gh-93453: No longer create an event loop in get_event_loop() (#98440)

asyncio.get_event_loop() now always return either running event loop or
the result of get_event_loop_policy().get_event_loop() call. The latter
should now raise an RuntimeError if no current event loop was set
instead of creating and setting a new event loop.

It affects also a number of asyncio functions and constructors which
call get_event_loop() implicitly: ensure_future(), shield(), gather(),
etc.

DeprecationWarning is no longer emitted if there is no running event loop but
the current event loop was set.

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

Lib/test/test_asyncio/test_events.py

@@ -2550,29 +2550,8 @@ class PolicyTests(unittest.TestCase):
     def test_get_event_loop(self):
         policy = asyncio.DefaultEventLoopPolicy()
         self.assertIsNone(policy._local._loop)
-
-        loop = policy.get_event_loop()
-        self.assertIsInstance(loop, asyncio.AbstractEventLoop)
-
-        self.assertIs(policy._local._loop, loop)
-        self.assertIs(loop, policy.get_event_loop())
-        loop.close()
-
-    def test_get_event_loop_calls_set_event_loop(self):
-        policy = asyncio.DefaultEventLoopPolicy()
-
-        with mock.patch.object(
-                policy, "set_event_loop",
-                wraps=policy.set_event_loop) as m_set_event_loop:
-
-            loop = policy.get_event_loop()
-
-            # policy._local._loop must be set through .set_event_loop()
-            # (the unix DefaultEventLoopPolicy needs this call to attach
-            # the child watcher correctly)
-            m_set_event_loop.assert_called_with(loop)
-
-        loop.close()
+        with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
+            policy.get_event_loop()
 
     def test_get_event_loop_after_set_none(self):
         policy = asyncio.DefaultEventLoopPolicy()
@@ -2599,7 +2578,8 @@ 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())
 
@@ -2716,15 +2696,11 @@ 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__)
+            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.filename, __file__)
+            with self.assertRaises(TestError):
+                asyncio.get_event_loop()
 
             with self.assertRaisesRegex(RuntimeError, 'no running'):
                 asyncio.get_running_loop()
@@ -2738,16 +2714,11 @@ 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__)
-
+            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.filename, __file__)
+            with self.assertRaises(TestError):
+                asyncio.get_event_loop()
 
         finally:
             asyncio.set_event_loop_policy(old_policy)
@@ -2766,15 +2737,11 @@ class GetEventLoopTestsMixin:
             loop = asyncio.new_event_loop()
             self.addCleanup(loop.close)
 
-            with self.assertWarns(DeprecationWarning) as cm:
-                loop2 = asyncio.get_event_loop()
-            self.addCleanup(loop2.close)
-            self.assertEqual(cm.filename, __file__)
+            with self.assertRaisesRegex(RuntimeError, 'no current'):
+                asyncio.get_event_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.filename, __file__)
+            with self.assertRaisesRegex(RuntimeError, 'no current'):
+                asyncio.get_event_loop()
 
             with self.assertRaisesRegex(RuntimeError, 'no running'):
                 asyncio.get_running_loop()
@@ -2788,15 +2755,11 @@ 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__)
+            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.filename, __file__)
+            with self.assertRaisesRegex(RuntimeError, 'no current'):
+                asyncio.get_event_loop()
 
         finally:
             asyncio.set_event_loop_policy(old_policy)