[3.11] gh-93453: Only emit deprecation warning in asyncio.get_event_loop when a new event loop is created (#99949)

It no longer emits a deprecation warning if the current event loop was set.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Doc/library/asyncio-eventloop.rst

@@ -43,10 +43,12 @@ an event loop:
 
    Get the current event loop.
 
-   If there is no current event loop set in the current OS thread,
+   When called from a coroutine or a callback (e.g. scheduled with
-   the OS thread is main, and :func:`set_event_loop` has not yet
+   call_soon or similar API), this function will always return the
-   been called, asyncio will create a new event loop and set it as the
+   running event loop.
-   current one.
+
+   If there is no running event loop set, the function will return
+   the result of ``get_event_loop_policy().get_event_loop()`` call.
 
    Because this function has rather complex behavior (especially
    when custom event loop policies are in use), using the
@@ -58,10 +60,14 @@ an event loop:
    event loop.
 
    .. deprecated:: 3.10
-      Emits a deprecation warning if there is no running event loop.
+      Deprecation warning is emitted if there is no current event loop.
-      In future Python releases, this function may become an alias of
+      In Python 3.12 it will be an error.
-      :func:`get_running_loop` and will accordingly raise a
+
-      :exc:`RuntimeError` if there is no running event loop.
+   .. note::
+      In Python versions 3.10.0--3.10.8 and 3.11.0 this function
+      (and other functions which used it implicitly) emitted a
+      :exc:`DeprecationWarning` if there was no running event loop, even if
+      the current loop was set.
 
 .. function:: set_event_loop(loop)
 

Doc/library/asyncio-llapi-index.rst

@@ -19,7 +19,7 @@ Obtaining the Event Loop
       - The **preferred** function to get the running event loop.
 
     * - :func:`asyncio.get_event_loop`
-      - Get an event loop instance (current or via the policy).
+      - Get an event loop instance (running or current via the current policy).
 
     * - :func:`asyncio.set_event_loop`
       - Set the event loop as current via the current policy.

Doc/library/asyncio-policy.rst

@@ -112,6 +112,11 @@ asyncio ships with the following built-in policies:
 
       On Windows, :class:`ProactorEventLoop` is now used by default.
 
+   .. deprecated:: 3.11.1
+      :meth:`get_event_loop` now emits a :exc:`DeprecationWarning` if there
+      is no current event loop set and a new event loop has been implicitly
+      created. In Python 3.12 it will be an error.
+
 
 .. class:: WindowsSelectorEventLoopPolicy
 

Lib/asyncio/events.py

@@ -671,6 +671,21 @@ class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy):
         if (self._local._loop is None and
                 not self._local._set_called and
                 threading.current_thread() is threading.main_thread()):
+            stacklevel = 2
+            try:
+                f = sys._getframe(1)
+            except AttributeError:
+                pass
+            else:
+                while f:
+                    module = f.f_globals.get('__name__')
+                    if not (module == 'asyncio' or module.startswith('asyncio.')):
+                        break
+                    f = f.f_back
+                    stacklevel += 1
+            import warnings
+            warnings.warn('There is no current event loop',
+                          DeprecationWarning, stacklevel=stacklevel)
             self.set_event_loop(self.new_event_loop())
 
         if self._local._loop is None:
@@ -786,12 +801,13 @@ def get_event_loop():
 
 
 def _get_event_loop(stacklevel=3):
+    # This internal method is going away in Python 3.12, left here only for
+    # backwards compatibility with 3.10.0 - 3.10.8 and 3.11.0.
+    # Similarly, this method's C equivalent in _asyncio is going away as well.
+    # See GH-99949 for more details.
     current_loop = _get_running_loop()
     if current_loop is not None:
         return current_loop
-    import warnings
-    warnings.warn('There is no current event loop',
-                  DeprecationWarning, stacklevel=stacklevel)
     return get_event_loop_policy().get_event_loop()
 
 

