Issue #28003: Implement PEP 525 -- Asynchronous Generators.

Include/ceval.h

@@ -25,6 +25,10 @@ PyAPI_FUNC(void) PyEval_SetProfile(Py_tracefunc, PyObject *);
 PyAPI_FUNC(void) PyEval_SetTrace(Py_tracefunc, PyObject *);
 PyAPI_FUNC(void) _PyEval_SetCoroutineWrapper(PyObject *);
 PyAPI_FUNC(PyObject *) _PyEval_GetCoroutineWrapper(void);
+PyAPI_FUNC(void) _PyEval_SetAsyncGenFirstiter(PyObject *);
+PyAPI_FUNC(PyObject *) _PyEval_GetAsyncGenFirstiter(void);
+PyAPI_FUNC(void) _PyEval_SetAsyncGenFinalizer(PyObject *);
+PyAPI_FUNC(PyObject *) _PyEval_GetAsyncGenFinalizer(void);
 #endif
 
 struct _frame; /* Avoid including frameobject.h */

Include/code.h

@@ -59,6 +59,7 @@ typedef struct {
    ``async def`` keywords) */
 #define CO_COROUTINE            0x0080
 #define CO_ITERABLE_COROUTINE   0x0100
+#define CO_ASYNC_GENERATOR      0x0200
 
 /* These are no longer used. */
 #if 0

Include/genobject.h

@@ -61,6 +61,37 @@ PyObject *_PyAIterWrapper_New(PyObject *aiter);
 PyObject *_PyCoro_GetAwaitableIter(PyObject *o);
 PyAPI_FUNC(PyObject *) PyCoro_New(struct _frame *,
     PyObject *name, PyObject *qualname);
+
+/* Asynchronous Generators */
+
+typedef struct {
+    _PyGenObject_HEAD(ag)
+    PyObject *ag_finalizer;
+
+    /* Flag is set to 1 when hooks set up by sys.set_asyncgen_hooks
+       were called on the generator, to avoid calling them more
+       than once. */
+    int ag_hooks_inited;
+
+    /* Flag is set to 1 when aclose() is called for the first time, or
+       when a StopAsyncIteration exception is raised. */
+    int ag_closed;
+} PyAsyncGenObject;
+
+PyAPI_DATA(PyTypeObject) PyAsyncGen_Type;
+PyAPI_DATA(PyTypeObject) _PyAsyncGenASend_Type;
+PyAPI_DATA(PyTypeObject) _PyAsyncGenWrappedValue_Type;
+PyAPI_DATA(PyTypeObject) _PyAsyncGenAThrow_Type;
+
+PyAPI_FUNC(PyObject *) PyAsyncGen_New(struct _frame *,
+    PyObject *name, PyObject *qualname);
+
+#define PyAsyncGen_CheckExact(op) (Py_TYPE(op) == &PyAsyncGen_Type)
+
+PyObject *_PyAsyncGenValueWrapperNew(PyObject *);
+
+int PyAsyncGen_ClearFreeLists(void);
+
 #endif
 
 #undef _PyGenObject_HEAD

Include/pylifecycle.h

@@ -107,6 +107,7 @@ PyAPI_FUNC(void) _PyGC_Fini(void);
 PyAPI_FUNC(void) PySlice_Fini(void);
 PyAPI_FUNC(void) _PyType_Fini(void);
 PyAPI_FUNC(void) _PyRandom_Fini(void);
+PyAPI_FUNC(void) PyAsyncGen_Fini(void);
 
 PyAPI_DATA(PyThreadState *) _Py_Finalizing;
 #endif

Include/pystate.h

@@ -148,6 +148,9 @@ typedef struct _ts {
     Py_ssize_t co_extra_user_count;
     freefunc co_extra_freefuncs[MAX_CO_EXTRA_USERS];
 
+    PyObject *async_gen_firstiter;
+    PyObject *async_gen_finalizer;
+
     /* XXX signal handlers should also be here */
 
 } PyThreadState;

Include/symtable.h

@@ -48,6 +48,7 @@ typedef struct _symtable_entry {
     unsigned ste_child_free : 1;  /* true if a child block has free vars,
                                      including free refs to globals */
     unsigned ste_generator : 1;   /* true if namespace is a generator */
+    unsigned ste_coroutine : 1;   /* true if namespace is a coroutine */
     unsigned ste_varargs : 1;     /* true if block has varargs */
     unsigned ste_varkeywords : 1; /* true if block has varkeywords */
     unsigned ste_returns_value : 1;  /* true if namespace uses return with

Lib/asyncio/base_events.py

@@ -13,7 +13,6 @@ conscious design decision, leaving the door open for keyword arguments
 to modify the meaning of the API call itself.
 """
 
-
 import collections
 import concurrent.futures
 import heapq
@@ -28,6 +27,7 @@ import time
 import traceback
 import sys
 import warnings
+import weakref
 
 from . import compat
 from . import coroutines
@@ -242,6 +242,13 @@ class BaseEventLoop(events.AbstractEventLoop):
         self._task_factory = None
         self._coroutine_wrapper_set = False
 
+        # A weak set of all asynchronous generators that are being iterated
+        # by the loop.
+        self._asyncgens = weakref.WeakSet()
+
+        # Set to True when `loop.shutdown_asyncgens` is called.
+        self._asyncgens_shutdown_called = False
+
     def __repr__(self):
         return ('<%s running=%s closed=%s debug=%s>'
                 % (self.__class__.__name__, self.is_running(),
@@ -333,6 +340,46 @@ class BaseEventLoop(events.AbstractEventLoop):
         if self._closed:
             raise RuntimeError('Event loop is closed')
 
