https://github.com/python/cpython/commit/ac430e328525dd803053dea05bda9701a4a651c2
commit: ac430e328525dd803053dea05bda9701a4a651c2
branch: main
author: Shamil <[email protected]>
committer: kumaraditya303 <[email protected]>
date: 2026-10-22T14:18:28+05:30
summary:
gh-158304: Avoid creating StopIteration for every value yielded by an async
generator (#158315)
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2026-09-28-10-12-25.gh-issue-158304.3pdcnJ.rst
M Objects/genobject.c
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2026-09-28-10-12-25.gh-issue-158304.3pdcnJ.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2026-09-28-10-12-25.gh-issue-158304.3pdcnJ.rst
new file mode 100644
index 00000000000000..394367d0dba8eb
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2026-09-28-10-12-25.gh-issue-158304.3pdcnJ.rst
@@ -0,0 +1,4 @@
+Speed up iteration over asynchronous generators: values yielded to
+``async for`` no longer create and discard a :exc:`StopIteration` exception
+at every nesting level. The ``async_generators`` pyperformance benchmark is
+about 30% faster. Patch by Shamil Abdulaev.
diff --git a/Objects/genobject.c b/Objects/genobject.c
index 05a331e88b88c9..1c95a569af8b9b 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -1965,8 +1965,10 @@ PyAsyncGen_New(PyFrameObject *f, PyObject *name,
PyObject *qualname)
return (PyObject*)ag;
}
-static PyObject *
-async_gen_unwrap_value(PyAsyncGenObject *gen, PyObject *result)
+// Report an async yield as PYGEN_RETURN with the yielded value.
+static PySendResult
+async_gen_unwrap_send(PyAsyncGenObject *gen, PyObject *result,
+ PyObject **presult)
{
if (result == NULL) {
if (!PyErr_Occurred()) {
@@ -1979,17 +1981,31 @@ async_gen_unwrap_value(PyAsyncGenObject *gen, PyObject
*result)
FT_ATOMIC_STORE_INT8_RELAXED(gen->ag_closed, 1);
}
- return NULL;
+ *presult = NULL;
+ return PYGEN_ERROR;
}
if (_PyAsyncGenWrappedValue_CheckExact(result)) {
/* async yield */
-
_PyGen_SetStopIterationValue(((_PyAsyncGenWrappedValue*)result)->agw_val);
+ *presult = Py_NewRef(((_PyAsyncGenWrappedValue*)result)->agw_val);
Py_DECREF(result);
- return NULL;
+ return PYGEN_RETURN;
}
- return result;
+ *presult = result;
+ return PYGEN_NEXT;
+}
+
+static PyObject *
+async_gen_unwrap_value(PyAsyncGenObject *gen, PyObject *result)
+{
+ PyObject *value;
+ if (async_gen_unwrap_send(gen, result, &value) == PYGEN_RETURN) {
+ _PyGen_SetStopIterationValue(value);
+ Py_DECREF(value);
+ return NULL;
+ }
+ return value;
}
@@ -2002,7 +2018,10 @@ async_gen_asend_dealloc(PyObject *self)
assert(PyAsyncGenASend_CheckExact(self));
PyAsyncGenASend *ags = _PyAsyncGenASend_CAST(self);
- if (PyObject_CallFinalizerFromDealloc(self)) {
+ // The finalizer only warns about an asend() that was never awaited.
+ if (ags->ags_state == AWAITABLE_STATE_INIT
+ && PyObject_CallFinalizerFromDealloc(self))
+ {
return;
}
@@ -2025,18 +2044,19 @@ async_gen_asend_traverse(PyObject *self, visitproc
visit, void *arg)
}
-static PyObject *
-async_gen_asend_send(PyObject *self, PyObject *arg)
+PySendResult
+_PyAsyncGenASend_Send(PyObject *self, PyObject *arg, PyObject **presult)
{
PyAsyncGenASend *o = _PyAsyncGenASend_CAST(self);
+ *presult = NULL;
int8_t state = FT_ATOMIC_LOAD_INT8_RELAXED(o->ags_state);
do {
if (state == AWAITABLE_STATE_CLOSED) {
PyErr_SetString(
PyExc_RuntimeError,
"cannot reuse already awaited __anext__()/asend()");
- return NULL;
+ return PYGEN_ERROR;
}
if (state == AWAITABLE_STATE_ITER) {
goto do_send;
@@ -2053,7 +2073,7 @@ async_gen_asend_send(PyObject *self, PyObject *arg)
PyErr_SetString(
PyExc_RuntimeError,
"anext(): asynchronous generator is already running");
- return NULL;
+ return PYGEN_ERROR;
}
if (arg == NULL || arg == Py_None) {
@@ -2061,29 +2081,29 @@ async_gen_asend_send(PyObject *self, PyObject *arg)
}
PyObject *result;
+ PySendResult res;
do_send:
result = gen_send((PyObject*)o->ags_gen, arg);
- result = async_gen_unwrap_value(o->ags_gen, result);
+ res = async_gen_unwrap_send(o->ags_gen, result, presult);
- if (result == NULL) {
+ if (res != PYGEN_NEXT) {
FT_ATOMIC_STORE_INT8_RELAXED(o->ags_state, AWAITABLE_STATE_CLOSED);
FT_ATOMIC_STORE_INT8_RELEASE(o->ags_gen->ag_running_async, 0);
}
- return result;
+ return res;
}
-PySendResult
-_PyAsyncGenASend_Send(PyObject *iter, PyObject *arg, PyObject **result)
+static PyObject *
+async_gen_asend_send(PyObject *self, PyObject *arg)
{
- *result = async_gen_asend_send(iter, arg);
- if (*result != NULL) {
- return PYGEN_NEXT;
- }
- if (_PyGen_FetchStopIterationValue(result) == 0) {
- return PYGEN_RETURN;
+ PyObject *result;
+ if (_PyAsyncGenASend_Send(self, arg, &result) == PYGEN_RETURN) {
+ _PyGen_SetStopIterationValue(result);
+ Py_DECREF(result);
+ return NULL;
}
- return PYGEN_ERROR;
+ return result;
}
@@ -2168,6 +2188,7 @@ static void
async_gen_asend_finalize(PyObject *self)
{
PyAsyncGenASend *ags = _PyAsyncGenASend_CAST(self);
+ // async_gen_asend_dealloc() calls this only in the INIT state.
if (ags->ags_state == AWAITABLE_STATE_INIT) {
_PyErr_WarnUnawaitedAgenMethod(ags->ags_gen, &_Py_ID(asend));
}
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