bpo-41842: Add a unregister function in _codecs module

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

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.

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__")

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.

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

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