+    def _asyncgen_finalizer_hook(self, agen):
+        self._asyncgens.discard(agen)
+        if not self.is_closed():
+            self.create_task(agen.aclose())
+
+    def _asyncgen_firstiter_hook(self, agen):
+        if self._asyncgens_shutdown_called:
+            warnings.warn(
+                "asynchronous generator {!r} was scheduled after "
+                "loop.shutdown_asyncgens() call".format(agen),
+                ResourceWarning, source=self)
+
+        self._asyncgens.add(agen)
+
+    @coroutine
+    def shutdown_asyncgens(self):
+        """Shutdown all active asynchronous generators."""
+        self._asyncgens_shutdown_called = True
+
+        if not len(self._asyncgens):
+            return
+
+        closing_agens = list(self._asyncgens)
+        self._asyncgens.clear()
+
+        shutdown_coro = tasks.gather(
+            *[ag.aclose() for ag in closing_agens],
+            return_exceptions=True,
+            loop=self)
+
+        results = yield from shutdown_coro
+        for result, agen in zip(results, closing_agens):
+            if isinstance(result, Exception):
+                self.call_exception_handler({
+                    'message': 'an error occurred during closing of '
+                               'asynchronous generator {!r}'.format(agen),
+                    'exception': result,
+                    'asyncgen': agen
+                })
+
     def run_forever(self):
         """Run until stop() is called."""
         self._check_closed()
@@ -340,6 +387,9 @@ class BaseEventLoop(events.AbstractEventLoop):
             raise RuntimeError('Event loop is running.')
         self._set_coroutine_wrapper(self._debug)
         self._thread_id = threading.get_ident()
+        old_agen_hooks = sys.get_asyncgen_hooks()
+        sys.set_asyncgen_hooks(firstiter=self._asyncgen_firstiter_hook,
+                               finalizer=self._asyncgen_finalizer_hook)
         try:
             while True:
                 self._run_once()
@@ -349,6 +399,7 @@ class BaseEventLoop(events.AbstractEventLoop):
             self._stopping = False
             self._thread_id = None
             self._set_coroutine_wrapper(False)
+            sys.set_asyncgen_hooks(*old_agen_hooks)
 
     def run_until_complete(self, future):
         """Run until the Future is done.
@@ -1179,7 +1230,9 @@ class BaseEventLoop(events.AbstractEventLoop):
         - 'handle' (optional): Handle instance;
         - 'protocol' (optional): Protocol instance;
         - 'transport' (optional): Transport instance;
-        - 'socket' (optional): Socket instance.
+        - 'socket' (optional): Socket instance;
+        - 'asyncgen' (optional): Asynchronous generator that caused
+                                 the exception.
 
         New keys maybe introduced in the future.
 

Lib/asyncio/coroutines.py

@@ -276,7 +276,10 @@ def _format_coroutine(coro):
     try:
         coro_code = coro.gi_code
     except AttributeError:
-        coro_code = coro.cr_code
+        try:
+            coro_code = coro.cr_code
+        except AttributeError:
+            return repr(coro)
 
     try:
         coro_frame = coro.gi_frame

Lib/asyncio/events.py

@@ -248,6 +248,10 @@ class AbstractEventLoop:
         """
         raise NotImplementedError
 
+    def shutdown_asyncgens(self):
+        """Shutdown all active asynchronous generators."""
+        raise NotImplementedError
+
     # Methods scheduling callbacks.  All these return Handles.
 
     def _timer_handle_cancelled(self, handle):

Lib/dis.py

@@ -87,6 +87,7 @@ COMPILER_FLAG_NAMES = {
     64: "NOFREE",
    128: "COROUTINE",
    256: "ITERABLE_COROUTINE",
+   512: "ASYNC_GENERATOR",
 }
 
 def pretty_flags(flags):

Lib/inspect.py

@@ -185,6 +185,13 @@ def iscoroutinefunction(object):
     return bool((isfunction(object) or ismethod(object)) and
                 object.__code__.co_flags & CO_COROUTINE)
 
+def isasyncgenfunction(object):
+    return bool((isfunction(object) or ismethod(object)) and
+                object.__code__.co_flags & CO_ASYNC_GENERATOR)
+
+def isasyncgen(object):
+    return isinstance(object, types.AsyncGeneratorType)
+
 def isgenerator(object):
     """Return true if the object is a generator.
 

