From fefda7041bd2a53d058a6f263cc9c52146741cf6 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Tue, 22 Sep 2020 21:12:22 +0800
Subject: [PATCH 01/14] add a _codecs.unregister in _codecs module

---
 Doc/library/codecs.rst                        | 13 +++++++++---
 Include/codecs.h                              |  4 ++++
 Lib/test/test_codecs.py                       |  6 ++++++
 .../2020-09-22-20-54-18.bpo-39337.zX2e5M.rst  |  2 ++
 Modules/_codecsmodule.c                       | 19 ++++++++++++++++++
 Modules/clinic/_codecsmodule.c.h              | 14 ++++++++++++-
 Python/codecs.c                               | 20 +++++++++++++++++++
 7 files changed, 74 insertions(+), 4 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst

diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index f071057293eece..9e6b86cf590202 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -165,9 +165,16 @@ function:
 
    .. note::
 
-      Search function registration is not currently reversible,
-      which may cause problems in some cases, such as unit testing or
-      module reloading.
+      Search function registration is reversible since Python 3.10.
+      You can use `codecs.unregister()` to unregister the search function.
+
+.. function:: unregister(search_function)
+
+   Unregister a codec search function from the codecs registry. If
+   the search function haven't registered, this function dose nothing.
+
+   .. versionadded:: 3.10
+
 
 While the builtin :func:`open` and the associated :mod:`io` module are the
 recommended approach for working with encoded text files, this module
diff --git a/Include/codecs.h b/Include/codecs.h
index 3ad0f2b5aae79c..e31bef8939c7ea 100644
--- a/Include/codecs.h
+++ b/Include/codecs.h
@@ -53,6 +53,10 @@ PyAPI_FUNC(PyObject *) _PyCodec_Lookup(
 PyAPI_FUNC(int) _PyCodec_Forget(
        const char *encoding
        );
+
+PyAPI_FUNC(PyObject *) _PyCodec_Unregister(
+       PyObject *search_function
+       );
 #endif
 
 /* Codec registry encoding check API.
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index 3dd56820cd1078..420391255d5a4f 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1641,6 +1641,12 @@ def test_register(self):
         self.assertRaises(TypeError, codecs.register)
         self.assertRaises(TypeError, codecs.register, 42)
 
+    def test_unregister(self):
+        def test_encoding(encoding):
+            return None
+        codecs.register(test_encoding)
+        self.assertEqual(codecs.unregister(test_encoding), None)
+
     def test_lookup(self):
         self.assertRaises(TypeError, codecs.lookup)
         self.assertRaises(LookupError, codecs.lookup, "__spam__")
diff --git a/Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst b/Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst
new file mode 100644
index 00000000000000..86b3d8dc13ce54
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst
@@ -0,0 +1,2 @@
+Add a :func:`_codecs.unregister` which could unregister a codec search
+function from the codec registry.
diff --git a/Modules/_codecsmodule.c b/Modules/_codecsmodule.c
index 952072102d5d8d..0d062baefc11b4 100644
--- a/Modules/_codecsmodule.c
+++ b/Modules/_codecsmodule.c
@@ -68,6 +68,24 @@ _codecs_register(PyObject *module, PyObject *search_function)
     Py_RETURN_NONE;
 }
 
+/*[clinic input]
+_codecs.unregister
+    search_function: object
+    /
+
+Unregister a codec search function.
+
+Unregister a codec search function from codec registry. If the search
+function haven't registered, this function does nothing.
+[clinic start generated code]*/
+
+static PyObject *
+_codecs_unregister(PyObject *module, PyObject *search_function)
+/*[clinic end generated code: output=1f0edee9cf246399 input=ecb26613941e6a47]*/
+{
+    return _PyCodec_Unregister(search_function);
+}
+
 /*[clinic input]
 _codecs.lookup
     encoding: str
@@ -992,6 +1010,7 @@ _codecs_lookup_error_impl(PyObject *module, const char *name)
 
 static PyMethodDef _codecs_functions[] = {
     _CODECS_REGISTER_METHODDEF
+    _CODECS_UNREGISTER_METHODDEF
     _CODECS_LOOKUP_METHODDEF
     _CODECS_ENCODE_METHODDEF
     _CODECS_DECODE_METHODDEF
diff --git a/Modules/clinic/_codecsmodule.c.h b/Modules/clinic/_codecsmodule.c.h
index 249065c9fd05a4..1810319b65e0ce 100644
--- a/Modules/clinic/_codecsmodule.c.h
+++ b/Modules/clinic/_codecsmodule.c.h
@@ -15,6 +15,18 @@ PyDoc_STRVAR(_codecs_register__doc__,
 #define _CODECS_REGISTER_METHODDEF    \
     {"register", (PyCFunction)_codecs_register, METH_O, _codecs_register__doc__},
 
