7 @@ def set_config(assume_finite=None, working_memory=None,
     config_context : Context manager for global scikit-learn configuration.
     get_config : Retrieve current values of the global configuration.
     """
-    local_config = _get_threadlocal()
+    local_config = get_config()
 
     if assume_finite is not None:
         local_config['assume_finite'] = assume_finite

From 319f007b3f62a7dfa766765ff32e972fcf7cf390 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Sat, 10 Apr 2021 08:29:29 -0400
Subject: [PATCH 13/16] CLN Better copy logic

---
 sklearn/_config.py | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index 3530d2b7beb41..ba584717373db 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -13,6 +13,14 @@
 _threadlocal = threading.local()
 
 
+def _get_threadlocal_config():
+    """Get the configuration that is local to the thread. If the configuration
+    does not exist, copy the default global configuration."""
+    if not hasattr(_threadlocal, 'global_config'):
+        _threadlocal.global_config = _global_config.copy()
+    return _threadlocal.global_config
+
+
 def get_config():
     """Retrieve current values for configuration set by :func:`set_config`
 
@@ -26,11 +34,7 @@ def get_config():
     config_context : Context manager for global scikit-learn configuration.
     set_config : Set global scikit-learn configuration.
     """
-    # Get the configuration that is local to the thread. If the configuration
-    # does not exist, copy the default global configuration.
-    if not hasattr(_threadlocal, 'global_config'):
-        _threadlocal.global_config = _global_config.copy()
-    return _threadlocal.global_config
+    return _get_threadlocal_config().copy()
 
 
 def set_config(assume_finite=None, working_memory=None,
@@ -78,7 +82,7 @@ def set_config(assume_finite=None, working_memory=None,
     config_context : Context manager for global scikit-learn configuration.
     get_config : Retrieve current values of the global configuration.
     """
-    local_config = get_config()
+    local_config = _get_threadlocal_config()
 
     if assume_finite is not None:
         local_config['assume_finite'] = assume_finite
@@ -148,7 +152,7 @@ def config_context(**new_config):
     set_config : Set global scikit-learn configuration.
     get_config : Retrieve current values of the global configuration.
     """
-    old_config = get_config().copy()
+    old_config = _get_threadlocal_config().copy()
     set_config(**new_config)
 
     try:

From 450a4568264c6a5852e5078a7cfe775e2ffac188 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Sat, 10 Apr 2021 08:34:29 -0400
Subject: [PATCH 14/16] CLN Do not need copy in config_context

---
 sklearn/_config.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index ba584717373db..feaf541ea3b9d 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -152,7 +152,7 @@ def config_context(**new_config):
     set_config : Set global scikit-learn configuration.
     get_config : Retrieve current values of the global configuration.
     """
-    old_config = _get_threadlocal_config().copy()
+    old_config = get_config()
     set_config(**new_config)
 
     try:

From 957b3c045686edfbfe5b8ed3276d6048880406a8 Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Tue, 27 Apr 2021 10:56:07 -0400
Subject: [PATCH 15/16] DOC Adds comment about mutable

---
 sklearn/_config.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index feaf541ea3b9d..b9aa9cf51d3b9 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -14,7 +14,7 @@
 
 
 def _get_threadlocal_config():
-    """Get the configuration that is local to the thread. If the configuration
+    """Get a threadlocal **mutable** configuration. If the configuration
     does not exist, copy the default global configuration."""
     if not hasattr(_threadlocal, 'global_config'):
         _threadlocal.global_config = _global_config.copy()

From 8ef81f73d86b10c83c2769bf2e4a64b4a97edf1d Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Tue, 27 Apr 2021 10:58:14 -0400
Subject: [PATCH 16/16] DOC Adds comment for copy

---
 sklearn/_config.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sklearn/_config.py b/sklearn/_config.py
index b9aa9cf51d3b9..e81d50849db05 100644
--- a/sklearn/_config.py
+++ b/sklearn/_config.py
@@ -34,6 +34,8 @@ def get_config():
     config_context : Context manager for global scikit-learn configuration.
     set_config : Set global scikit-learn configuration.
     """
+    # Return a copy of the threadlocal configuration so that users will
+    # not be able to modify the configuration with the returned dict.
     return _get_threadlocal_config().copy()