Lib/test/badsyntax_async6.py

@@ -1,2 +0,0 @@
-async def foo():
-    yield

Lib/test/test_asyncgen.py

@@ -0,0 +1,823 @@
+import asyncio
+import inspect
+import sys
+import types
+import unittest
+
+from unittest import mock
+
+
+class AwaitException(Exception):
+    pass
+
+
+@types.coroutine
+def awaitable(*, throw=False):
+    if throw:
+        yield ('throw',)
+    else:
+        yield ('result',)
+
+
+def run_until_complete(coro):
+    exc = False
+    while True:
+        try:
+            if exc:
+                exc = False
+                fut = coro.throw(AwaitException)
+            else:
+                fut = coro.send(None)
+        except StopIteration as ex:
+            return ex.args[0]
+
+        if fut == ('throw',):
+            exc = True
+
+
+def to_list(gen):
+    async def iterate():
+        res = []
+        async for i in gen:
+            res.append(i)
+        return res
+
+    return run_until_complete(iterate())
+
+
+class AsyncGenSyntaxTest(unittest.TestCase):
+
+    def test_async_gen_syntax_01(self):
+        code = '''async def foo():
+            await abc
+            yield from 123
+        '''
+
+        with self.assertRaisesRegex(SyntaxError, 'yield from.*inside async'):
+            exec(code, {}, {})
+
+    def test_async_gen_syntax_02(self):
+        code = '''async def foo():
+            yield from 123
+        '''
+
+        with self.assertRaisesRegex(SyntaxError, 'yield from.*inside async'):
+            exec(code, {}, {})
+
+    def test_async_gen_syntax_03(self):
+        code = '''async def foo():
+            await abc
+            yield
+            return 123
+        '''
+
+        with self.assertRaisesRegex(SyntaxError, 'return.*value.*async gen'):
+            exec(code, {}, {})
+
+    def test_async_gen_syntax_04(self):
+        code = '''async def foo():
+            yield
+            return 123
+        '''
+
+        with self.assertRaisesRegex(SyntaxError, 'return.*value.*async gen'):
+            exec(code, {}, {})
+
+    def test_async_gen_syntax_05(self):
+        code = '''async def foo():
+            if 0:
+                yield
+            return 12
+        '''
+
+        with self.assertRaisesRegex(SyntaxError, 'return.*value.*async gen'):
+            exec(code, {}, {})
+
+
+class AsyncGenTest(unittest.TestCase):
+
+    def compare_generators(self, sync_gen, async_gen):
+        def sync_iterate(g):
+            res = []
+            while True:
+                try:
+                    res.append(g.__next__())
+                except StopIteration:
+                    res.append('STOP')
+                    break
+                except Exception as ex:
+                    res.append(str(type(ex)))
+            return res
+
+        def async_iterate(g):
+            res = []
+            while True:
+                try:
+                    g.__anext__().__next__()
+                except StopAsyncIteration:
+                    res.append('STOP')
+                    break
+                except StopIteration as ex:
+                    if ex.args:
+                        res.append(ex.args[0])
+                    else:
+                        res.append('EMPTY StopIteration')
+                        break
+                except Exception as ex:
+                    res.append(str(type(ex)))
+            return res
+
+        sync_gen_result = sync_iterate(sync_gen)
+        async_gen_result = async_iterate(async_gen)
+        self.assertEqual(sync_gen_result, async_gen_result)
+        return async_gen_result
+
+    def test_async_gen_iteration_01(self):
+        async def gen():
+            await awaitable()
+            a = yield 123
+            self.assertIs(a, None)
+            await awaitable()
+            yield 456
+            await awaitable()
+            yield 789
+
+        self.assertEqual(to_list(gen()), [123, 456, 789])
+
+    def test_async_gen_iteration_02(self):
+        async def gen():
+            await awaitable()
+            yield 123
+            await awaitable()
+
+        g = gen()
+        ai = g.__aiter__()
+        self.assertEqual(ai.__anext__().__next__(), ('result',))
+
+        try:
+            ai.__anext__().__next__()
+        except StopIteration as ex:
+            self.assertEqual(ex.args[0], 123)
+        else:
+            self.fail('StopIteration was not raised')
+
+        self.assertEqual(ai.__anext__().__next__(), ('result',))
+
+        try:
+            ai.__anext__().__next__()
+        except StopAsyncIteration as ex:
+            self.assertFalse(ex.args)
+        else:
+            self.fail('StopAsyncIteration was not raised')
+
+    def test_async_gen_exception_03(self):
+        async def gen():
+            await awaitable()
+            yield 123
+            await awaitable(throw=True)
+            yield 456
+
+        with self.assertRaises(AwaitException):
+            to_list(gen())
+
+    def test_async_gen_exception_04(self):
+        async def gen():
+            await awaitable()
+            yield 123
+            1 / 0
+
+        g = gen()
+        ai = g.__aiter__()
+        self.assertEqual(ai.__anext__().__next__(), ('result',))
+
+        try:
+            ai.__anext__().__next__()
+        except StopIteration as ex:
+            self.assertEqual(ex.args[0], 123)
+        else:
+            self.fail('StopIteration was not raised')
+
+        with self.assertRaises(ZeroDivisionError):
+            ai.__anext__().__next__()
+
+    def test_async_gen_exception_05(self):
+        async def gen():
+            yield 123
+            raise StopAsyncIteration
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'async generator.*StopAsyncIteration'):
+            to_list(gen())
+
+    def test_async_gen_exception_06(self):
+        async def gen():
+            yield 123
+            raise StopIteration
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'async generator.*StopIteration'):
+            to_list(gen())
+
+    def test_async_gen_exception_07(self):
+        def sync_gen():
+            try:
+                yield 1
+                1 / 0
+            finally:
+                yield 2
+                yield 3
+
+            yield 100
+
+        async def async_gen():
+            try:
+                yield 1
+                1 / 0
+            finally:
+                yield 2
+                yield 3
+
+            yield 100
+
+        self.compare_generators(sync_gen(), async_gen())
+
+    def test_async_gen_exception_08(self):
+        def sync_gen():
+            try:
+                yield 1
+            finally:
+                yield 2
+                1 / 0
+                yield 3
+
+            yield 100
+
+        async def async_gen():
+            try:
+                yield 1
+                await awaitable()
+            finally:
+                await awaitable()
+                yield 2
+                1 / 0
+                yield 3
+
+            yield 100
+
+        self.compare_generators(sync_gen(), async_gen())
+
+    def test_async_gen_exception_09(self):
+        def sync_gen():
+            try:
+                yield 1
+                1 / 0
+            finally:
+                yield 2
+                yield 3
+
+            yield 100
+
+        async def async_gen():
+            try:
+                await awaitable()
+                yield 1
+                1 / 0
+            finally:
+                yield 2
+                await awaitable()
+                yield 3
+
+            yield 100
+
+        self.compare_generators(sync_gen(), async_gen())
+
+    def test_async_gen_exception_10(self):
+        async def gen():
+            yield 123
+        with self.assertRaisesRegex(TypeError,
+                                    "non-None value .* async generator"):
+            gen().__anext__().send(100)
+
+    def test_async_gen_api_01(self):
+        async def gen():
+            yield 123
+
+        g = gen()
+
+        self.assertEqual(g.__name__, 'gen')
+        g.__name__ = '123'
+        self.assertEqual(g.__name__, '123')
+
+        self.assertIn('.gen', g.__qualname__)
+        g.__qualname__ = '123'
+        self.assertEqual(g.__qualname__, '123')
+
+        self.assertIsNone(g.ag_await)
+        self.assertIsInstance(g.ag_frame, types.FrameType)
+        self.assertFalse(g.ag_running)
+        self.assertIsInstance(g.ag_code, types.CodeType)
+
+        self.assertTrue(inspect.isawaitable(g.aclose()))
+
+
+class AsyncGenAsyncioTest(unittest.TestCase):
+
+    def setUp(self):
+        self.loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(None)
+
+    def tearDown(self):
+        self.loop.close()
+        self.loop = None
+
+    async def to_list(self, gen):
+        res = []
+        async for i in gen:
+            res.append(i)
+        return res
+
+    def test_async_gen_asyncio_01(self):
+        async def gen():
+            yield 1
+            await asyncio.sleep(0.01, loop=self.loop)
+            yield 2
+            await asyncio.sleep(0.01, loop=self.loop)
+            return
+            yield 3
+
+        res = self.loop.run_until_complete(self.to_list(gen()))
+        self.assertEqual(res, [1, 2])
+
+    def test_async_gen_asyncio_02(self):
+        async def gen():
+            yield 1
+            await asyncio.sleep(0.01, loop=self.loop)
+            yield 2
+            1 / 0
+            yield 3
+
+        with self.assertRaises(ZeroDivisionError):
+            self.loop.run_until_complete(self.to_list(gen()))
+
+    def test_async_gen_asyncio_03(self):
+        loop = self.loop
+
+        class Gen:
+            async def __aiter__(self):
+                yield 1
+                await asyncio.sleep(0.01, loop=loop)
+                yield 2
+
+        res = loop.run_until_complete(self.to_list(Gen()))
+        self.assertEqual(res, [1, 2])
+
+    def test_async_gen_asyncio_anext_04(self):
+        async def foo():
+            yield 1
+            await asyncio.sleep(0.01, loop=self.loop)
+            try:
+                yield 2
+                yield 3
+            except ZeroDivisionError:
+                yield 1000
+            await asyncio.sleep(0.01, loop=self.loop)
+            yield 4
+
+        async def run1():
+            it = foo().__aiter__()
+
+            self.assertEqual(await it.__anext__(), 1)
+            self.assertEqual(await it.__anext__(), 2)
+            self.assertEqual(await it.__anext__(), 3)
+            self.assertEqual(await it.__anext__(), 4)
+            with self.assertRaises(StopAsyncIteration):
+                await it.__anext__()
+            with self.assertRaises(StopAsyncIteration):
+                await it.__anext__()
+
+        async def run2():
+            it = foo().__aiter__()
+
+            self.assertEqual(await it.__anext__(), 1)
+            self.assertEqual(await it.__anext__(), 2)
+            try:
+                it.__anext__().throw(ZeroDivisionError)
+            except StopIteration as ex:
+                self.assertEqual(ex.args[0], 1000)
+            else:
+                self.fail('StopIteration was not raised')
+            self.assertEqual(await it.__anext__(), 4)
+            with self.assertRaises(StopAsyncIteration):
