[3.10] gh-93453: Only emit deprecation warning in asyncio.get_event_loop when a new event loop is created (#100059)

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

(cherry picked from commit 3fae04b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Author: ambv

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,
-   the OS thread is main, and :func:`set_event_loop` has not yet
-   been called, asyncio will create a new event loop and set it as the
-   current one.
+   When called from a coroutine or a callback (e.g. scheduled with
+   call_soon or similar API), this function will always return the
+   running event loop.
+
+   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.
-      In future Python releases, this function may become an alias of
-      :func:`get_running_loop` and will accordingly raise a
-      :exc:`RuntimeError` if there is no running event loop.
+      Deprecation warning is emitted if there is no current event loop.
+      In Python 3.12 it will be an error.
+
+   .. note::
+      In Python versions 3.10.0--3.10.8 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:: 3.10.9
+      :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

@@ -650,6 +650,21 @@ def get_event_loop(self):
         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:
@@ -763,12 +778,13 @@ 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

@@ -752,7 +752,7 @@ async def coro():
     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

@@ -2561,8 +2561,9 @@ def test_event_loop_policy(self):
     def test_get_event_loop(self):
         policy = asyncio.DefaultEventLoopPolicy()
         self.assertIsNone(policy._local._loop)
-
-        loop = policy.get_event_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)
@@ -2576,7 +2577,10 @@ def test_get_event_loop_calls_set_event_loop(self):
                 policy, "set_event_loop",
                 wraps=policy.set_event_loop) as m_set_event_loop:
 
-            loop = policy.get_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
@@ -2610,7 +2614,8 @@ def test_new_event_loop(self):
 
     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(AssertionError, policy.set_event_loop, object())
 
@@ -2723,15 +2728,11 @@ def get_event_loop(self):
             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.warnings[0].filename, __file__)
+            with self.assertRaises(TestError):
+                asyncio.get_event_loop()
             asyncio.set_event_loop(None)
-            with self.assertWarns(DeprecationWarning) as cm:
-                with self.assertRaises(TestError):
-                    asyncio.get_event_loop()
-            self.assertEqual(cm.warnings[0].filename, __file__)
+            with self.assertRaises(TestError):
+                asyncio.get_event_loop()
 
             with self.assertRaisesRegex(RuntimeError, 'no running'):
                 asyncio.get_running_loop()
@@ -2745,16 +2746,11 @@ async def func():
             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.warnings[0].filename, __file__)
-
+            with self.assertRaises(TestError):
+                asyncio.get_event_loop()
             asyncio.set_event_loop(None)
-            with self.assertWarns(DeprecationWarning) as cm:
-                with self.assertRaises(TestError):
-                    asyncio.get_event_loop()
-            self.assertEqual(cm.warnings[0].filename, __file__)
+            with self.assertRaises(TestError):
+                asyncio.get_event_loop()
 
         finally:
             asyncio.set_event_loop_policy(old_policy)
@@ -2778,10 +2774,8 @@ def test_get_event_loop_returns_running_loop2(self):
             self.addCleanup(loop2.close)
             self.assertEqual(cm.warnings[0].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.warnings[0].filename, __file__)
+            with self.assertRaisesRegex(RuntimeError, 'no current'):
+                asyncio.get_event_loop()
 
             with self.assertRaisesRegex(RuntimeError, 'no running'):
                 asyncio.get_running_loop()
@@ -2795,15 +2789,11 @@ async def func():
             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.warnings[0].filename, __file__)
+            self.assertIs(asyncio.get_event_loop(), loop)
 
             asyncio.set_event_loop(None)
-            with self.assertWarns(DeprecationWarning) as cm:
-                with self.assertRaisesRegex(RuntimeError, 'no current'):
-                    asyncio.get_event_loop()
-            self.assertEqual(cm.warnings[0].filename, __file__)
+            with self.assertRaisesRegex(RuntimeError, 'no current'):
+                asyncio.get_event_loop()
 
         finally:
             asyncio.set_event_loop_policy(old_policy)

Lib/test/test_asyncio/test_futures.py

@@ -145,10 +145,8 @@ def test_initial_state(self):
         self.assertTrue(f.cancelled())
 
     def test_constructor_without_loop(self):
-        with self.assertWarns(DeprecationWarning) as cm:
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
-                self._new_future()
-        self.assertEqual(cm.warnings[0].filename, __file__)
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
+            self._new_future()
 
     def test_constructor_use_running_loop(self):
         async def test():
@@ -158,12 +156,10 @@ async def test():
         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.warnings[0].filename, __file__)
+        f = self._new_future()
         self.assertIs(f._loop, self.loop)
         self.assertIs(f.get_loop(), self.loop)
 
@@ -499,10 +495,8 @@ def run(arg):
             return (arg, threading.get_ident())
         ex = concurrent.futures.ThreadPoolExecutor(1)
         f1 = ex.submit(run, 'oi')
-        with self.assertWarns(DeprecationWarning) as cm:
-            with self.assertRaises(RuntimeError):
-                asyncio.wrap_future(f1)
-        self.assertEqual(cm.warnings[0].filename, __file__)
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
+            asyncio.wrap_future(f1)
         ex.shutdown(wait=True)
 
     def test_wrap_future_use_running_loop(self):
@@ -517,16 +511,14 @@ async def test():
         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.warnings[0].filename, __file__)
+        f2 = asyncio.wrap_future(f1)
         self.assertIs(self.loop, f2._loop)
         ex.shutdown(wait=True)
 

Lib/test/test_asyncio/test_streams.py

@@ -747,10 +747,8 @@ def test_read_all_from_pipe_reader(self):
         self.assertEqual(data, b'data')
 
     def test_streamreader_constructor_without_loop(self):
-        with self.assertWarns(DeprecationWarning) as cm:
-            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
-                asyncio.StreamReader()
-        self.assertEqual(cm.warnings[0].filename, __file__)
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
+            asyncio.StreamReader()
 
     def test_streamreader_constructor_use_running_loop(self):
         # asyncio issue #184: Ensure that StreamReaderProtocol constructor
@@ -764,21 +762,17 @@ async def test():
     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.warnings[0].filename, __file__)
+        reader = asyncio.StreamReader()
         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, 'There is no current event loop'):
-                asyncio.StreamReaderProtocol(reader)
-        self.assertEqual(cm.warnings[0].filename, __file__)
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
+            asyncio.StreamReaderProtocol(reader)
 
     def test_streamreaderprotocol_constructor_use_running_loop(self):
         # asyncio issue #184: Ensure that StreamReaderProtocol constructor
@@ -792,13 +786,11 @@ async def test():
     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.warnings[0].filename, __file__)
+        protocol = asyncio.StreamReaderProtocol(reader)
         self.assertIs(protocol._loop, self.loop)
 
     def test_multiple_drain(self):