From a6724dd561f2f629e3b3d709024e33bd640e4bc7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Mon, 13 Jan 2025 13:55:09 +0100
Subject: [PATCH] gh-128078: Clear exception in `anext` before calling
 `_PyGen_SetStopIterationValue` (GH-128780) (cherry picked from commit
 76ffaef729c91bb79da6df2ade48f3ec51118300)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>
---
 Lib/test/test_asyncgen.py                       | 17 +++++++++++++++++
 ...25-01-13-12-48-30.gh-issue-128078.qOsl9B.rst |  2 ++
 Objects/genobject.c                             |  1 +
 Objects/iterobject.c                            |  2 ++
 4 files changed, 22 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-01-13-12-48-30.gh-issue-128078.qOsl9B.rst

diff --git a/Lib/test/test_asyncgen.py b/Lib/test/test_asyncgen.py
index 9e199806da6b15..7c868f367b3e8c 100644
--- a/Lib/test/test_asyncgen.py
+++ b/Lib/test/test_asyncgen.py
@@ -1100,6 +1100,23 @@ async def run():
 
         self.loop.run_until_complete(run())
 
+    def test_async_gen_asyncio_anext_tuple_no_exceptions(self):
+        # StopAsyncIteration exceptions should be cleared.
+        # See: https://github.com/python/cpython/issues/128078.
+
+        async def foo():
+            if False:
+                yield (1, 2)
+
+        async def run():
+            it = foo().__aiter__()
+            with self.assertRaises(StopAsyncIteration):
+                await it.__anext__()
+            res = await anext(it, ('a', 'b'))
+            self.assertEqual(res, ('a', 'b'))
+
+        self.loop.run_until_complete(run())
+
     def test_async_gen_asyncio_anext_stopiteration(self):
         async def foo():
             try:
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-01-13-12-48-30.gh-issue-128078.qOsl9B.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-01-13-12-48-30.gh-issue-128078.qOsl9B.rst
new file mode 100644
index 00000000000000..498864a0aa3145
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-01-13-12-48-30.gh-issue-128078.qOsl9B.rst
@@ -0,0 +1,2 @@
+Fix a :exc:`SystemError` when using :func:`anext` with a default tuple
+value. Patch by Bénédikt Tran.
diff --git a/Objects/genobject.c b/Objects/genobject.c
index 474abe1094be72..d56a9ad5d73ef5 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -618,6 +618,7 @@ gen_iternext(PyGenObject *gen)
 int
 _PyGen_SetStopIterationValue(PyObject *value)
 {
+    assert(!PyErr_Occurred());
     PyObject *e;
 
     if (value == NULL ||
diff --git a/Objects/iterobject.c b/Objects/iterobject.c
index 7cb17a6ca4ab56..66e4490766abbd 100644
--- a/Objects/iterobject.c
+++ b/Objects/iterobject.c
@@ -382,6 +382,7 @@ anextawaitable_iternext(anextawaitableobject *obj)
         return result;
     }
     if (PyErr_ExceptionMatches(PyExc_StopAsyncIteration)) {
+        PyErr_Clear();
         _PyGen_SetStopIterationValue(obj->default_value);
     }
     return NULL;
@@ -405,6 +406,7 @@ anextawaitable_proxy(anextawaitableobject *obj, char *meth, PyObject *arg) {
          * exception we replace it with a `StopIteration(default)`, as if
          * it was the return value of `__anext__()` coroutine.
          */
+        PyErr_Clear();
         _PyGen_SetStopIterationValue(obj->default_value);
     }
     return NULL;