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>

Doc/library/asyncio-eventloop.rst

@@ -48,7 +48,7 @@ an event loop:
    running event loop.
 
    If there is no running event loop set, the function will return
-   the result of calling ``get_event_loop_policy().get_event_loop()``.
+   the result of the ``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
@@ -59,11 +59,9 @@ an event loop:
    instead of using these lower level functions to manually create and close an
    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.
+   .. deprecated:: 3.12
+      Deprecation warning is emitted if there is no current event loop.
+      In some future Python release this will become an error.
 
 .. function:: set_event_loop(loop)
 

Doc/library/asyncio-policy.rst

@@ -116,9 +116,11 @@ asyncio ships with the following built-in policies:
 
       On Windows, :class:`ProactorEventLoop` is now used by default.
 
-   .. versionchanged:: 3.12
-      :meth:`get_event_loop` now raises a :exc:`RuntimeError` if there is no
-      current event loop set.
+   .. deprecated:: 3.12
+      The :meth:`get_event_loop` method of the default asyncio policy now emits
+      a :exc:`DeprecationWarning` if there is no current event loop set and it
+      decides to create one.
+      In some future Python release this will become an error.
 
 
 .. class:: WindowsSelectorEventLoopPolicy

Doc/whatsnew/3.10.rst

@@ -1709,19 +1709,6 @@ Deprecated
   scheduled for removal in Python 3.12.
   (Contributed by Erlend E. Aasland in :issue:`42264`.)
 
-* :func:`asyncio.get_event_loop` now emits a deprecation warning if there is
-  no running event loop. In the future it will be an alias of
-  :func:`~asyncio.get_running_loop`.
-  :mod:`asyncio` functions which implicitly create :class:`~asyncio.Future`
-  or :class:`~asyncio.Task` objects now emit
-  a deprecation warning if there is no running event loop and no explicit
-  *loop* argument is passed: :func:`~asyncio.ensure_future`,
-  :func:`~asyncio.wrap_future`, :func:`~asyncio.gather`,
-  :func:`~asyncio.shield`, :func:`~asyncio.as_completed` and constructors of
-  :class:`~asyncio.Future`, :class:`~asyncio.Task`,
-  :class:`~asyncio.StreamReader`, :class:`~asyncio.StreamReaderProtocol`.
-  (Contributed by Serhiy Storchaka in :issue:`39529`.)
-
 * The undocumented built-in function ``sqlite3.enable_shared_cache`` is now
   deprecated, scheduled for removal in Python 3.12.  Its use is strongly
   discouraged by the SQLite3 documentation.  See `the SQLite3 docs

Doc/whatsnew/3.12.rst

@@ -408,6 +408,11 @@ Deprecated
   :exc:`ImportWarning`).
   (Contributed by Brett Cannon in :gh:`65961`.)
 
+* The :meth:`~asyncio.DefaultEventLoopPolicy.get_event_loop` method of the
+  default event loop policy now emits a :exc:`DeprecationWarning` if there
+  is no current event loop set and it decides to create one.
+  (Contributed by Serhiy Storchaka and Guido van Rossum in :gh:`100160`.)
+
 
 Pending Removal in Python 3.13
 ------------------------------
@@ -710,18 +715,6 @@ Changes in the Python API
   around process-global resources, which are best managed from the main interpreter.
   (Contributed by Dong-hee Na in :gh:`99127`.)
 
-* :func:`asyncio.get_event_loop` and many other :mod:`asyncio` functions like
-  :func:`~asyncio.ensure_future`, :func:`~asyncio.shield` or
-  :func:`~asyncio.gather`, and also the
-  :meth:`~asyncio.BaseDefaultEventLoopPolicy.get_event_loop` method of
-  :class:`~asyncio.BaseDefaultEventLoopPolicy` now raise a :exc:`RuntimeError`
-  if called when there is no running event loop and the current event loop was
-  not set.
-  Previously they implicitly created and set a new current event loop.
-  :exc:`DeprecationWarning` is no longer emitted if there is no running
-  event loop but the current event loop is set in the policy.
-  (Contributed by Serhiy Storchaka in :gh:`93453`.)
-
 
 Build Changes
 =============

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)

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()

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')

Misc/NEWS.d/next/Library/2022-12-21-17-49-50.gh-issue-100160.N0NHRj.rst

@@ -0,0 +1,3 @@
+Emit a deprecation warning in
+:meth:`asyncio.DefaultEventLoopPolicy.get_event_loop` if there is no current
+event loop set and it decides to create one.