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 <[email protected]>

Author: serhiy-storchaka

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 calling ``get_event_loop_policy().get_event_loop()``.
 
    Because this function has rather complex behavior (especially
    when custom event loop policies are in use), using the
@@ -57,11 +59,11 @@ an event loop:
    instead of using these lower level functions to manually create and close an
    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.
+   .. 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

@@ -116,6 +116,10 @@ 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.
+
 
 .. class:: WindowsSelectorEventLoopPolicy
 

Doc/whatsnew/3.12.rst

@@ -686,6 +686,18 @@ 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 @@ def get_event_loop(self):
 
         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 and the current policy does not specify to create one.
+        current context.
 
         It should never return None."""
         raise NotImplementedError
@@ -672,11 +672,6 @@ def get_event_loop(self):
 
         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()):
-            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)
@@ -786,16 +781,9 @@ def get_event_loop():
     the result of `get_event_loop_policy().get_event_loop()` call.
     """
     # NOTE: this function is implemented in C (see _asynciomodule.c)
-    return _py__get_event_loop()
-
-
-def _get_event_loop(stacklevel=3):
     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()
 
 
@@ -825,15 +813,14 @@ def set_child_watcher(watcher):
 _py__set_running_loop = _set_running_loop
 _py_get_running_loop = get_running_loop
 _py_get_event_loop = get_event_loop
-_py__get_event_loop = _get_event_loop
 
 
 try:
     # get_event_loop() is one of the most frequently called
     # functions in asyncio.  Pure Python implementation is
     # about 4 times slower than C-accelerated.
     from _asyncio import (_get_running_loop, _set_running_loop,
-                          get_running_loop, get_event_loop, _get_event_loop)
+                          get_running_loop, get_event_loop)
 except ImportError:
     pass
 else:
@@ -842,7 +829,6 @@ def set_child_watcher(watcher):
     _c__set_running_loop = _set_running_loop
     _c_get_running_loop = get_running_loop
     _c_get_event_loop = get_event_loop
-    _c__get_event_loop = _get_event_loop
 
 
 if hasattr(os, 'fork'):

Lib/asyncio/futures.py

@@ -77,7 +77,7 @@ def __init__(self, *, loop=None):
         the default event loop.
         """
         if loop is None:
-            self._loop = events._get_event_loop()
+            self._loop = events.get_event_loop()
         else:
             self._loop = loop
         self._callbacks = []
@@ -413,7 +413,7 @@ def wrap_future(future, *, loop=None):
     assert isinstance(future, concurrent.futures.Future), \
         f'concurrent.futures.Future is expected, got {future!r}'
     if loop is None:
-        loop = events._get_event_loop()
+        loop = events.get_event_loop()
     new_future = loop.create_future()
     _chain_future(future, new_future)
     return new_future

Lib/asyncio/streams.py

@@ -125,7 +125,7 @@ class FlowControlMixin(protocols.Protocol):
 
     def __init__(self, loop=None):
         if loop is None:
-            self._loop = events._get_event_loop(stacklevel=4)
+            self._loop = events.get_event_loop()
         else:
             self._loop = loop
         self._paused = False
@@ -404,7 +404,7 @@ def __init__(self, limit=_DEFAULT_LIMIT, loop=None):
 
         self._limit = limit
         if loop is None:
-            self._loop = events._get_event_loop()
+            self._loop = events.get_event_loop()
         else:
             self._loop = loop
         self._buffer = bytearray()

Lib/asyncio/tasks.py

@@ -582,7 +582,7 @@ def as_completed(fs, *, timeout=None):
     from .queues import Queue  # Import here to avoid circular import problem.
     done = Queue()
 
-    loop = events._get_event_loop()
+    loop = events.get_event_loop()
     todo = {ensure_future(f, loop=loop) for f in set(fs)}
     timeout_handle = None
 
@@ -668,7 +668,7 @@ def _ensure_future(coro_or_future, *, loop=None):
                             'is required')
 
     if loop is None:
-        loop = events._get_event_loop(stacklevel=4)
+        loop = events.get_event_loop()
     try:
         return loop.create_task(coro_or_future)
     except RuntimeError:
@@ -749,7 +749,7 @@ def gather(*coros_or_futures, return_exceptions=False):
     gather won't cancel any other awaitables.
     """
     if not coros_or_futures:
-        loop = events._get_event_loop()
+        loop = events.get_event_loop()
         outer = loop.create_future()
         outer.set_result([])
         return outer

Lib/test/test_asyncio/test_base_events.py

@@ -746,7 +746,7 @@ 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

@@ -2550,29 +2550,8 @@ 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()
-        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 @@ 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(TypeError, policy.set_event_loop, object())
 
@@ -2716,15 +2696,11 @@ 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.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 @@ 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.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 @@ def test_get_event_loop_returns_running_loop2(self):
             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 @@ 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.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)