Make get_event_loop() return the running loop if called from coroutines

Also:

* Raise when a user attempts to run a loop within another loop;
* Clarify error message when the loop is already running.

asyncio/base_events.py

@@ -393,21 +393,26 @@ def run_forever(self):
         """Run until stop() is called."""
         self._check_closed()
         if self.is_running():
-            raise RuntimeError('Event loop is running.')
+            raise RuntimeError('This event loop is already running')
+        if events._get_running_loop() is not None:
+            raise RuntimeError(
+                'Cannot run the event loop while another loop is running')
         self._set_coroutine_wrapper(self._debug)
         self._thread_id = threading.get_ident()
         if self._asyncgens is not None:
             old_agen_hooks = sys.get_asyncgen_hooks()
             sys.set_asyncgen_hooks(firstiter=self._asyncgen_firstiter_hook,
                                    finalizer=self._asyncgen_finalizer_hook)
         try:
+            events._set_running_loop(self)
             while True:
                 self._run_once()
                 if self._stopping:
                     break
         finally:
             self._stopping = False
             self._thread_id = None
+            events._set_running_loop(None)
             self._set_coroutine_wrapper(False)
             if self._asyncgens is not None:
                 sys.set_asyncgen_hooks(*old_agen_hooks)

asyncio/events.py

@@ -607,6 +607,30 @@ def new_event_loop(self):
 _lock = threading.Lock()
 
 
+# A TLS for the running event loop, used by _get_running_loop.
+class _RunningLoop(threading.local):
+    _loop = None
+_running_loop = _RunningLoop()
+
+
+def _get_running_loop():
+    """Return the running event loop or None.
+
+    This is a low-level function intended to be used by event loops.
+    This function is thread-specific.
+    """
+    return _running_loop._loop
+
+
+def _set_running_loop(loop):
+    """Set the running event loop.
+
+    This is a low-level function intended to be used by event loops.
+    This function is thread-specific.
+    """
+    _running_loop._loop = loop
+
+
 def _init_event_loop_policy():
     global _event_loop_policy
     with _lock:
@@ -632,7 +656,17 @@ def set_event_loop_policy(policy):
 
 
 def get_event_loop():
-    """Equivalent to calling get_event_loop_policy().get_event_loop()."""
+    """Return an asyncio event loop.
+
+    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.
+    """
+    current_loop = _get_running_loop()
+    if current_loop is not None:
+        return current_loop
     return get_event_loop_policy().get_event_loop()
 
 

asyncio/test_utils.py

@@ -449,7 +449,13 @@ def new_test_loop(self, gen=None):
         self.set_event_loop(loop)
         return loop
 
+    def setUp(self):
+        self._get_running_loop = events._get_running_loop
+        events._get_running_loop = lambda: None
+
     def tearDown(self):
+        events._get_running_loop = self._get_running_loop
+
         events.set_event_loop(None)
 
         # Detect CPython bug #23353: ensure that yield/yield-from is not used

tests/test_base_events.py

@@ -154,6 +154,7 @@ def test_ipaddr_info_no_inet_pton(self, m_socket):
 class BaseEventLoopTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = base_events.BaseEventLoop()
         self.loop._selector = mock.Mock()
         self.loop._selector.select.return_value = ()
@@ -976,6 +977,7 @@ def connection_lost(self, exc):
 class BaseEventLoopWithSelectorTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = asyncio.new_event_loop()
         self.set_event_loop(self.loop)
 
@@ -1692,5 +1694,23 @@ def stop_loop_coro(loop):
                          "took .* seconds$")
 
 
+class RunningLoopTests(unittest.TestCase):
+
+    def test_running_loop_within_a_loop(self):
+        @asyncio.coroutine
+        def runner(loop):
+            loop.run_forever()
+
+        loop = asyncio.new_event_loop()
+        outer_loop = asyncio.new_event_loop()
+        try:
+            with self.assertRaisesRegex(RuntimeError,
+                                        'while another loop is running'):
+                outer_loop.run_until_complete(runner(loop))
+        finally:
+            loop.close()
+            outer_loop.close()
+
+
 if __name__ == '__main__':
     unittest.main()

tests/test_events.py

@@ -2233,6 +2233,7 @@ def noop(*args, **kwargs):
 class HandleTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = mock.Mock()
         self.loop.get_debug.return_value = True
 
@@ -2411,6 +2412,7 @@ def __await__(self):
 class TimerTests(unittest.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = mock.Mock()
 
     def test_hash(self):
@@ -2719,6 +2721,27 @@ def test_set_event_loop_policy(self):
         self.assertIs(policy, asyncio.get_event_loop_policy())
         self.assertIsNot(policy, old_policy)
 
+    def test_get_event_loop_returns_running_loop(self):
+        class Policy(asyncio.DefaultEventLoopPolicy):
+            def get_event_loop(self):
+                raise NotImplementedError
+
+        loop = None
+
+        old_policy = asyncio.get_event_loop_policy()
+        try:
+            asyncio.set_event_loop_policy(Policy())
+            loop = asyncio.new_event_loop()
+
+            async def func():
+                self.assertIs(asyncio.get_event_loop(), loop)
+
+            loop.run_until_complete(func())
+        finally:
+            asyncio.set_event_loop_policy(old_policy)
+            if loop is not None:
+                loop.close()
+
 
 if __name__ == '__main__':
     unittest.main()

tests/test_futures.py

@@ -79,6 +79,7 @@ def __iter__(self):
 class DuckTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.addCleanup(self.loop.close)
 