Lib/test/test_asyncio/test_base_events.py

@@ -746,7 +746,7 @@ class BaseEventLoopTests(test_utils.TestCase):
     def test_env_var_debug(self):
         code = '\n'.join((
             'import asyncio',
-            'loop = asyncio.get_event_loop()',
+            'loop = asyncio.new_event_loop()',
             'print(loop.get_debug())'))
 
         # Test with -E to not fail if the unit test was run with

Lib/test/test_asyncio/test_events.py

@@ -2547,8 +2547,9 @@ class PolicyTests(unittest.TestCase):
     def test_get_event_loop(self):
         policy = asyncio.DefaultEventLoopPolicy()
         self.assertIsNone(policy._local._loop)
-
+        with self.assertWarns(DeprecationWarning) as cm:
             loop = policy.get_event_loop()
+        self.assertEqual(cm.filename, __file__)
         self.assertIsInstance(loop, asyncio.AbstractEventLoop)
 
         self.assertIs(policy._local._loop, loop)
@@ -2562,7 +2563,10 @@ class PolicyTests(unittest.TestCase):
                 policy, "set_event_loop",
                 wraps=policy.set_event_loop) as m_set_event_loop:
 
+            with self.assertWarns(DeprecationWarning) as cm:
                 loop = policy.get_event_loop()
+            self.addCleanup(loop.close)
+            self.assertEqual(cm.filename, __file__)
 
             # policy._local._loop must be set through .set_event_loop()
             # (the unix DefaultEventLoopPolicy needs this call to attach
@@ -2596,7 +2600,8 @@ class PolicyTests(unittest.TestCase):
 
     def test_set_event_loop(self):
         policy = asyncio.DefaultEventLoopPolicy()
-        old_loop = policy.get_event_loop()
+        old_loop = policy.new_event_loop()
+        policy.set_event_loop(old_loop)
 
         self.assertRaises(TypeError, policy.set_event_loop, object())
 
@@ -2709,15 +2714,11 @@ class GetEventLoopTestsMixin:
             asyncio.set_event_loop_policy(Policy())
             loop = asyncio.new_event_loop()
 
-            with self.assertWarns(DeprecationWarning) as cm:
             with self.assertRaises(TestError):
                 asyncio.get_event_loop()
-            self.assertEqual(cm.filename, __file__)
             asyncio.set_event_loop(None)
-            with self.assertWarns(DeprecationWarning) as cm:
             with self.assertRaises(TestError):
                 asyncio.get_event_loop()
-            self.assertEqual(cm.filename, __file__)
 
             with self.assertRaisesRegex(RuntimeError, 'no running'):
                 asyncio.get_running_loop()
@@ -2731,16 +2732,11 @@ class GetEventLoopTestsMixin:
             loop.run_until_complete(func())
 
             asyncio.set_event_loop(loop)
-            with self.assertWarns(DeprecationWarning) as cm:
             with self.assertRaises(TestError):
                 asyncio.get_event_loop()
-            self.assertEqual(cm.filename, __file__)
-
             asyncio.set_event_loop(None)
-            with self.assertWarns(DeprecationWarning) as cm:
             with self.assertRaises(TestError):
                 asyncio.get_event_loop()
-            self.assertEqual(cm.filename, __file__)
 
         finally:
             asyncio.set_event_loop_policy(old_policy)
@@ -2764,10 +2760,8 @@ class GetEventLoopTestsMixin:
             self.addCleanup(loop2.close)
             self.assertEqual(cm.filename, __file__)
             asyncio.set_event_loop(None)
-            with self.assertWarns(DeprecationWarning) as cm:
             with self.assertRaisesRegex(RuntimeError, 'no current'):
                 asyncio.get_event_loop()
-            self.assertEqual(cm.filename, __file__)
 
             with self.assertRaisesRegex(RuntimeError, 'no running'):
                 asyncio.get_running_loop()
@@ -2781,15 +2775,11 @@ class GetEventLoopTestsMixin:
             loop.run_until_complete(func())
 
             asyncio.set_event_loop(loop)
-            with self.assertWarns(DeprecationWarning) as cm:
             self.assertIs(asyncio.get_event_loop(), loop)
-            self.assertEqual(cm.filename, __file__)
 
             asyncio.set_event_loop(None)
-            with self.assertWarns(DeprecationWarning) as cm:
             with self.assertRaisesRegex(RuntimeError, 'no current'):
                 asyncio.get_event_loop()
-            self.assertEqual(cm.filename, __file__)
 
         finally:
             asyncio.set_event_loop_policy(old_policy)