+PyDoc_STRVAR(_codecs_unregister__doc__,
+"unregister($module, search_function, /)\n"
+"--\n"
+"\n"
+"Unregister a codec search function.\n"
+"\n"
+"Unregister a codec search function from codec registry. If the search\n"
+"function haven\'t registered, this function does nothing.");
+
+#define _CODECS_UNREGISTER_METHODDEF    \
+    {"unregister", (PyCFunction)_codecs_unregister, METH_O, _codecs_unregister__doc__},
+
 PyDoc_STRVAR(_codecs_lookup__doc__,
 "lookup($module, encoding, /)\n"
 "--\n"
@@ -2827,4 +2839,4 @@ _codecs_lookup_error(PyObject *module, PyObject *arg)
 #ifndef _CODECS_CODE_PAGE_ENCODE_METHODDEF
     #define _CODECS_CODE_PAGE_ENCODE_METHODDEF
 #endif /* !defined(_CODECS_CODE_PAGE_ENCODE_METHODDEF) */
-/*[clinic end generated code: output=eeead01414be6e42 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=b2d6d8e9caa87da7 input=a9049054013a1b77]*/
diff --git a/Python/codecs.c b/Python/codecs.c
index 0f18c27e5fe461..57edc2851055b4 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -50,6 +50,26 @@ int PyCodec_Register(PyObject *search_function)
     return -1;
 }
 
+PyObject *
+_PyCodec_Unregister(PyObject *search_function)
+{
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    if (interp->codec_search_path == NULL) {
+        Py_RETURN_NONE;
+    }
+
+    Py_ssize_t n = PyList_Size(interp->codec_search_path);
+    PyObject *list = PyList_New(0);
+    for (Py_ssize_t i = 0; i < n; i++) {
+        PyObject *item = PyList_GetItem(interp->codec_search_path, i);
+        if (item != search_function) {
+            PyList_Append(list, item);
+        }
+    }
+    Py_SETREF(interp->codec_search_path, list);
+    Py_RETURN_NONE;
+}
+
 extern int _Py_normalize_encoding(const char *, char *, size_t);
 
 /* Convert a string to a normalized Python string(decoded from UTF-8): all characters are

From f7775c09869738ca0341098ee0bea203a6345fae Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Wed, 23 Sep 2020 22:54:00 +0800
Subject: [PATCH 02/14] apply victor's comment

---
 Doc/library/codecs.rst                             |  9 +++------
 Doc/whatsnew/3.10.rst                              |  5 +++++
 Include/codecs.h                                   |  2 +-
 .../2020-09-22-20-54-18.bpo-39337.zX2e5M.rst       |  2 --
 .../2020-09-23-22-52-24.bpo-41842.lIuhC9.rst       |  1 +
 Modules/_codecsmodule.c                            | 11 +++++++----
 Modules/clinic/_codecsmodule.c.h                   |  5 ++---
 Python/codecs.c                                    | 14 +++++++-------
 8 files changed, 26 insertions(+), 23 deletions(-)
 delete mode 100644 Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst
 create mode 100644 Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst

diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index 9e6b86cf590202..9fb90761febbc4 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -163,15 +163,12 @@ function:
    :class:`CodecInfo` object. In case a search function cannot find
    a given encoding, it should return ``None``.
 
-   .. note::
-
-      Search function registration is reversible since Python 3.10.
-      You can use `codecs.unregister()` to unregister the search function.
 
 .. function:: unregister(search_function)
 
-   Unregister a codec search function from the codecs registry. If
-   the search function haven't registered, this function dose nothing.
+   Unregister a codec search function from the codecs registry.
+   If the search function is not registered, do nothing.
+   Clear the registry's cache if the codec search function is removed.
 
    .. versionadded:: 3.10
 
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index ce888fec1d8c97..832e2dc5dc171a 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -109,6 +109,11 @@ base64
 Add :func:`base64.b32hexencode` and :func:`base64.b32hexdecode` to support the
 Base32 Encoding with Extended Hex Alphabet.
 