@@ -96,6 +97,7 @@ def test_ensure_future(self):
 class FutureTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.addCleanup(self.loop.close)
 
@@ -468,6 +470,7 @@ def test_set_result_unless_cancelled(self):
 class FutureDoneCallbackTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
 
     def run_briefly(self):

tests/test_locks.py

@@ -19,6 +19,7 @@
 class LockTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
 
     def test_ctor_loop(self):
@@ -235,6 +236,7 @@ def test_context_manager_no_yield(self):
 class EventTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
 
     def test_ctor_loop(self):
@@ -364,6 +366,7 @@ def c1(result):
 class ConditionTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
 
     def test_ctor_loop(self):
@@ -699,6 +702,7 @@ def test_ambiguous_loops(self):
 class SemaphoreTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
 
     def test_ctor_loop(self):

tests/test_pep492.py

@@ -17,6 +17,7 @@
 class BaseTest(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = asyncio.BaseEventLoop()
         self.loop._process_events = mock.Mock()
         self.loop._selector = mock.Mock()

tests/test_proactor_events.py

@@ -24,6 +24,7 @@ def close_transport(transport):
 class ProactorSocketTransportTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.addCleanup(self.loop.close)
         self.proactor = mock.Mock()
@@ -436,6 +437,8 @@ def test_dont_pause_writing(self):
 class BaseProactorEventLoopTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
+
         self.sock = test_utils.mock_nonblocking_socket()
         self.proactor = mock.Mock()
 

tests/test_queues.py

@@ -10,6 +10,7 @@
 class _QueueTestBase(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
 
 

tests/test_selector_events.py

@@ -51,6 +51,7 @@ def close_transport(transport):
 class BaseSelectorEventLoopTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.selector = mock.Mock()
         self.selector.select.return_value = []
         self.loop = TestBaseSelectorEventLoop(self.selector)
@@ -698,6 +699,7 @@ def test_accept_connection_multiple(self):
 class SelectorTransportTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.protocol = test_utils.make_test_protocol(asyncio.Protocol)
         self.sock = mock.Mock(socket.socket)
@@ -793,6 +795,7 @@ def test_connection_lost(self):
 class SelectorSocketTransportTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.protocol = test_utils.make_test_protocol(asyncio.Protocol)
         self.sock = mock.Mock(socket.socket)
@@ -1141,6 +1144,7 @@ def test_transport_close_remove_writer(self, m_log):
 class SelectorSslTransportTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.protocol = test_utils.make_test_protocol(asyncio.Protocol)
         self.sock = mock.Mock(socket.socket)
@@ -1501,6 +1505,7 @@ def test_ssl_transport_requires_ssl_module(self):
 class SelectorDatagramTransportTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.protocol = test_utils.make_test_protocol(asyncio.DatagramProtocol)
         self.sock = mock.Mock(spec_set=socket.socket)

tests/test_sslproto.py

@@ -18,6 +18,7 @@
 class SslProtoHandshakeTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = asyncio.new_event_loop()
         self.set_event_loop(self.loop)
 

tests/test_streams.py

@@ -22,6 +22,7 @@ class StreamReaderTests(test_utils.TestCase):
     DATA = b'line1\nline2\nline3\n'
 
     def setUp(self):
+        super().setUp()
         self.loop = asyncio.new_event_loop()
         self.set_event_loop(self.loop)
 

tests/test_subprocess.py

@@ -35,6 +35,7 @@ def _start(self, *args, **kwargs):
 
 class SubprocessTransportTests(test_utils.TestCase):
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
         self.set_event_loop(self.loop)
 
@@ -466,6 +467,7 @@ class SubprocessWatcherMixin(SubprocessMixin):
         Watcher = None
 
         def setUp(self):
+            super().setUp()
             policy = asyncio.get_event_loop_policy()
             self.loop = policy.new_event_loop()
             self.set_event_loop(self.loop)
@@ -490,6 +492,7 @@ class SubprocessFastWatcherTests(SubprocessWatcherMixin,
     class SubprocessProactorTests(SubprocessMixin, test_utils.TestCase):
 
         def setUp(self):
+            super().setUp()
             self.loop = asyncio.ProactorEventLoop()
             self.set_event_loop(self.loop)
 

tests/test_tasks.py

@@ -75,6 +75,7 @@ def __call__(self, *args):
 class TaskTests(test_utils.TestCase):
 
     def setUp(self):
+        super().setUp()
         self.loop = self.new_test_loop()
 
     def test_other_loop_future(self):
@@ -1933,6 +1934,7 @@ def cancelling_callback(_):
 class GatherTestsBase:
 
     def setUp(self):
+        super().setUp()
         self.one_loop = self.new_test_loop()
         self.other_loop = self.new_test_loop()
         self.set_event_loop(self.one_loop, cleanup=False)
@@ -2216,6 +2218,7 @@ class RunCoroutineThreadsafeTests(test_utils.TestCase):
     """Test case for asyncio.run_coroutine_threadsafe."""
 
     def setUp(self):
+        super().setUp()
         self.loop = asyncio.new_event_loop()
         self.set_event_loop(self.loop) # Will cleanup properly
 
@@ -2306,12 +2309,14 @@ def test_run_coroutine_threadsafe_task_factory_exception(self):
 
 class SleepTests(test_utils.TestCase):
     def setUp(self):
+        super().setUp()
         self.loop = asyncio.new_event_loop()
         asyncio.set_event_loop(None)
 
     def tearDown(self):
         self.loop.close()
         self.loop = None
+        super().tearDown()
 
     def test_sleep_zero(self):
         result = 0