gh-141510: Use frozendict for errno.errorcode

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

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):
 

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.

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;
 }