+                await it.__anext__()
+
+        self.loop.run_until_complete(run1())
+        self.loop.run_until_complete(run2())
+
+    def test_async_gen_asyncio_anext_05(self):
+        async def foo():
+            v = yield 1
+            v = yield v
+            yield v * 100
+
+        async def run():
+            it = foo().__aiter__()
+
+            try:
+                it.__anext__().send(None)
+            except StopIteration as ex:
+                self.assertEqual(ex.args[0], 1)
+            else:
+                self.fail('StopIteration was not raised')
+
+            try:
+                it.__anext__().send(10)
+            except StopIteration as ex:
+                self.assertEqual(ex.args[0], 10)
+            else:
+                self.fail('StopIteration was not raised')
+
+            try:
+                it.__anext__().send(12)
+            except StopIteration as ex:
+                self.assertEqual(ex.args[0], 1200)
+            else:
+                self.fail('StopIteration was not raised')
+
+            with self.assertRaises(StopAsyncIteration):
+                await it.__anext__()
+
+        self.loop.run_until_complete(run())
+
+    def test_async_gen_asyncio_aclose_06(self):
+        async def foo():
+            try:
+                yield 1
+                1 / 0
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                yield 12
+
+        async def run():
+            gen = foo()
+            it = gen.__aiter__()
+            await it.__anext__()
+            await gen.aclose()
+
+        with self.assertRaisesRegex(
+                RuntimeError,
+                "async generator ignored GeneratorExit"):
+            self.loop.run_until_complete(run())
+
+    def test_async_gen_asyncio_aclose_07(self):
+        DONE = 0
+
+        async def foo():
+            nonlocal DONE
+            try:
+                yield 1
+                1 / 0
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE += 1
+            DONE += 1000
+
+        async def run():
+            gen = foo()
+            it = gen.__aiter__()
+            await it.__anext__()
+            await gen.aclose()
+
+        self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+    def test_async_gen_asyncio_aclose_08(self):
+        DONE = 0
+
+        fut = asyncio.Future(loop=self.loop)
+
+        async def foo():
+            nonlocal DONE
+            try:
+                yield 1
+                await fut
+                DONE += 1000
+                yield 2
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE += 1
+            DONE += 1000
+
+        async def run():
+            gen = foo()
+            it = gen.__aiter__()
+            self.assertEqual(await it.__anext__(), 1)
+            t = self.loop.create_task(it.__anext__())
+            await asyncio.sleep(0.01, loop=self.loop)
+            await gen.aclose()
+            return t
+
+        t = self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+        # Silence ResourceWarnings
+        fut.cancel()
+        t.cancel()
+        self.loop.run_until_complete(asyncio.sleep(0.01, loop=self.loop))
+
+    def test_async_gen_asyncio_gc_aclose_09(self):
+        DONE = 0
+
+        async def gen():
+            nonlocal DONE
+            try:
+                while True:
+                    yield 1
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE = 1
+
+        async def run():
+            g = gen()
+            await g.__anext__()
+            await g.__anext__()
+            del g
+
+            await asyncio.sleep(0.1, loop=self.loop)
+
+        self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+    def test_async_gen_asyncio_asend_01(self):
+        DONE = 0
+
+        # Sanity check:
+        def sgen():
+            v = yield 1
+            yield v * 2
+        sg = sgen()
+        v = sg.send(None)
+        self.assertEqual(v, 1)
+        v = sg.send(100)
+        self.assertEqual(v, 200)
+
+        async def gen():
+            nonlocal DONE
+            try:
+                await asyncio.sleep(0.01, loop=self.loop)
+                v = yield 1
+                await asyncio.sleep(0.01, loop=self.loop)
+                yield v * 2
+                await asyncio.sleep(0.01, loop=self.loop)
+                return
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE = 1
+
+        async def run():
+            g = gen()
+
+            v = await g.asend(None)
+            self.assertEqual(v, 1)
+
+            v = await g.asend(100)
+            self.assertEqual(v, 200)
+
+            with self.assertRaises(StopAsyncIteration):
+                await g.asend(None)
+
+        self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+    def test_async_gen_asyncio_asend_02(self):
+        DONE = 0
+
+        async def sleep_n_crash(delay):
+            await asyncio.sleep(delay, loop=self.loop)
+            1 / 0
+
+        async def gen():
+            nonlocal DONE
+            try:
+                await asyncio.sleep(0.01, loop=self.loop)
+                v = yield 1
+                await sleep_n_crash(0.01)
+                DONE += 1000
+                yield v * 2
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE = 1
+
+        async def run():
+            g = gen()
+
+            v = await g.asend(None)
+            self.assertEqual(v, 1)
+
+            await g.asend(100)
+
+        with self.assertRaises(ZeroDivisionError):
+            self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+    def test_async_gen_asyncio_asend_03(self):
+        DONE = 0
+
+        async def sleep_n_crash(delay):
+            fut = asyncio.ensure_future(asyncio.sleep(delay, loop=self.loop),
+                                        loop=self.loop)
+            self.loop.call_later(delay / 2, lambda: fut.cancel())
+            return await fut
+
+        async def gen():
+            nonlocal DONE
+            try:
+                await asyncio.sleep(0.01, loop=self.loop)
+                v = yield 1
+                await sleep_n_crash(0.01)
+                DONE += 1000
+                yield v * 2
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE = 1
+
+        async def run():
+            g = gen()
+
+            v = await g.asend(None)
+            self.assertEqual(v, 1)
+
+            await g.asend(100)
+
+        with self.assertRaises(asyncio.CancelledError):
+            self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+    def test_async_gen_asyncio_athrow_01(self):
+        DONE = 0
+
+        class FooEr(Exception):
+            pass
+
+        # Sanity check:
+        def sgen():
+            try:
+                v = yield 1
+            except FooEr:
+                v = 1000
+            yield v * 2
+        sg = sgen()
+        v = sg.send(None)
+        self.assertEqual(v, 1)
+        v = sg.throw(FooEr)
+        self.assertEqual(v, 2000)
+        with self.assertRaises(StopIteration):
+            sg.send(None)
+
+        async def gen():
+            nonlocal DONE
+            try:
+                await asyncio.sleep(0.01, loop=self.loop)
+                try:
+                    v = yield 1
+                except FooEr:
+                    v = 1000
+                    await asyncio.sleep(0.01, loop=self.loop)
+                yield v * 2
+                await asyncio.sleep(0.01, loop=self.loop)
+                # return
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE = 1
+
+        async def run():
+            g = gen()
+
+            v = await g.asend(None)
+            self.assertEqual(v, 1)
+
+            v = await g.athrow(FooEr)
+            self.assertEqual(v, 2000)
+
+            with self.assertRaises(StopAsyncIteration):
+                await g.asend(None)
+
+        self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+    def test_async_gen_asyncio_athrow_02(self):
+        DONE = 0
+
+        class FooEr(Exception):
+            pass
+
+        async def sleep_n_crash(delay):
+            fut = asyncio.ensure_future(asyncio.sleep(delay, loop=self.loop),
+                                        loop=self.loop)
+            self.loop.call_later(delay / 2, lambda: fut.cancel())
+            return await fut
+
+        async def gen():
+            nonlocal DONE
+            try:
+                await asyncio.sleep(0.01, loop=self.loop)
+                try:
+                    v = yield 1
+                except FooEr:
+                    await sleep_n_crash(0.01)
+                yield v * 2
+                await asyncio.sleep(0.01, loop=self.loop)
+                # return
+            finally:
+                await asyncio.sleep(0.01, loop=self.loop)
+                await asyncio.sleep(0.01, loop=self.loop)
+                DONE = 1
+
+        async def run():
+            g = gen()
+
+            v = await g.asend(None)
+            self.assertEqual(v, 1)
+
+            try:
+                await g.athrow(FooEr)
+            except asyncio.CancelledError:
+                self.assertEqual(DONE, 1)
+                raise
+            else:
+                self.fail('CancelledError was not raised')
+
+        with self.assertRaises(asyncio.CancelledError):
+            self.loop.run_until_complete(run())
+        self.assertEqual(DONE, 1)
+
+    def test_async_gen_asyncio_shutdown_01(self):
+        finalized = 0
+
+        async def waiter(timeout):
+            nonlocal finalized
+            try:
+                await asyncio.sleep(timeout, loop=self.loop)
+                yield 1
+            finally:
+                await asyncio.sleep(0, loop=self.loop)
+                finalized += 1
+
+        async def wait():
+            async for _ in waiter(1):
+                pass
+
+        t1 = self.loop.create_task(wait())
+        t2 = self.loop.create_task(wait())
+
+        self.loop.run_until_complete(asyncio.sleep(0.1, loop=self.loop))
+
+        self.loop.run_until_complete(self.loop.shutdown_asyncgens())
+        self.assertEqual(finalized, 2)
+
+        # Silence warnings
+        t1.cancel()
+        t2.cancel()
+        self.loop.run_until_complete(asyncio.sleep(0.1, loop=self.loop))
+
+    def test_async_gen_asyncio_shutdown_02(self):
+        logged = 0
+
+        def logger(loop, context):
+            nonlocal logged
+            self.assertIn('asyncgen', context)
+            expected = 'an error occurred during closing of asynchronous'
+            if expected in context['message']:
+                logged += 1
+
+        async def waiter(timeout):
+            try:
+                await asyncio.sleep(timeout, loop=self.loop)
+                yield 1
+            finally:
+                1 / 0
+
+        async def wait():
+            async for _ in waiter(1):
+                pass
+
+        t = self.loop.create_task(wait())
+        self.loop.run_until_complete(asyncio.sleep(0.1, loop=self.loop))
+
+        self.loop.set_exception_handler(logger)
+        self.loop.run_until_complete(self.loop.shutdown_asyncgens())
+
+        self.assertEqual(logged, 1)
+
+        # Silence warnings
+        t.cancel()
+        self.loop.run_until_complete(asyncio.sleep(0.1, loop=self.loop))
+
+if __name__ == "__main__":
+    unittest.main()

