gh-149211: Preserve custom warning handler during finalization

Include/internal/pycore_interp_structs.h

@@ -660,6 +660,7 @@ struct _warnings_runtime_state {
     PyObject *filters;  /* List */
     PyObject *once_registry;  /* Dict */
     PyObject *default_action; /* String */
+    PyObject *module;  /* Weakref, used after sys.modules cleanup */
     _PyRecursiveMutex lock;
     long filters_version;
     PyObject *context;

Lib/test/test_warnings/__init__.py

@@ -1693,6 +1693,30 @@ def __del__(self):
         self.assertEqual(err.decode().rstrip(),
                          '<string>:7: UserWarning: test')
 
+    def test_showwarning_during_finalization(self):
+        # gh-149211: warnings.warn() should use a custom showwarning()
+        # during Python finalization.
+        code = """
+import sys
+import warnings
+
+def showwarning(message, category, filename, lineno,
+                file=None, line=None, stderr=sys.stderr):
+    print(f"custom showwarning: {message}", file=file or stderr)
+
+warnings.showwarning = showwarning
+
+class A:
+    def __del__(self):
+        warnings.warn("test")
+
+a = A()
+        """
+        rc, out, err = assert_python_ok("-c", code)
+        self.assertEqual(out, b"")
+        self.assertEqual(err.decode().rstrip(),
+                         "custom showwarning: test")
+
     def test_late_resource_warning(self):
         # Issue #21925: Emitting a ResourceWarning late during the Python
         # shutdown must be logged.

Lib/warnings.py

@@ -68,6 +68,7 @@
         _filters_mutated_lock_held,
         _onceregistry as onceregistry,
         _release_lock,
+        _set_module as _set_c_module,
         _warnings_context,
         filters,
         warn,
@@ -91,9 +92,13 @@ def __exit__(self, *args):
 
 # Module initialization
 _set_module(sys.modules[__name__])
+if _warnings_defaults:
+    _set_c_module(sys.modules[__name__])
 _processoptions(sys.warnoptions)
 if not _warnings_defaults:
     _setup_defaults()
 
 del _warnings_defaults
+if "_set_c_module" in globals():
+    del _set_c_module
 del _setup_defaults

Misc/NEWS.d/next/Library/2026-06-05-21-05-00.gh-issue-149211.a1b2c3.rst

@@ -0,0 +1,2 @@
+Fix :func:`warnings.warn` to call a custom :func:`warnings.showwarning`
+during Python finalization.

Python/_warnings.c

@@ -69,6 +69,7 @@ warnings_clear_state(WarningsState *st)
     Py_CLEAR(st->filters);
     Py_CLEAR(st->once_registry);
     Py_CLEAR(st->default_action);
+    Py_CLEAR(st->module);
     Py_CLEAR(st->context);
 }
 
@@ -245,8 +246,13 @@ get_warnings_attr(PyInterpreterState *interp, PyObject *attr, int try_import)
             return NULL;
         }
         warnings_module = PyImport_GetModule(&_Py_ID(warnings));
-        if (warnings_module == NULL)
-            return NULL;
+        if (warnings_module == NULL) {
+            WarningsState *st = warnings_get_state(interp);
+            if (st->module == NULL ||
+                    PyWeakref_GetRef(st->module, &warnings_module) <= 0) {
+                return NULL;
+            }
+        }
     }
 
     (void)PyObject_GetOptionalAttr(warnings_module, attr, &obj);
@@ -375,6 +381,25 @@ warnings_release_lock_impl(PyObject *module)
     Py_RETURN_NONE;
 }
 
+static PyObject *
+warnings_set_module(PyObject *Py_UNUSED(module), PyObject *warnings_module)
+{
+    PyInterpreterState *interp = get_current_interp();
+    if (interp == NULL) {
+        return NULL;
+    }
+    WarningsState *st = warnings_get_state(interp);
+    if (st == NULL) {
+        return NULL;
+    }
+    PyObject *ref = PyWeakref_NewRef(warnings_module, NULL);
+    if (ref == NULL) {
+        return NULL;
+    }
+    Py_XSETREF(st->module, ref);
+    Py_RETURN_NONE;
+}
+
 static PyObject *
 get_once_registry(PyInterpreterState *interp)
 {
@@ -1593,6 +1618,7 @@ static PyMethodDef warnings_functions[] = {
     WARNINGS_FILTERS_MUTATED_LOCK_HELD_METHODDEF
     WARNINGS_ACQUIRE_LOCK_METHODDEF
     WARNINGS_RELEASE_LOCK_METHODDEF
+    {"_set_module", warnings_set_module, METH_O, NULL},
     /* XXX(brett.cannon): add showwarning? */
     /* XXX(brett.cannon): Reasonable to add formatwarning? */
     {NULL, NULL}                /* sentinel */