+codecs
+------
+
+Add :func:`_codecs.unregister` to unregister a codec search function.
+
 curses
 ------
 
diff --git a/Include/codecs.h b/Include/codecs.h
index e31bef8939c7ea..4ed664a058275d 100644
--- a/Include/codecs.h
+++ b/Include/codecs.h
@@ -54,7 +54,7 @@ PyAPI_FUNC(int) _PyCodec_Forget(
        const char *encoding
        );
 
-PyAPI_FUNC(PyObject *) _PyCodec_Unregister(
+PyAPI_FUNC(int) _PyCodec_Unregister(
        PyObject *search_function
        );
 #endif
diff --git a/Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst b/Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst
deleted file mode 100644
index 86b3d8dc13ce54..00000000000000
--- a/Misc/NEWS.d/next/Library/2020-09-22-20-54-18.bpo-39337.zX2e5M.rst
+++ /dev/null
@@ -1,2 +0,0 @@
-Add a :func:`_codecs.unregister` which could unregister a codec search
-function from the codec registry.
diff --git a/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
new file mode 100644
index 00000000000000..bfc279aba3d090
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
@@ -0,0 +1 @@
+Add a :func:`_codecs.unregister` to unregister a codec search function.
diff --git a/Modules/_codecsmodule.c b/Modules/_codecsmodule.c
index 0d062baefc11b4..fa12e68b9bc714 100644
--- a/Modules/_codecsmodule.c
+++ b/Modules/_codecsmodule.c
@@ -75,15 +75,18 @@ _codecs.unregister
 
 Unregister a codec search function.
 
-Unregister a codec search function from codec registry. If the search
-function haven't registered, this function does nothing.
+If the search function is not registered, do nothing.
 [clinic start generated code]*/
 
 static PyObject *
 _codecs_unregister(PyObject *module, PyObject *search_function)
-/*[clinic end generated code: output=1f0edee9cf246399 input=ecb26613941e6a47]*/
+/*[clinic end generated code: output=1f0edee9cf246399 input=8bfb99685e00d653]*/
 {
-    return _PyCodec_Unregister(search_function);
+    if (_PyCodec_Unregister(search_function)) {
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
 }
 
 /*[clinic input]
diff --git a/Modules/clinic/_codecsmodule.c.h b/Modules/clinic/_codecsmodule.c.h
index 1810319b65e0ce..7974af32f4556e 100644
--- a/Modules/clinic/_codecsmodule.c.h
+++ b/Modules/clinic/_codecsmodule.c.h
@@ -21,8 +21,7 @@ PyDoc_STRVAR(_codecs_unregister__doc__,
 "\n"
 "Unregister a codec search function.\n"
 "\n"
-"Unregister a codec search function from codec registry. If the search\n"
-"function haven\'t registered, this function does nothing.");
+"If the search function is not registered, do nothing.");
 
 #define _CODECS_UNREGISTER_METHODDEF    \
     {"unregister", (PyCFunction)_codecs_unregister, METH_O, _codecs_unregister__doc__},
@@ -2839,4 +2838,4 @@ _codecs_lookup_error(PyObject *module, PyObject *arg)
 #ifndef _CODECS_CODE_PAGE_ENCODE_METHODDEF
     #define _CODECS_CODE_PAGE_ENCODE_METHODDEF
 #endif /* !defined(_CODECS_CODE_PAGE_ENCODE_METHODDEF) */
-/*[clinic end generated code: output=b2d6d8e9caa87da7 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=47d275cbfe79eea2 input=a9049054013a1b77]*/
diff --git a/Python/codecs.c b/Python/codecs.c
index 57edc2851055b4..39af3e6c36a4ec 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -50,24 +50,24 @@ int PyCodec_Register(PyObject *search_function)
     return -1;
 }
 
-PyObject *
+int
 _PyCodec_Unregister(PyObject *search_function)
 {
     PyInterpreterState *interp = PyInterpreterState_Get();
+    /* Do nothing if codec_search_path is not created yet or was created. */
     if (interp->codec_search_path == NULL) {
-        Py_RETURN_NONE;
+        return 0;
     }
 
     Py_ssize_t n = PyList_Size(interp->codec_search_path);
-    PyObject *list = PyList_New(0);
     for (Py_ssize_t i = 0; i < n; i++) {
         PyObject *item = PyList_GetItem(interp->codec_search_path, i);
-        if (item != search_function) {
-            PyList_Append(list, item);
+        if (item == search_function) {
+            return PyList_SetSlice(interp->codec_search_path, i, i+1, NULL);
         }
     }
-    Py_SETREF(interp->codec_search_path, list);
-    Py_RETURN_NONE;
+    PyDict_Clear(interp->codec_search_cache);
+    return 0;
 }
 
 extern int _Py_normalize_encoding(const char *, char *, size_t);

From 71b624e3b6d09884b80670b98896d6638826db42 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 00:59:30 +0800
Subject: [PATCH 03/14] apply victor's comment