Lib/test/test_coroutines.py

@@ -88,12 +88,6 @@ class AsyncBadSyntaxTest(unittest.TestCase):
         with self.assertRaisesRegex(SyntaxError, 'invalid syntax'):
             import test.badsyntax_async5
 
-    def test_badsyntax_6(self):
-        with self.assertRaisesRegex(
-            SyntaxError, "'yield' inside async function"):
-
-            import test.badsyntax_async6
-
     def test_badsyntax_7(self):
         with self.assertRaisesRegex(
             SyntaxError, "'yield from' inside async function"):

Lib/test/test_dis.py

@@ -574,7 +574,7 @@ Argument count:    0
 Kw-only arguments: 0
 Number of locals:  2
 Stack size:        17
-Flags:             OPTIMIZED, NEWLOCALS, GENERATOR, NOFREE, COROUTINE
+Flags:             OPTIMIZED, NEWLOCALS, NOFREE, COROUTINE
 Constants:
    0: None
    1: 1"""

Lib/test/test_inspect.py

@@ -65,7 +65,8 @@ class IsTestBase(unittest.TestCase):
                       inspect.isframe, inspect.isfunction, inspect.ismethod,
                       inspect.ismodule, inspect.istraceback,
                       inspect.isgenerator, inspect.isgeneratorfunction,
-                      inspect.iscoroutine, inspect.iscoroutinefunction])
+                      inspect.iscoroutine, inspect.iscoroutinefunction,
+                      inspect.isasyncgen, inspect.isasyncgenfunction])
 
     def istest(self, predicate, exp):
         obj = eval(exp)
@@ -73,6 +74,7 @@ class IsTestBase(unittest.TestCase):
 
         for other in self.predicates - set([predicate]):
             if (predicate == inspect.isgeneratorfunction or \
+               predicate == inspect.isasyncgenfunction or \
                predicate == inspect.iscoroutinefunction) and \
                other == inspect.isfunction:
                 continue
@@ -82,6 +84,10 @@ def generator_function_example(self):
     for i in range(2):
         yield i
 