Lib/test/test_asyncio/test_futures.py

@@ -145,10 +145,8 @@ class BaseFutureTests:
         self.assertTrue(f.cancelled())
 
     def test_constructor_without_loop(self):
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
             self._new_future()
-        self.assertEqual(cm.filename, __file__)
 
     def test_constructor_use_running_loop(self):
         async def test():
@@ -158,12 +156,10 @@ class BaseFutureTests:
         self.assertIs(f.get_loop(), self.loop)
 
     def test_constructor_use_global_loop(self):
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         asyncio.set_event_loop(self.loop)
         self.addCleanup(asyncio.set_event_loop, None)
-        with self.assertWarns(DeprecationWarning) as cm:
         f = self._new_future()
-        self.assertEqual(cm.filename, __file__)
         self.assertIs(f._loop, self.loop)
         self.assertIs(f.get_loop(), self.loop)
 
@@ -499,10 +495,8 @@ class BaseFutureTests:
             return (arg, threading.get_ident())
         ex = concurrent.futures.ThreadPoolExecutor(1)
         f1 = ex.submit(run, 'oi')
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaises(RuntimeError):
             asyncio.wrap_future(f1)
-        self.assertEqual(cm.filename, __file__)
         ex.shutdown(wait=True)
 
     def test_wrap_future_use_running_loop(self):
@@ -517,16 +511,14 @@ class BaseFutureTests:
         ex.shutdown(wait=True)
 
     def test_wrap_future_use_global_loop(self):
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         asyncio.set_event_loop(self.loop)
         self.addCleanup(asyncio.set_event_loop, None)
         def run(arg):
             return (arg, threading.get_ident())
         ex = concurrent.futures.ThreadPoolExecutor(1)
         f1 = ex.submit(run, 'oi')
-        with self.assertWarns(DeprecationWarning) as cm:
         f2 = asyncio.wrap_future(f1)
-        self.assertEqual(cm.filename, __file__)
         self.assertIs(self.loop, f2._loop)
         ex.shutdown(wait=True)
 

Lib/test/test_asyncio/test_streams.py

@@ -810,10 +810,8 @@ os.close(fd)
         self.assertEqual(data, b'data')
 
     def test_streamreader_constructor_without_loop(self):
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
             asyncio.StreamReader()
-        self.assertEqual(cm.filename, __file__)
 
     def test_streamreader_constructor_use_running_loop(self):
         # asyncio issue #184: Ensure that StreamReaderProtocol constructor
@@ -827,21 +825,17 @@ os.close(fd)
     def test_streamreader_constructor_use_global_loop(self):
         # asyncio issue #184: Ensure that StreamReaderProtocol constructor
         # retrieves the current loop if the loop parameter is not set
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         self.addCleanup(asyncio.set_event_loop, None)
         asyncio.set_event_loop(self.loop)
-        with self.assertWarns(DeprecationWarning) as cm:
         reader = asyncio.StreamReader()
-        self.assertEqual(cm.filename, __file__)
         self.assertIs(reader._loop, self.loop)
 
 
     def test_streamreaderprotocol_constructor_without_loop(self):
         reader = mock.Mock()
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
             asyncio.StreamReaderProtocol(reader)
-        self.assertEqual(cm.filename, __file__)
 
