From a4834a8c07377e12a555b8d30397356fc1398254 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Tue, 17 Feb 2026 12:23:55 +0100
Subject: [PATCH] gh-141510: Use frozendict for errno.errorcode

---
 Doc/library/errno.rst                         |  3 +++
 Lib/test/test_errno.py                        |  4 ++++
 ...-02-17-12-26-53.gh-issue-141510.tQcHWz.rst |  2 ++
 Modules/errnomodule.c                         | 20 +++++++++----------
 4 files changed, 19 insertions(+), 10 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-02-17-12-26-53.gh-issue-141510.tQcHWz.rst

diff --git a/Doc/library/errno.rst b/Doc/library/errno.rst
index 48b9762d85c2e1..210ff8ac0a1a42 100644
--- a/Doc/library/errno.rst
+++ b/Doc/library/errno.rst
@@ -18,6 +18,9 @@ all-inclusive.
    underlying system.  For instance, ``errno.errorcode[errno.EPERM]`` maps to
    ``'EPERM'``.
 
+   .. versionchanged:: next
+      The dictionary type is now a :class:`frozendict`.
+
 To translate a numeric error code to an error message, use :func:`os.strerror`.
 
 Of the following list, symbols that are not used on the current platform are not
diff --git a/Lib/test/test_errno.py b/Lib/test/test_errno.py
index e7f185c6b1a181..ee92b6f7017cc7 100644
--- a/Lib/test/test_errno.py
+++ b/Lib/test/test_errno.py
@@ -19,6 +19,10 @@ def test_using_errorcode(self):
         for value in errno.errorcode.values():
             self.assertHasAttr(errno, value)
 
+    def test_readonly_errorcode(self):
+        with self.assertRaises(TypeError):
+            errno.errorcode[1] = 'hack'
+
 
 class ErrorcodeTests(unittest.TestCase):
 
diff --git a/Misc/NEWS.d/next/Library/2026-02-17-12-26-53.gh-issue-141510.tQcHWz.rst b/Misc/NEWS.d/next/Library/2026-02-17-12-26-53.gh-issue-141510.tQcHWz.rst
new file mode 100644
index 00000000000000..1804fa9355e928
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-02-17-12-26-53.gh-issue-141510.tQcHWz.rst
@@ -0,0 +1,2 @@
+:data:`errno.errorcode` is now read-only: use :class:`frozendict` type
+instead of :class:`dict`. Patch by Victor Stinner.
diff --git a/Modules/errnomodule.c b/Modules/errnomodule.c
index 9557d68e759497..97959b05b04ed1 100644
--- a/Modules/errnomodule.c
+++ b/Modules/errnomodule.c
@@ -1,11 +1,5 @@
 /* Errno module */
 
-// Need limited C API version 3.13 for Py_mod_gil
-#include "pyconfig.h"   // Py_GIL_DISABLED
-#ifndef Py_GIL_DISABLED
-#  define Py_LIMITED_API 0x030d0000
-#endif
-
 #include "Python.h"
 #include <errno.h>                // EPIPE
 
@@ -96,10 +90,6 @@ errno_exec(PyObject *module)
     if (error_dict == NULL) {
         return -1;
     }
-    if (PyDict_SetItemString(module_dict, "errorcode", error_dict) < 0) {
-        Py_DECREF(error_dict);
-        return -1;
-    }
 
 /* Macro so I don't have to edit each and every line below... */
 #define add_errcode(name, code, comment)                               \
@@ -947,7 +937,17 @@ errno_exec(PyObject *module)
     add_errcode("ENOTCAPABLE", ENOTCAPABLE, "Capabilities insufficient");
 #endif
 
+    PyObject *frozendict = PyFrozenDict_New(error_dict);
     Py_DECREF(error_dict);
+    if (frozendict == NULL) {
+        return -1;
+    }
+    if (PyDict_SetItemString(module_dict, "errorcode", frozendict) < 0) {
+        Py_DECREF(frozendict);
+        return -1;
+    }
+    Py_DECREF(frozendict);
+
     return 0;
 }