gh-100160: Restore and deprecate implicit creation of an event loop (GH-100410)

Partially revert changes made in GH-93453.

asyncio.DefaultEventLoopPolicy.get_event_loop() now emits a
DeprecationWarning and creates and sets a new event loop instead of
raising a RuntimeError if there is no current event loop set.

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>

3 files changed, 60 insertions, 7 deletions

diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index 6cff8c5..ce44942 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -619,7 +619,7 @@ class AbstractEventLoopPolicy:
 
         Returns an event loop object implementing the BaseEventLoop interface,
         or raises an exception in case no event loop has been set for the
-        current context.
+        current context and the current policy does not specify to create one.
 
         It should never return None."""
         raise NotImplementedError
@@ -672,6 +672,28 @@ class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy):
 
         Returns an instance of EventLoop or raises an exception.
         """
+        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:
+                # Move up the call stack so that the warning is attached
+                # to the line outside asyncio itself.
+                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:
             raise RuntimeError('There is no current event loop in thread %r.'
                                % threading.current_thread().name)
diff --git a/Lib/test/test_asyncio/test_events.py b/Lib/test/test_asyncio/test_events.py
index 093f8c1..214544b 100644
--- a/Lib/test/test_asyncio/test_events.py
+++ b/Lib/test/test_asyncio/test_events.py
@@ -2614,8 +2614,33 @@ class PolicyTests(unittest.TestCase):
     def test_get_event_loop(self):
         policy = asyncio.DefaultEventLoopPolicy()
         self.assertIsNone(policy._local._loop)
-        with self.assertRaisesRegex(RuntimeError, 'no current event 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)
+        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:
+
+            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
+            # the child watcher correctly)
+            m_set_event_loop.assert_called_with(loop)
+
+        loop.close()
 
     def test_get_event_loop_after_set_none(self):
         policy = asyncio.DefaultEventLoopPolicy()
@@ -2801,8 +2826,10 @@ class GetEventLoopTestsMixin:
             loop = asyncio.new_event_loop()
             self.addCleanup(loop.close)
 
-            with self.assertRaisesRegex(RuntimeError, 'no current'):
-                asyncio.get_event_loop()
+            with self.assertWarns(DeprecationWarning) as cm:
+                loop2 = asyncio.get_event_loop()
+            self.addCleanup(loop2.close)
+            self.assertEqual(cm.filename, __file__)
             asyncio.set_event_loop(None)
             with self.assertRaisesRegex(RuntimeError, 'no current'):
                 asyncio.get_event_loop()
diff --git a/Lib/test/test_asyncio/test_unix_events.py b/Lib/test/test_asyncio/test_unix_events.py
index 600a590..33d0ea1 100644
--- a/Lib/test/test_asyncio/test_unix_events.py
+++ b/Lib/test/test_asyncio/test_unix_events.py
@@ -1884,7 +1884,9 @@ class TestFork(unittest.IsolatedAsyncioTestCase):
         if pid == 0:
             # child
             try:
-                loop = asyncio.get_event_loop_policy().get_event_loop()
+                with self.assertWarns(DeprecationWarning):
+                    loop = asyncio.get_event_loop_policy().get_event_loop()
+                os.write(w, b'LOOP:' + str(id(loop)).encode())
             except RuntimeError:
                 os.write(w, b'NO LOOP')
             except:
@@ -1893,7 +1895,9 @@ class TestFork(unittest.IsolatedAsyncioTestCase):
                 os._exit(0)
         else:
             # parent
-            self.assertEqual(os.read(r, 100), b'NO LOOP')
+            result = os.read(r, 100)
+            self.assertEqual(result[:5], b'LOOP:', result)
+            self.assertNotEqual(int(result[5:]), id(loop))
             wait_process(pid, exitcode=0)
 
     @hashlib_helper.requires_hashdigest('md5')