From be739c11614af097168925ef8842de7054a5d7cb Mon Sep 17 00:00:00 2001
From: snoopuppy582 <mnb0968@naver.com>
Date: Tue, 14 Jul 2026 21:31:53 +0900
Subject: [PATCH] gh-153128: Preserve once warnings across filter changes

Keep the C implementation's once registry independent of filter version changes, matching the pure Python implementation. Add a shared regression test and a Library news entry.
---
 Lib/test/test_warnings/__init__.py            | 10 +++
 ...-07-14-21-30-00.gh-issue-153128.Xm2QvA.rst |  3 +
 Python/_warnings.c                            | 66 ++++++++++++-------
 3 files changed, 56 insertions(+), 23 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-14-21-30-00.gh-issue-153128.Xm2QvA.rst

diff --git a/Lib/test/test_warnings/__init__.py b/Lib/test/test_warnings/__init__.py
index bf1bcf8e6ed5d9..697963e11dcc44 100644
--- a/Lib/test/test_warnings/__init__.py
+++ b/Lib/test/test_warnings/__init__.py
@@ -242,6 +242,16 @@ def test_once(self):
                                     42)
             self.assertEqual(len(w), 0)
 
+    def test_once_after_filter_change(self):
+        with self.module.catch_warnings(record=True) as w:
+            self.module.resetwarnings()
+            message = "FilterTests.test_once_after_filter_change"
+            self.module.filterwarnings("once", message, UserWarning)
+            self.module.warn(message, UserWarning)
+            self.module.filterwarnings("ignore", "unrelated")
+            self.module.warn(message, UserWarning)
+            self.assertEqual(len(w), 1)
+
     def test_filter_module(self):
         MS_WINDOWS = (sys.platform == 'win32')
         with self.module.catch_warnings(record=True) as w:
diff --git a/Misc/NEWS.d/next/Library/2026-07-14-21-30-00.gh-issue-153128.Xm2QvA.rst b/Misc/NEWS.d/next/Library/2026-07-14-21-30-00.gh-issue-153128.Xm2QvA.rst
new file mode 100644
index 00000000000000..ea116555153207
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-14-21-30-00.gh-issue-153128.Xm2QvA.rst
@@ -0,0 +1,3 @@
+Fix the C implementation of :mod:`warnings` so the ``"once"`` action remains
+suppressed after unrelated warning filters are changed, matching the pure
+Python implementation.
diff --git a/Python/_warnings.c b/Python/_warnings.c
index 4f6de50efa14a8..35babb2ebf5f79 100644
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -574,11 +574,34 @@ get_filter(PyInterpreterState *interp, PyObject *category,
 
 
 static int
-already_warned(PyInterpreterState *interp, PyObject *registry, PyObject *key,
-               int should_set)
+already_warned_in_registry(PyObject *registry, PyObject *key, int should_set)
 {
     PyObject *already_warned;
 
+    if (key == NULL)
+        return -1;
+
+    if (PyDict_GetItemRef(registry, key, &already_warned) < 0) {
+        return -1;
+    }
+    if (already_warned != NULL) {
+        int rc = PyObject_IsTrue(already_warned);
+        Py_DECREF(already_warned);
+        if (rc != 0)
+            return rc;
+    }
+
+    /* This warning wasn't found in the registry, set it. */
+    if (should_set)
+        return PyDict_SetItem(registry, key, Py_True);
+    return 0;
+}
+
+
+static int
+already_warned(PyInterpreterState *interp, PyObject *registry, PyObject *key,
+               int should_set)
+{
     if (key == NULL)
         return -1;
 
@@ -607,22 +630,8 @@ already_warned(PyInterpreterState *interp, PyObject *registry, PyObject *key,
         }
         Py_DECREF(version_obj);
     }
-    else {
-        if (PyDict_GetItemRef(registry, key, &already_warned) < 0) {
-            return -1;
-        }
-        if (already_warned != NULL) {
-            int rc = PyObject_IsTrue(already_warned);
-            Py_DECREF(already_warned);
-            if (rc != 0)
-                return rc;
-        }
-    }
 
-    /* This warning wasn't found in the registry, set it. */
-    if (should_set)
-        return PyDict_SetItem(registry, key, Py_True);
-    return 0;
+    return already_warned_in_registry(registry, key, should_set);
 }
 
 static int
@@ -642,6 +651,22 @@ update_registry(PyInterpreterState *interp, PyObject *registry, PyObject *text,
     return rc;
 }
 
+
+static int
+update_once_registry(PyInterpreterState *interp, PyObject *text,
+                     PyObject *category)
+{
+    PyObject *registry = get_once_registry(interp);
+    if (registry == NULL) {
+        return -1;
+    }
+
+    PyObject *key = _PyTuple_FromPair(text, category);
+    int rc = already_warned_in_registry(registry, key, 1);
+    Py_XDECREF(key);
+    return rc;
+}
+
 static void
 show_warning(PyThreadState *tstate, PyObject *filename, int lineno,
              PyObject *text, PyObject *category, PyObject *sourceline)
@@ -854,13 +879,8 @@ warn_explicit(PyThreadState *tstate, PyObject *category, PyObject *message,
         }
 
         if (_PyUnicode_EqualToASCIIString(action, "once")) {
-            if (registry == NULL || registry == Py_None) {
-                registry = get_once_registry(interp);
-                if (registry == NULL)
-                    goto cleanup;
-            }
             /* WarningsState.once_registry[(text, category)] = 1 */
-            rc = update_registry(interp, registry, text, category, 0);
+            rc = update_once_registry(interp, text, category);
         }
         else if (_PyUnicode_EqualToASCIIString(action, "module")) {
             /* registry[(text, category, 0)] = 1 */