---
 Doc/library/codecs.rst                             |  3 +--
 Doc/whatsnew/3.10.rst                              |  2 +-
 Include/codecs.h                                   | 14 ++++++++++----
 Lib/test/test_codecs.py                            |  9 +++++----
 .../2020-09-23-22-52-24.bpo-41842.lIuhC9.rst       |  2 +-
 Modules/_codecsmodule.c                            |  6 +++---
 Modules/clinic/_codecsmodule.c.h                   |  4 ++--
 Python/codecs.c                                    |  4 ++--
 8 files changed, 25 insertions(+), 19 deletions(-)

diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index 9fb90761febbc4..a0265135205901 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -166,9 +166,8 @@ function:
 
 .. function:: unregister(search_function)
 
-   Unregister a codec search function from the codecs registry.
+   Unregister a codec search function and clear the registry's cache.
    If the search function is not registered, do nothing.
-   Clear the registry's cache if the codec search function is removed.
 
    .. versionadded:: 3.10
 
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 832e2dc5dc171a..fac139d77ee354 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -112,7 +112,7 @@ Base32 Encoding with Extended Hex Alphabet.
 codecs
 ------
 
-Add :func:`_codecs.unregister` to unregister a codec search function.
+Add :func:`codecs.unregister` to unregister a codec search function.
 
 curses
 ------
diff --git a/Include/codecs.h b/Include/codecs.h
index 4ed664a058275d..1cfcf8c4fd1274 100644
--- a/Include/codecs.h
+++ b/Include/codecs.h
@@ -27,6 +27,16 @@ PyAPI_FUNC(int) PyCodec_Register(
        PyObject *search_function
        );
 
+/* Unregister a codec search function.
+
+   Unregister a codec search function and clear the registry's cache.
+   If the search function is not registered, do nothing.
+   */
+
+PyAPI_FUNC(int) PyCodec_Unregister(
+       PyObject *search_function
+       );
+
 /* Codec registry lookup API.
 
    Looks up the given encoding and returns a CodecInfo object with
@@ -53,10 +63,6 @@ PyAPI_FUNC(PyObject *) _PyCodec_Lookup(
 PyAPI_FUNC(int) _PyCodec_Forget(
        const char *encoding
        );
-
-PyAPI_FUNC(int) _PyCodec_Unregister(
-       PyObject *search_function
-       );
 #endif
 
 /* Codec registry encoding check API.
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index 420391255d5a4f..1acafc2f1a2f1b 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1642,10 +1642,11 @@ def test_register(self):
         self.assertRaises(TypeError, codecs.register, 42)
 
     def test_unregister(self):
-        def test_encoding(encoding):
-            return None
-        codecs.register(test_encoding)
-        self.assertEqual(codecs.unregister(test_encoding), None)
+        search_function = mock.Mock(return_value=(1, 2, 3, 4))
+        codecs.register(search_function)
+        codecs.unregister(search_function)
+        self.assertRaises(LookupError, codecs.lookup, "test")
+        search_function.assert_not_called()
 
     def test_lookup(self):
         self.assertRaises(TypeError, codecs.lookup)
diff --git a/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
index bfc279aba3d090..e214175f49929c 100644
--- a/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
+++ b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
@@ -1 +1 @@
-Add a :func:`_codecs.unregister` to unregister a codec search function.
+Add a :func:`codecs.unregister` to unregister a codec search function.
diff --git a/Modules/_codecsmodule.c b/Modules/_codecsmodule.c
index fa12e68b9bc714..08a3d4ab024cc2 100644
--- a/Modules/_codecsmodule.c
+++ b/Modules/_codecsmodule.c
@@ -73,16 +73,16 @@ _codecs.unregister
     search_function: object
     /
 
-Unregister a codec search function.
+Unregister a codec search function and clear the registry's cache.
 
 If the search function is not registered, do nothing.
 [clinic start generated code]*/
 
 static PyObject *
 _codecs_unregister(PyObject *module, PyObject *search_function)
-/*[clinic end generated code: output=1f0edee9cf246399 input=8bfb99685e00d653]*/
+/*[clinic end generated code: output=1f0edee9cf246399 input=dd7c004c652d345e]*/
 {
-    if (_PyCodec_Unregister(search_function)) {
+    if (PyCodec_Unregister(search_function) < 0) {
         return NULL;
     }
 
diff --git a/Modules/clinic/_codecsmodule.c.h b/Modules/clinic/_codecsmodule.c.h
index 7974af32f4556e..e2ebb6861299e4 100644
--- a/Modules/clinic/_codecsmodule.c.h
+++ b/Modules/clinic/_codecsmodule.c.h
@@ -19,7 +19,7 @@ PyDoc_STRVAR(_codecs_unregister__doc__,
 "unregister($module, search_function, /)\n"
 "--\n"
 "\n"
-"Unregister a codec search function.\n"
+"Unregister a codec search function and clear the registry\'s cache.\n"
 "\n"
 "If the search function is not registered, do nothing.");
 