+async def async_generator_function_example(self):
+    async for i in range(2):
+        yield i
+
 async def coroutine_function_example(self):
     return 'spam'
 
@@ -122,6 +128,10 @@ class TestPredicates(IsTestBase):
         self.istest(inspect.isdatadescriptor, 'collections.defaultdict.default_factory')
         self.istest(inspect.isgenerator, '(x for x in range(2))')
         self.istest(inspect.isgeneratorfunction, 'generator_function_example')
+        self.istest(inspect.isasyncgen,
+                    'async_generator_function_example(1)')
+        self.istest(inspect.isasyncgenfunction,
+                    'async_generator_function_example')
 
         with warnings.catch_warnings():
             warnings.simplefilter("ignore")

Lib/test/test_sys.py

@@ -1192,6 +1192,32 @@ class SizeofTest(unittest.TestCase):
         # sys.flags
         check(sys.flags, vsize('') + self.P * len(sys.flags))
 
+    def test_asyncgen_hooks(self):
+        old = sys.get_asyncgen_hooks()
+        self.assertIsNone(old.firstiter)
+        self.assertIsNone(old.finalizer)
+
+        firstiter = lambda *a: None
+        sys.set_asyncgen_hooks(firstiter=firstiter)
+        hooks = sys.get_asyncgen_hooks()
+        self.assertIs(hooks.firstiter, firstiter)
+        self.assertIs(hooks[0], firstiter)
+        self.assertIs(hooks.finalizer, None)
+        self.assertIs(hooks[1], None)
+
+        finalizer = lambda *a: None
+        sys.set_asyncgen_hooks(finalizer=finalizer)
+        hooks = sys.get_asyncgen_hooks()
+        self.assertIs(hooks.firstiter, firstiter)
+        self.assertIs(hooks[0], firstiter)
+        self.assertIs(hooks.finalizer, finalizer)
+        self.assertIs(hooks[1], finalizer)
+
+        sys.set_asyncgen_hooks(*old)
+        cur = sys.get_asyncgen_hooks()
+        self.assertIsNone(cur.firstiter)
+        self.assertIsNone(cur.finalizer)
+
 
 def test_main():
     test.support.run_unittest(SysModuleTest, SizeofTest)

Lib/types.py

@@ -24,6 +24,11 @@ _c = _c()
 CoroutineType = type(_c)
 _c.close()  # Prevent ResourceWarning
 
+async def _ag():
+    yield
+_ag = _ag()
+AsyncGeneratorType = type(_ag)
+
 class _C:
     def _m(self): pass
 MethodType = type(_C()._m)

Misc/NEWS

@@ -103,6 +103,9 @@ Core and Builtins
 - Issue #27985: Implement PEP 526 -- Syntax for Variable Annotations.
   Patch by Ivan Levkivskyi.
 
+- Issue #28003: Implement PEP 525 -- Asynchronous Generators.
+
+
 Library
 -------
 

Modules/gcmodule.c

@@ -892,6 +892,7 @@ clear_freelists(void)
     (void)PyList_ClearFreeList();
     (void)PyDict_ClearFreeList();
     (void)PySet_ClearFreeList();
+    (void)PyAsyncGen_ClearFreeLists();
 }
 
 /* This is the main function.  Read this to understand how the

Python/ceval.c

@@ -1204,7 +1204,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
     f->f_stacktop = NULL;       /* remains NULL unless yield suspends frame */
     f->f_executing = 1;
 
-    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE)) {
+    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE | CO_ASYNC_GENERATOR)) {
         if (!throwflag && f->f_exc_type != NULL && f->f_exc_type != Py_None) {
             /* We were in an except handler when we left,
                restore the exception state which was put aside
@@ -2083,36 +2083,45 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
             PyObject *aiter = TOP();
             PyTypeObject *type = Py_TYPE(aiter);
 
-            if (type->tp_as_async != NULL)
-                getter = type->tp_as_async->am_anext;
-
-            if (getter != NULL) {
-                next_iter = (*getter)(aiter);
-                if (next_iter == NULL) {
+            if (PyAsyncGen_CheckExact(aiter)) {
+                awaitable = type->tp_as_async->am_anext(aiter);
+                if (awaitable == NULL) {
                     goto error;
                 }
-            }
-            else {
-                PyErr_Format(
-                    PyExc_TypeError,
-                    "'async for' requires an iterator with "
-                    "__anext__ method, got %.100s",
-                    type->tp_name);
-                goto error;
-            }
+            } else {
+                if (type->tp_as_async != NULL){
+                    getter = type->tp_as_async->am_anext;
+                }
 
-            awaitable = _PyCoro_GetAwaitableIter(next_iter);
-            if (awaitable == NULL) {
-                PyErr_Format(
-                    PyExc_TypeError,
-                    "'async for' received an invalid object "
-                    "from __anext__: %.100s",
-                    Py_TYPE(next_iter)->tp_name);
+                if (getter != NULL) {
+                    next_iter = (*getter)(aiter);
+                    if (next_iter == NULL) {
+                        goto error;
+                    }
+                }
+                else {
+                    PyErr_Format(
+                        PyExc_TypeError,
+                        "'async for' requires an iterator with "
+                        "__anext__ method, got %.100s",
+                        type->tp_name);
+                    goto error;
+                }
 
-                Py_DECREF(next_iter);
-                goto error;
-            } else
-                Py_DECREF(next_iter);
+                awaitable = _PyCoro_GetAwaitableIter(next_iter);
+                if (awaitable == NULL) {
+                    PyErr_Format(
+                        PyExc_TypeError,
+                        "'async for' received an invalid object "
+                        "from __anext__: %.100s",
+                        Py_TYPE(next_iter)->tp_name);
+
+                    Py_DECREF(next_iter);
+                    goto error;
+                } else {
+                    Py_DECREF(next_iter);
+                }
+            }
 
             PUSH(awaitable);
             PREDICT(LOAD_CONST);
@@ -2187,6 +2196,17 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
 