     def test_streamreaderprotocol_constructor_use_running_loop(self):
         # asyncio issue #184: Ensure that StreamReaderProtocol constructor
@@ -855,13 +849,11 @@ os.close(fd)
     def test_streamreaderprotocol_constructor_use_global_loop(self):
         # asyncio issue #184: Ensure that StreamReaderProtocol constructor
         # retrieves the current loop if the loop parameter is not set
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         self.addCleanup(asyncio.set_event_loop, None)
         asyncio.set_event_loop(self.loop)
         reader = mock.Mock()
-        with self.assertWarns(DeprecationWarning) as cm:
         protocol = asyncio.StreamReaderProtocol(reader)
-        self.assertEqual(cm.filename, __file__)
         self.assertIs(protocol._loop, self.loop)
 
     def test_multiple_drain(self):

Lib/test/test_asyncio/test_tasks.py

@@ -200,10 +200,8 @@ class BaseTaskTests:
 
         a = notmuch()
         self.addCleanup(a.close)
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
             asyncio.ensure_future(a)
-        self.assertEqual(cm.filename, __file__)
 
         async def test():
             return asyncio.ensure_future(notmuch())
@@ -213,12 +211,10 @@ class BaseTaskTests:
         self.assertTrue(t.done())
         self.assertEqual(t.result(), 'ok')
 
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         asyncio.set_event_loop(self.loop)
         self.addCleanup(asyncio.set_event_loop, None)
-        with self.assertWarns(DeprecationWarning) as cm:
         t = asyncio.ensure_future(notmuch())
-        self.assertEqual(cm.filename, __file__)
         self.assertIs(t._loop, self.loop)
         self.loop.run_until_complete(t)
         self.assertTrue(t.done())
@@ -1536,10 +1532,8 @@ class BaseTaskTests:
         self.addCleanup(a.close)
 
         futs = asyncio.as_completed([a])
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
             list(futs)
-        self.assertEqual(cm.filename, __file__)
 
     def test_as_completed_coroutine_use_running_loop(self):
         loop = self.new_test_loop()
@@ -1969,10 +1963,8 @@ class BaseTaskTests:
 
         inner = coro()
         self.addCleanup(inner.close)
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
             asyncio.shield(inner)
-        self.assertEqual(cm.filename, __file__)
 
     def test_shield_coroutine_use_running_loop(self):
         async def coro():
@@ -1986,15 +1978,13 @@ class BaseTaskTests:
         self.assertEqual(res, 42)
 
     def test_shield_coroutine_use_global_loop(self):
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         async def coro():
             return 42
 
         asyncio.set_event_loop(self.loop)
         self.addCleanup(asyncio.set_event_loop, None)
-        with self.assertWarns(DeprecationWarning) as cm:
         outer = asyncio.shield(coro())
-        self.assertEqual(cm.filename, __file__)
         self.assertEqual(outer._loop, self.loop)
         res = self.loop.run_until_complete(outer)
         self.assertEqual(res, 42)
@@ -2820,7 +2810,7 @@ class BaseCurrentLoopTests:
         self.assertIsNone(asyncio.current_task(loop=self.loop))
 
     def test_current_task_no_running_loop_implicit(self):
-        with self.assertRaises(RuntimeError):
+        with self.assertRaisesRegex(RuntimeError, 'no running event loop'):
             asyncio.current_task()
 
     def test_current_task_with_implicit_loop(self):
@@ -2984,10 +2974,8 @@ class FutureGatherTests(GatherTestsBase, test_utils.TestCase):
         return asyncio.gather(*args, **kwargs)
 
     def test_constructor_empty_sequence_without_loop(self):
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaises(RuntimeError):
             asyncio.gather()
-        self.assertEqual(cm.filename, __file__)
 
     def test_constructor_empty_sequence_use_running_loop(self):
         async def gather():
@@ -3000,12 +2988,10 @@ class FutureGatherTests(GatherTestsBase, test_utils.TestCase):
         self.assertEqual(fut.result(), [])
 