@@ -2838,4 +2838,4 @@ _codecs_lookup_error(PyObject *module, PyObject *arg)
 #ifndef _CODECS_CODE_PAGE_ENCODE_METHODDEF
     #define _CODECS_CODE_PAGE_ENCODE_METHODDEF
 #endif /* !defined(_CODECS_CODE_PAGE_ENCODE_METHODDEF) */
-/*[clinic end generated code: output=47d275cbfe79eea2 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=9a97e2ddf3e69072 input=a9049054013a1b77]*/
diff --git a/Python/codecs.c b/Python/codecs.c
index 39af3e6c36a4ec..05dfa013edaea1 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -51,7 +51,7 @@ int PyCodec_Register(PyObject *search_function)
 }
 
 int
-_PyCodec_Unregister(PyObject *search_function)
+PyCodec_Unregister(PyObject *search_function)
 {
     PyInterpreterState *interp = PyInterpreterState_Get();
     /* Do nothing if codec_search_path is not created yet or was created. */
@@ -63,10 +63,10 @@ _PyCodec_Unregister(PyObject *search_function)
     for (Py_ssize_t i = 0; i < n; i++) {
         PyObject *item = PyList_GetItem(interp->codec_search_path, i);
         if (item == search_function) {
+            PyDict_Clear(interp->codec_search_cache);
             return PyList_SetSlice(interp->codec_search_path, i, i+1, NULL);
         }
     }
-    PyDict_Clear(interp->codec_search_cache);
     return 0;
 }
 

From d186726753b556ccb75646b7d6ee7235e29bc489 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 12:45:12 +0800
Subject: [PATCH 04/14] apply victor's comment

---
 Doc/c-api/codec.rst                                    | 10 ++++++++++
 Doc/whatsnew/3.10.rst                                  |  3 ++-
 Include/codecs.h                                       |  2 +-
 Lib/test/test_codecs.py                                |  2 +-
 .../Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst   |  3 ++-
 Python/codecs.c                                        |  2 +-
 6 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/Doc/c-api/codec.rst b/Doc/c-api/codec.rst
index 172dcb326a4bc4..ed07254e5a64d1 100644
--- a/Doc/c-api/codec.rst
+++ b/Doc/c-api/codec.rst
@@ -10,6 +10,16 @@ Codec registry and support functions
    As side effect, this tries to load the :mod:`encodings` package, if not yet
    done, to make sure that it is always first in the list of search functions.
 
+.. c:function:: int PyCodec_Unregister(PyObject *search_function)
+
+   Unregister a codec search function.
+
+   Unregister a codec search function and clear the registry's cache.
+   If the search function is not registered, do nothing.
+   Return 0 on success. Raise an exception and return -1 on success.
+
+   .. versionadded:: 3.10
+
 .. c:function:: int PyCodec_KnownEncoding(const char *encoding)
 
    Return ``1`` or ``0`` depending on whether there is a registered codec for
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index fac139d77ee354..8c1b1ddd8a3388 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -112,7 +112,8 @@ Base32 Encoding with Extended Hex Alphabet.
 codecs
 ------
 
-Add :func:`codecs.unregister` to unregister a codec search function.
+Add :func:`codecs.unregister` and :c:func:`PyCodec_Unregister` to unregister
+a codec search function.
 
 curses
 ------
diff --git a/Include/codecs.h b/Include/codecs.h
index 1cfcf8c4fd1274..a17ebfbb24b39b 100644
--- a/Include/codecs.h
+++ b/Include/codecs.h
@@ -31,7 +31,7 @@ PyAPI_FUNC(int) PyCodec_Register(
 
    Unregister a codec search function and clear the registry's cache.
    If the search function is not registered, do nothing.
-   */
+   Return 0 on success. Raise an exception and return -1 on success. */
 