         TARGET(YIELD_VALUE) {
             retval = POP();
+
+            if (co->co_flags & CO_ASYNC_GENERATOR) {
+                PyObject *w = _PyAsyncGenValueWrapperNew(retval);
+                Py_DECREF(retval);
+                if (w == NULL) {
+                    retval = NULL;
+                    goto error;
+                }
+                retval = w;
+            }
+
             f->f_stacktop = stack_pointer;
             why = WHY_YIELD;
             goto fast_yield;
@@ -3712,7 +3732,7 @@ fast_block_end:
     assert((retval != NULL) ^ (PyErr_Occurred() != NULL));
 
 fast_yield:
-    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE)) {
+    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE | CO_ASYNC_GENERATOR)) {
 
         /* The purpose of this block is to put aside the generator's exception
            state and restore that of the calling frame. If the current
@@ -4156,8 +4176,8 @@ _PyEval_EvalCodeWithName(PyObject *_co, PyObject *globals, PyObject *locals,
         freevars[PyTuple_GET_SIZE(co->co_cellvars) + i] = o;
     }
 
-    /* Handle generator/coroutine */
-    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE)) {
+    /* Handle generator/coroutine/asynchronous generator */
+    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE | CO_ASYNC_GENERATOR)) {
         PyObject *gen;
         PyObject *coro_wrapper = tstate->coroutine_wrapper;
         int is_coro = co->co_flags & CO_COROUTINE;
@@ -4182,6 +4202,8 @@ _PyEval_EvalCodeWithName(PyObject *_co, PyObject *globals, PyObject *locals,
          * and return that as the value. */
         if (is_coro) {
             gen = PyCoro_New(f, name, qualname);
+        } else if (co->co_flags & CO_ASYNC_GENERATOR) {
+            gen = PyAsyncGen_New(f, name, qualname);
         } else {
             gen = PyGen_NewWithQualName(f, name, qualname);
         }
@@ -4660,6 +4682,38 @@ _PyEval_GetCoroutineWrapper(void)
     return tstate->coroutine_wrapper;
 }
 
+void
+_PyEval_SetAsyncGenFirstiter(PyObject *firstiter)
+{
+    PyThreadState *tstate = PyThreadState_GET();
+
+    Py_XINCREF(firstiter);
+    Py_XSETREF(tstate->async_gen_firstiter, firstiter);
+}
+
+PyObject *
+_PyEval_GetAsyncGenFirstiter(void)
+{
+    PyThreadState *tstate = PyThreadState_GET();
+    return tstate->async_gen_firstiter;
+}
+
+void
+_PyEval_SetAsyncGenFinalizer(PyObject *finalizer)
+{
+    PyThreadState *tstate = PyThreadState_GET();
+
+    Py_XINCREF(finalizer);
+    Py_XSETREF(tstate->async_gen_finalizer, finalizer);
+}
+
+PyObject *
+_PyEval_GetAsyncGenFinalizer(void)
+{
+    PyThreadState *tstate = PyThreadState_GET();
+    return tstate->async_gen_finalizer;
+}
+
 PyObject *
 PyEval_GetBuiltins(void)
 {

Python/compile.c

@@ -1886,8 +1886,6 @@ compiler_function(struct compiler *c, stmt_ty s, int is_async)
         return 0;
     }
 
-    if (is_async)
-        co->co_flags |= CO_COROUTINE;
     compiler_make_closure(c, co, funcflags, qualname);
     Py_DECREF(qualname);
     Py_DECREF(co);
@@ -2801,6 +2799,9 @@ compiler_visit_stmt(struct compiler *c, stmt_ty s)
         if (c->u->u_ste->ste_type != FunctionBlock)
             return compiler_error(c, "'return' outside function");
         if (s->v.Return.value) {
+            if (c->u->u_ste->ste_coroutine && c->u->u_ste->ste_generator)
+                return compiler_error(
+                    c, "'return' with value in async generator");
             VISIT(c, expr, s->v.Return.value);
         }
         else
@@ -4115,8 +4116,6 @@ compiler_visit_expr(struct compiler *c, expr_ty e)
     case Yield_kind:
         if (c->u->u_ste->ste_type != FunctionBlock)
             return compiler_error(c, "'yield' outside function");
-        if (c->u->u_scope_type == COMPILER_SCOPE_ASYNC_FUNCTION)
-            return compiler_error(c, "'yield' inside async function");
         if (e->v.Yield.value) {
             VISIT(c, expr, e->v.Yield.value);
         }
@@ -4992,8 +4991,12 @@ compute_code_flags(struct compiler *c)
         flags |= CO_NEWLOCALS | CO_OPTIMIZED;
         if (ste->ste_nested)
             flags |= CO_NESTED;
-        if (ste->ste_generator)
+        if (ste->ste_generator && !ste->ste_coroutine)
             flags |= CO_GENERATOR;
+        if (!ste->ste_generator && ste->ste_coroutine)
+            flags |= CO_COROUTINE;
+        if (ste->ste_generator && ste->ste_coroutine)
+            flags |= CO_ASYNC_GENERATOR;
         if (ste->ste_varargs)
             flags |= CO_VARARGS;
         if (ste->ste_varkeywords)

Python/pylifecycle.c

@@ -694,6 +694,7 @@ Py_FinalizeEx(void)
     _PyGC_Fini();
     _PyRandom_Fini();
     _PyArg_Fini();
+    PyAsyncGen_Fini();
 
     /* Cleanup Unicode implementation */
     _PyUnicode_Fini();

Python/pystate.c

@@ -229,6 +229,9 @@ new_threadstate(PyInterpreterState *interp, int init)
         tstate->in_coroutine_wrapper = 0;
         tstate->co_extra_user_count = 0;
 
+        tstate->async_gen_firstiter = NULL;
+        tstate->async_gen_finalizer = NULL;
+
         if (init)
             _PyThreadState_Init(tstate);
 
@@ -408,6 +411,8 @@ PyThreadState_Clear(PyThreadState *tstate)
     Py_CLEAR(tstate->c_traceobj);
 
     Py_CLEAR(tstate->coroutine_wrapper);
+    Py_CLEAR(tstate->async_gen_firstiter);
+    Py_CLEAR(tstate->async_gen_finalizer);
 }
 
 

Python/symtable.c

@@ -63,6 +63,7 @@ ste_new(struct symtable *st, identifier name, _Py_block_ty block,
         ste->ste_nested = 1;
     ste->ste_child_free = 0;
     ste->ste_generator = 0;
+    ste->ste_coroutine = 0;
     ste->ste_returns_value = 0;
     ste->ste_needs_class_closure = 0;
 