     def test_constructor_empty_sequence_use_global_loop(self):
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         asyncio.set_event_loop(self.one_loop)
         self.addCleanup(asyncio.set_event_loop, None)
-        with self.assertWarns(DeprecationWarning) as cm:
         fut = asyncio.gather()
-        self.assertEqual(cm.filename, __file__)
         self.assertIsInstance(fut, asyncio.Future)
         self.assertIs(fut._loop, self.one_loop)
         self._run_loop(self.one_loop)
@@ -3093,10 +3079,8 @@ class CoroutineGatherTests(GatherTestsBase, test_utils.TestCase):
         self.addCleanup(gen1.close)
         gen2 = coro()
         self.addCleanup(gen2.close)
-        with self.assertWarns(DeprecationWarning) as cm:
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
-            with self.assertRaises(RuntimeError):
             asyncio.gather(gen1, gen2)
-        self.assertEqual(cm.filename, __file__)
 
     def test_constructor_use_running_loop(self):
         async def coro():
@@ -3110,16 +3094,14 @@ class CoroutineGatherTests(GatherTestsBase, test_utils.TestCase):
         self.one_loop.run_until_complete(fut)
 
     def test_constructor_use_global_loop(self):
-        # Deprecated in 3.10
+        # Deprecated in 3.10, undeprecated in 3.11.1
         async def coro():
             return 'abc'
         asyncio.set_event_loop(self.other_loop)
         self.addCleanup(asyncio.set_event_loop, None)
         gen1 = coro()
         gen2 = coro()
-        with self.assertWarns(DeprecationWarning) as cm:
         fut = asyncio.gather(gen1, gen2)
-        self.assertEqual(cm.filename, __file__)
         self.assertIs(fut._loop, self.other_loop)
         self.other_loop.run_until_complete(fut)
 

Lib/test/test_asyncio/test_unix_events.py

@@ -1740,7 +1740,8 @@ class PolicyTests(unittest.TestCase):
 
     def test_child_watcher_replace_mainloop_existing(self):
         policy = self.create_policy()
-        loop = policy.get_event_loop()
+        loop = policy.new_event_loop()
+        policy.set_event_loop(loop)
 
         # Explicitly setup SafeChildWatcher,
         # default ThreadedChildWatcher has no _loop property

Lib/test/test_coroutines.py

@@ -2411,7 +2411,8 @@ class UnawaitedWarningDuringShutdownTest(unittest.TestCase):
     def test_unawaited_warning_during_shutdown(self):
         code = ("import asyncio\n"
                 "async def f(): pass\n"
-                "asyncio.gather(f())\n")
+                "async def t(): asyncio.gather(f())\n"
+                "asyncio.run(t())\n")
         assert_python_ok("-c", code)
 
         code = ("import sys\n"

Misc/NEWS.d/next/Library/2022-12-02-13-05-00.gh-issue-93453.EFj1NN.rst

@@ -0,0 +1,3 @@
+:func:`asyncio.get_event_loop` now only emits a deprecation warning when a
+new event loop was created implicitly. It no longer emits a deprecation
+warning if the current event loop was set.

Modules/_asynciomodule.c

@@ -339,13 +339,6 @@ get_event_loop(int stacklevel)
         return loop;
     }
 
-    if (PyErr_WarnEx(PyExc_DeprecationWarning,
-                     "There is no current event loop",
-                     stacklevel))
-    {
-        return NULL;
-    }
-
     policy = PyObject_CallNoArgs(asyncio_get_event_loop_policy);
     if (policy == NULL) {
         return NULL;
@@ -3129,6 +3122,11 @@ _asyncio_get_event_loop_impl(PyObject *module)
     return get_event_loop(1);
 }
 
+// This internal method is going away in Python 3.12, left here only for
+// backwards compatibility with 3.10.0 - 3.10.8 and 3.11.0.
+// Similarly, this method's Python equivalent in asyncio.events is going
+// away as well.
+// See GH-99949 for more details.
 /*[clinic input]
 _asyncio._get_event_loop
     stacklevel: int = 3