 PyAPI_FUNC(int) PyCodec_Unregister(
        PyObject *search_function
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index 1acafc2f1a2f1b..bc78ced28f74d3 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1645,7 +1645,7 @@ def test_unregister(self):
         search_function = mock.Mock(return_value=(1, 2, 3, 4))
         codecs.register(search_function)
         codecs.unregister(search_function)
-        self.assertRaises(LookupError, codecs.lookup, "test")
+        self.assertRaises(LookupError, codecs.lookup, "nonexistent_codec_name")
         search_function.assert_not_called()
 
     def test_lookup(self):
diff --git a/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
index e214175f49929c..790227417b3c31 100644
--- a/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
+++ b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
@@ -1 +1,2 @@
-Add a :func:`codecs.unregister` to unregister a codec search function.
+Add :func:`codecs.unregister` and :c:func:`PyCodec_Unregister` to unregister
+a codec search function.
diff --git a/Python/codecs.c b/Python/codecs.c
index 05dfa013edaea1..c876cf87726013 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -54,7 +54,7 @@ int
 PyCodec_Unregister(PyObject *search_function)
 {
     PyInterpreterState *interp = PyInterpreterState_Get();
-    /* Do nothing if codec_search_path is not created yet or was created. */
+    /* Do nothing if codec_search_path is not created yet or was cleared. */
     if (interp->codec_search_path == NULL) {
         return 0;
     }

From f8e912bdd87cd0288005a04b9459250ae78dd0f9 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 13:37:00 +0800
Subject: [PATCH 05/14] retrigger checks


From 40666fc49e51ab73a19048d8d90968b6bf00463d Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 19:27:51 +0800
Subject: [PATCH 06/14] apply victor's comment

---
 Doc/c-api/codec.rst     | 2 --
 Doc/whatsnew/3.10.rst   | 6 ++++--
 Include/codecs.h        | 2 +-
 Lib/test/test_codecs.py | 8 ++++++--
 Misc/ACKS               | 1 +
 5 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/Doc/c-api/codec.rst b/Doc/c-api/codec.rst
index ed07254e5a64d1..5c3a60ec1fd529 100644
--- a/Doc/c-api/codec.rst
+++ b/Doc/c-api/codec.rst
@@ -12,8 +12,6 @@ Codec registry and support functions
 
 .. c:function:: int PyCodec_Unregister(PyObject *search_function)
 
-   Unregister a codec search function.
-
    Unregister a codec search function and clear the registry's cache.
    If the search function is not registered, do nothing.
    Return 0 on success. Raise an exception and return -1 on success.
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 8c1b1ddd8a3388..a5d830da594df6 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -112,8 +112,7 @@ Base32 Encoding with Extended Hex Alphabet.
 codecs
 ------
 
-Add :func:`codecs.unregister` and :c:func:`PyCodec_Unregister` to unregister
-a codec search function.
+Add a :func:`codecs.unregister` to unregister a codec search function.
 
 curses
 ------
@@ -229,6 +228,9 @@ New Features
   Python executable.
   (Contributed by Victor Stinner in :issue:`23427`.)
 
+* Add a c:func:`PyCodec_Unregister` to unregister a codec search function.
+  (Contributed by Hai Shi in :issue:`41842`.)
+
 Porting to Python 3.10
 ----------------------
 
diff --git a/Include/codecs.h b/Include/codecs.h
index a17ebfbb24b39b..f9e305ad665bbe 100644
--- a/Include/codecs.h
+++ b/Include/codecs.h
@@ -31,7 +31,7 @@ PyAPI_FUNC(int) PyCodec_Register(
 
    Unregister a codec search function and clear the registry's cache.
    If the search function is not registered, do nothing.
-   Return 0 on success. Raise an exception and return -1 on success. */
+   Return 0 on success. Raise an exception and return -1 on error. */
 
 PyAPI_FUNC(int) PyCodec_Unregister(
        PyObject *search_function
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index bc78ced28f74d3..d2a27b8159eec5 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1642,10 +1642,14 @@ def test_register(self):
         self.assertRaises(TypeError, codecs.register, 42)
 
     def test_unregister(self):
-        search_function = mock.Mock(return_value=(1, 2, 3, 4))
+        name = "nonexistent_codec_name"
+        search_function = mock.Mock()
         codecs.register(search_function)
+        self.assertRaises(TypeError, codecs.lookup, name)
+        search_function.assert_called_with(name)
+        search_function.reset_mock()
         codecs.unregister(search_function)
-        self.assertRaises(LookupError, codecs.lookup, "nonexistent_codec_name")
+        self.assertRaises(LookupError, codecs.lookup, name)
         search_function.assert_not_called()
 
     def test_lookup(self):
diff --git a/Misc/ACKS b/Misc/ACKS
index 01ee1cb42d39d0..566ab9f5c4f488 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1574,6 +1574,7 @@ Akash Shende
 Charlie Shepherd
 Bruce Sherwood
 Gregory Shevchenko
+Hai Shi
 Alexander Shigin
 Pete Shinners
 Michael Shiplett

From 3257b6c2ca5fbcdf6c1bbc4a131a1efcca8f1787 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 19:35:56 +0800
Subject: [PATCH 07/14] update doc