@@ -378,7 +379,7 @@ error_at_directive(PySTEntryObject *ste, PyObject *name)
                                        PyLong_AsLong(PyTuple_GET_ITEM(data, 2)));
 
             return 0;
-        }   
+        }
     }
     PyErr_SetString(PyExc_RuntimeError,
                     "BUG: internal directive bookkeeping broken");
@@ -1397,6 +1398,7 @@ symtable_visit_stmt(struct symtable *st, stmt_ty s)
                                   FunctionBlock, (void *)s, s->lineno,
                                   s->col_offset))
             VISIT_QUIT(st, 0);
+        st->st_cur->ste_coroutine = 1;
         VISIT(st, arguments, s->v.AsyncFunctionDef.args);
         VISIT_SEQ(st, stmt, s->v.AsyncFunctionDef.body);
         if (!symtable_exit_block(st, s))
@@ -1492,7 +1494,7 @@ symtable_visit_expr(struct symtable *st, expr_ty e)
         break;
     case Await_kind:
         VISIT(st, expr, e->v.Await.value);
-        st->st_cur->ste_generator = 1;
+        st->st_cur->ste_coroutine = 1;
         break;
     case Compare_kind:
         VISIT(st, expr, e->v.Compare.left);

Python/sysmodule.c

@@ -717,6 +717,113 @@ Return the wrapper for coroutine objects set by sys.set_coroutine_wrapper."
 );
 
 
+static PyTypeObject AsyncGenHooksType;
+
+PyDoc_STRVAR(asyncgen_hooks_doc,
+"asyncgen_hooks\n\
+\n\
+A struct sequence providing information about asynhronous\n\
+generators hooks.  The attributes are read only.");
+
+static PyStructSequence_Field asyncgen_hooks_fields[] = {
+    {"firstiter", "Hook to intercept first iteration"},
+    {"finalizer", "Hook to intercept finalization"},
+    {0}
+};
+
+static PyStructSequence_Desc asyncgen_hooks_desc = {
+    "asyncgen_hooks",          /* name */
+    asyncgen_hooks_doc,        /* doc */
+    asyncgen_hooks_fields ,    /* fields */
+    2
+};
+
+
+static PyObject *
+sys_set_asyncgen_hooks(PyObject *self, PyObject *args, PyObject *kw)
+{
+    static char *keywords[] = {"firstiter", "finalizer", NULL};
+    PyObject *firstiter = NULL;
+    PyObject *finalizer = NULL;
+
+    if (!PyArg_ParseTupleAndKeywords(
+            args, kw, "|OO", keywords,
+            &firstiter, &finalizer)) {
+        return NULL;
+    }
+
+    if (finalizer && finalizer != Py_None) {
+        if (!PyCallable_Check(finalizer)) {
+            PyErr_Format(PyExc_TypeError,
+                         "callable finalizer expected, got %.50s",
+                         Py_TYPE(finalizer)->tp_name);
+            return NULL;
+        }
+        _PyEval_SetAsyncGenFinalizer(finalizer);
+    }
+    else if (finalizer == Py_None) {
+        _PyEval_SetAsyncGenFinalizer(NULL);
+    }
+
+    if (firstiter && firstiter != Py_None) {
+        if (!PyCallable_Check(firstiter)) {
+            PyErr_Format(PyExc_TypeError,
+                         "callable firstiter expected, got %.50s",
+                         Py_TYPE(firstiter)->tp_name);
+            return NULL;
+        }
+        _PyEval_SetAsyncGenFirstiter(firstiter);
+    }
+    else if (firstiter == Py_None) {
+        _PyEval_SetAsyncGenFirstiter(NULL);
+    }
+
+    Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(set_asyncgen_hooks_doc,
+"set_asyncgen_hooks(*, firstiter=None, finalizer=None)\n\
+\n\
+Set a finalizer for async generators objects."
+);
+
+static PyObject *
+sys_get_asyncgen_hooks(PyObject *self, PyObject *args)
+{
+    PyObject *res;
+    PyObject *firstiter = _PyEval_GetAsyncGenFirstiter();
+    PyObject *finalizer = _PyEval_GetAsyncGenFinalizer();
+
+    res = PyStructSequence_New(&AsyncGenHooksType);
+    if (res == NULL) {
+        return NULL;
+    }
+
+    if (firstiter == NULL) {
+        firstiter = Py_None;
+    }
+
+    if (finalizer == NULL) {
+        finalizer = Py_None;
+    }
+
+    Py_INCREF(firstiter);
+    PyStructSequence_SET_ITEM(res, 0, firstiter);
+
+    Py_INCREF(finalizer);
+    PyStructSequence_SET_ITEM(res, 1, finalizer);
+
+    return res;
+}
+
+PyDoc_STRVAR(get_asyncgen_hooks_doc,
+"get_asyncgen_hooks()\n\
+\n\
+Return a namedtuple of installed asynchronous generators hooks \
+(firstiter, finalizer)."
+);
+
+
 static PyTypeObject Hash_InfoType;
 
 PyDoc_STRVAR(hash_info_doc,
@@ -1315,6 +1422,10 @@ static PyMethodDef sys_methods[] = {
      set_coroutine_wrapper_doc},
     {"get_coroutine_wrapper", sys_get_coroutine_wrapper, METH_NOARGS,
      get_coroutine_wrapper_doc},
+    {"set_asyncgen_hooks", sys_set_asyncgen_hooks,
+     METH_VARARGS | METH_KEYWORDS, set_asyncgen_hooks_doc},
+    {"get_asyncgen_hooks", sys_get_asyncgen_hooks, METH_NOARGS,
+     get_asyncgen_hooks_doc},
     {NULL,              NULL}           /* sentinel */
 };
 
@@ -1950,6 +2061,14 @@ _PySys_Init(void)
     SET_SYS_FROM_STRING("thread_info", PyThread_GetInfo());
 #endif
 
+    /* initialize asyncgen_hooks */
+    if (AsyncGenHooksType.tp_name == NULL) {
+        if (PyStructSequence_InitType2(
+                &AsyncGenHooksType, &asyncgen_hooks_desc) < 0) {
+            return NULL;
+        }
+    }
+
 #undef SET_SYS_FROM_STRING
 #undef SET_SYS_FROM_STRING_BORROW
     if (PyErr_Occurred())