apply victor and Zackery's comment

vstinner · Sep 28, 2020 · Sep 22, 2020 · Sep 23, 2020 · Sep 23, 2020 · Sep 24, 2020
commit 8f6dc4d62b0f9c5b8750a2206179ef20a965e3fa
diff --git 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
@@ -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);
         }
     }