---
 Include/codecs.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/Include/codecs.h b/Include/codecs.h
index f9e305ad665bbe..37ecfb4ab757b4 100644
--- a/Include/codecs.h
+++ b/Include/codecs.h
@@ -27,9 +27,7 @@ PyAPI_FUNC(int) PyCodec_Register(
        PyObject *search_function
        );
 
-/* Unregister a codec search function.
-
-   Unregister a codec search function and clear the registry's cache.
+/* Unregister a codec search function and clear the registry's cache.
    If the search function is not registered, do nothing.
    Return 0 on success. Raise an exception and return -1 on error. */
 

From a493a35bc56d4cc1437cdce7ee157ac3191761f8 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 19:37:12 +0800
Subject: [PATCH 08/14] update doc

---
 Doc/c-api/codec.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/c-api/codec.rst b/Doc/c-api/codec.rst
index 5c3a60ec1fd529..235c77c945cc5b 100644
--- a/Doc/c-api/codec.rst
+++ b/Doc/c-api/codec.rst
@@ -14,7 +14,7 @@ Codec registry and support functions
 
    Unregister a codec search function and clear the registry's cache.
    If the search function is not registered, do nothing.
-   Return 0 on success. Raise an exception and return -1 on success.
+   Return 0 on success. Raise an exception and return -1 on error.
 
    .. versionadded:: 3.10
 

From db4631ef2edc122eb472b56e2f363cf7f9d5f20c Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 20:54:08 +0800
Subject: [PATCH 09/14] update doc

---
 Doc/whatsnew/3.10.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 68619945401368..8a3e33ace1f020 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -242,7 +242,7 @@ New Features
   :class:`datetime.time` objects.
   (Contributed by Zackery Spytz in :issue:`30155`.)
 
-* Add a c:func:`PyCodec_Unregister` to unregister a codec search function.
+* Add a :c:func:`PyCodec_Unregister` to unregister a codec search function.
   (Contributed by Hai Shi in :issue:`41842`.)
 
 =======

From 5efff220fb0ba9c3bac3454ecbce56fb7f87edf0 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Thu, 24 Sep 2020 21:20:50 +0800
Subject: [PATCH 10/14] apply victor's comment

---
 Doc/whatsnew/3.10.rst   | 3 +--
 Lib/test/test_codecs.py | 1 +
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 8a3e33ace1f020..69a9d40a34762e 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -113,6 +113,7 @@ codecs
 ------
 
 Add a :func:`codecs.unregister` to unregister a codec search function.
+(Contributed by Hai Shi in :issue:`41842`.)
 
 curses
 ------
@@ -245,8 +246,6 @@ New Features
 * Add a :c:func:`PyCodec_Unregister` to unregister a codec search function.
   (Contributed by Hai Shi in :issue:`41842`.)
 
-=======
-
 Porting to Python 3.10
 ----------------------
 
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index d2a27b8159eec5..ed508f36ad4234 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1648,6 +1648,7 @@ def test_unregister(self):
         self.assertRaises(TypeError, codecs.lookup, name)
         search_function.assert_called_with(name)
         search_function.reset_mock()
+
         codecs.unregister(search_function)
         self.assertRaises(LookupError, codecs.lookup, name)
         search_function.assert_not_called()

From f18cd4f9f2dc12476069975c4b9ab955cd5c13f9 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Sat, 26 Sep 2020 08:45:41 +0800
Subject: [PATCH 11/14] apply Dong-hee Na's comment

---
 Python/codecs.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/Python/codecs.c b/Python/codecs.c
index c876cf87726013..f3a75cf110af4f 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -54,17 +54,18 @@ int
 PyCodec_Unregister(PyObject *search_function)
 {
     PyInterpreterState *interp = PyInterpreterState_Get();
+    PyObject *codec_search_path = interp->codec_search_path;
     /* Do nothing if codec_search_path is not created yet or was cleared. */
-    if (interp->codec_search_path == NULL) {
+    if (codec_search_path == NULL) {
         return 0;
     }
 
-    Py_ssize_t n = PyList_Size(interp->codec_search_path);
+    Py_ssize_t n = PyList_Size(codec_search_path);
     for (Py_ssize_t i = 0; i < n; i++) {
-        PyObject *item = PyList_GetItem(interp->codec_search_path, i);
+        PyObject *item = PyList_GetItem(codec_search_path, i);
         if (item == search_function) {
             PyDict_Clear(interp->codec_search_cache);
-            return PyList_SetSlice(interp->codec_search_path, i, i+1, NULL);
+            return PyList_SetSlice(codec_search_path, i, i+1, NULL);
         }
     }
     return 0;

