deprecate sys._enablelegacywindowsfsencoding()

Author: methane

Doc/library/sys.rst

@@ -1644,11 +1644,21 @@ always available.
    See also :func:`sys.getfilesystemencoding` and
    :func:`sys.getfilesystemencodeerrors`.
 
+   .. note::
+      Changing the filesystem encoding after Python startup is dangerous.
+      This is because the old fsencoding or paths encoded by the old fsencoding
+      might be cached somewhere.
+      Use :envvar:`PYTHONLEGACYWINDOWSFSENCODING` instead of this API.
+
    .. availability:: Windows.
 
    .. versionadded:: 3.6
       See :pep:`529` for more details.
 
+   .. deprecated-removed:: 3.13 3.16
+      Use :envvar:`PYTHONLEGACYWINDOWSFSENCODING` instead.
+
+
 .. data:: stdin
           stdout
           stderr

Doc/whatsnew/3.13.rst

@@ -187,6 +187,10 @@ Deprecated
   practice.
   (Contributed by Victor Stinner in :gh:`106535`.)
 
+* :mod:`sys`: :func:`sys._enablelegacywindowsfsencoding` function.
+  Replace it with :envvar:`PYTHONLEGACYWINDOWSFSENCODING` environment variable.
+  (Contributed by Inada Naoki in :gh:`73427`.)
+
 * :mod:`typing`: Creating a :class:`typing.NamedTuple` class using keyword arguments to denote
   the fields (``NT = NamedTuple("NT", x=int, y=int)``) is deprecated, and will
   be disallowed in Python 3.15. Use the class-based syntax or the functional

Python/sysmodule.c

@@ -1676,6 +1676,13 @@ static PyObject *
 sys__enablelegacywindowsfsencoding_impl(PyObject *module)
 /*[clinic end generated code: output=f5c3855b45e24fe9 input=2bfa931a20704492]*/
 {
+    if (PyErr_WarnEx(
+        PyExc_DeprecationWarning,
+        "sys._enablelegacywindowsfsencoding() is deprecated and will be removed in Python 3.16. Use PYTHONLEGACYWINDOWSFSENCODING instead.",
+        1
+    )) {
+        return NULL;
+    }
     if (_PyUnicode_EnableLegacyWindowsFSEncoding() < 0) {
         return NULL;
     }