From 8f6dc4d62b0f9c5b8750a2206179ef20a965e3fa Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Sun, 27 Sep 2020 00:02:16 +0800
Subject: [PATCH 12/14] apply victor and Zackery's comment

---
 Doc/whatsnew/3.10.rst |  5 +++--
 Python/codecs.c       | 10 +++++++---
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 69a9d40a34762e..f74dd1aa247a34 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -112,7 +112,7 @@ Base32 Encoding with Extended Hex Alphabet.
 codecs
 ------
 
-Add a :func:`codecs.unregister` to unregister a codec search function.
+Add a :func:`codecs.unregister` function to unregister a codec search function.
 (Contributed by Hai Shi in :issue:`41842`.)
 
 curses
@@ -243,7 +243,8 @@ New Features
   :class:`datetime.time` objects.
   (Contributed by Zackery Spytz in :issue:`30155`.)
 
-* Add a :c:func:`PyCodec_Unregister` to unregister a codec search function.
+* Add a :c:func:`PyCodec_Unregister` function to unregister a codec
+  search function.
   (Contributed by Hai Shi in :issue:`41842`.)
 
 Porting to Python 3.10
diff --git a/Python/codecs.c b/Python/codecs.c
index f3a75cf110af4f..a2e3eac36d06db 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -60,11 +60,15 @@ PyCodec_Unregister(PyObject *search_function)
         return 0;
     }
 
-    Py_ssize_t n = PyList_Size(codec_search_path);
+    assert(PyList_CheckExact(codec_search_path));
+    Py_ssize_t n = PyList_GET_SIZE(codec_search_path);
     for (Py_ssize_t i = 0; i < n; i++) {
-        PyObject *item = PyList_GetItem(codec_search_path, i);
+        PyObject *item = PyList_GET_ITEM(codec_search_path, i);
         if (item == search_function) {
-            PyDict_Clear(interp->codec_search_cache);
+            if (interp->codec_search_cache != NULL) {
+                assert(PyDict_Check(interp->codec_search_cache));
+                PyDict_Clear(interp->codec_search_cache);
+            }
             return PyList_SetSlice(codec_search_path, i, i+1, NULL);
         }
     }

From 4308ea8fc744e377bfeab38157767cecfe377e10 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Sun, 27 Sep 2020 00:20:50 +0800
Subject: [PATCH 13/14] apply victor's comment

---
 Python/codecs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Python/codecs.c b/Python/codecs.c
index a2e3eac36d06db..a8233a73c4ed3f 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -66,7 +66,7 @@ PyCodec_Unregister(PyObject *search_function)
         PyObject *item = PyList_GET_ITEM(codec_search_path, i);
         if (item == search_function) {
             if (interp->codec_search_cache != NULL) {
-                assert(PyDict_Check(interp->codec_search_cache));
+                assert(PyDict_CheckExact(interp->codec_search_cache));
                 PyDict_Clear(interp->codec_search_cache);
             }
             return PyList_SetSlice(codec_search_path, i, i+1, NULL);

From c9a0c99c4d380b548c17b2933003c687f69245a5 Mon Sep 17 00:00:00 2001
From: Hai Shi <shihai1992@gmail.com>
Date: Sun, 27 Sep 2020 20:43:53 +0800
Subject: [PATCH 14/14] apply victor's comment

---
 .../NEWS.d/next/C API/2020-09-27-20-43-16.bpo-41842.bCakAj.rst | 2 ++
 .../next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst      | 3 +--
 2 files changed, 3 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/C API/2020-09-27-20-43-16.bpo-41842.bCakAj.rst

diff --git a/Misc/NEWS.d/next/C API/2020-09-27-20-43-16.bpo-41842.bCakAj.rst b/Misc/NEWS.d/next/C API/2020-09-27-20-43-16.bpo-41842.bCakAj.rst
new file mode 100644
index 00000000000000..116d08f4908596
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2020-09-27-20-43-16.bpo-41842.bCakAj.rst	
@@ -0,0 +1,2 @@
+Add :c:func:`PyCodec_Unregister` function to unregister a codec search
+function.
diff --git a/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
index 790227417b3c31..306b02d76fffe2 100644
--- a/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
+++ b/Misc/NEWS.d/next/Library/2020-09-23-22-52-24.bpo-41842.lIuhC9.rst
@@ -1,2 +1 @@
-Add :func:`codecs.unregister` and :c:func:`PyCodec_Unregister` to unregister
-a codec search function.
+Add :func:`codecs.unregister` function to unregister a codec search function.