gh-127949: deprecate asyncio.set_event_loop_policy (#128024)

First step towards deprecating the asyncio policy system.
This deprecates `asyncio.set_event_loop_policy` and will be removed in Python 3.16.

Author: kumaraditya303

Doc/library/asyncio-policy.rst

@@ -46,6 +46,10 @@ for the current process:
 
    If *policy* is set to ``None``, the default policy is restored.
 
+   .. deprecated:: next
+      The :func:`set_event_loop_policy` function is deprecated and
+      will be removed in Python 3.16.
+
 
 .. _asyncio-policy-objects:
 

Lib/asyncio/events.py

@@ -8,7 +8,9 @@
     'AbstractEventLoopPolicy',
     'AbstractEventLoop', 'AbstractServer',
     'Handle', 'TimerHandle',
-    'get_event_loop_policy', 'set_event_loop_policy',
+    'get_event_loop_policy',
+    '_set_event_loop_policy',
+    'set_event_loop_policy',
     'get_event_loop', 'set_event_loop', 'new_event_loop',
     '_set_running_loop', 'get_running_loop',
     '_get_running_loop',
@@ -21,6 +23,7 @@
 import subprocess
 import sys
 import threading
+import warnings
 
 from . import format_helpers
 
@@ -765,7 +768,7 @@ def get_event_loop_policy():
     return _event_loop_policy
 
 
-def set_event_loop_policy(policy):
+def _set_event_loop_policy(policy):
     """Set the current event loop policy.
 
     If policy is None, the default policy is restored."""
@@ -774,6 +777,9 @@ def set_event_loop_policy(policy):
         raise TypeError(f"policy must be an instance of AbstractEventLoopPolicy or None, not '{type(policy).__name__}'")
     _event_loop_policy = policy
 
+def set_event_loop_policy(policy):
+    warnings._deprecated('set_event_loop_policy', remove=(3,16))
+    _set_event_loop_policy(policy)
 
 def get_event_loop():
     """Return an asyncio event loop.

Lib/test/libregrtest/save_env.py

@@ -97,7 +97,7 @@ def get_asyncio_events__event_loop_policy(self):
         return support.maybe_get_event_loop_policy()
     def restore_asyncio_events__event_loop_policy(self, policy):
         asyncio = self.get_module('asyncio')
-        asyncio.set_event_loop_policy(policy)
+        asyncio._set_event_loop_policy(policy)
 
     def get_sys_argv(self):
         return id(sys.argv), sys.argv, sys.argv[:]

Lib/test/test_asyncgen.py

@@ -629,7 +629,7 @@ def setUp(self):
     def tearDown(self):
         self.loop.close()
         self.loop = None
-        asyncio.set_event_loop_policy(None)
+        asyncio._set_event_loop_policy(None)
 
     def check_async_iterator_anext(self, ait_class):
         with self.subTest(anext="pure-Python"):

Lib/test/test_asyncio/test_base_events.py

@@ -25,7 +25,7 @@
 
 
 def tearDownModule():
-    asyncio.set_event_loop_policy(None)
+    asyncio._set_event_loop_policy(None)
 
 
 def mock_socket_module():

Lib/test/test_asyncio/test_buffered_proto.py

@@ -5,7 +5,7 @@
 
 
 def tearDownModule():
-    asyncio.set_event_loop_policy(None)
+    asyncio._set_event_loop_policy(None)
 
 
 class ReceiveStuffProto(asyncio.BufferedProtocol):

Lib/test/test_asyncio/test_context.py

@@ -4,7 +4,7 @@
 
 
 def tearDownModule():
-    asyncio.set_event_loop_policy(None)
+    asyncio._set_event_loop_policy(None)
 
 
 @unittest.skipUnless(decimal.HAVE_CONTEXTVAR, "decimal is built with a thread-local context")

Lib/test/test_asyncio/test_eager_task_factory.py

@@ -13,7 +13,7 @@
 
 
 def tearDownModule():
-    asyncio.set_event_loop_policy(None)
+    asyncio._set_event_loop_policy(None)
 
 
 class EagerTaskFactoryLoopTests:

Lib/test/test_asyncio/test_events.py

@@ -36,10 +36,10 @@
 from test.support import socket_helper
 from test.support import threading_helper
 from test.support import ALWAYS_EQ, LARGEST, SMALLEST
-
+from test.support import warnings_helper
 
 def tearDownModule():
-    asyncio.set_event_loop_policy(None)
+    asyncio._set_event_loop_policy(None)
 
 
 def broken_unix_getsockname():
@@ -2764,13 +2764,17 @@ def test_get_event_loop_policy(self):
         self.assertIs(policy, asyncio.get_event_loop_policy())
 
     def test_set_event_loop_policy(self):
-        self.assertRaises(
-            TypeError, asyncio.set_event_loop_policy, object())
+        with self.assertWarnsRegex(
+                DeprecationWarning, "'set_event_loop_policy' is deprecated"):
+            self.assertRaises(
+                TypeError, asyncio.set_event_loop_policy, object())
 
         old_policy = asyncio.get_event_loop_policy()
 
         policy = asyncio.DefaultEventLoopPolicy()
-        asyncio.set_event_loop_policy(policy)
+        with self.assertWarnsRegex(
+                DeprecationWarning, "'set_event_loop_policy' is deprecated"):
+            asyncio.set_event_loop_policy(policy)
         self.assertIs(policy, asyncio.get_event_loop_policy())
         self.assertIsNot(policy, old_policy)
 
@@ -2857,7 +2861,7 @@ def get_event_loop(self):
 
         old_policy = asyncio.get_event_loop_policy()
         try:
-            asyncio.set_event_loop_policy(Policy())
+            asyncio._set_event_loop_policy(Policy())
             loop = asyncio.new_event_loop()
 
             with self.assertRaises(TestError):
@@ -2885,7 +2889,7 @@ async def func():
                 asyncio.get_event_loop()
 
         finally:
-            asyncio.set_event_loop_policy(old_policy)
+            asyncio._set_event_loop_policy(old_policy)
             if loop is not None:
                 loop.close()
 
@@ -2897,7 +2901,7 @@ async def func():
     def test_get_event_loop_returns_running_loop2(self):
         old_policy = asyncio.get_event_loop_policy()
         try:
-            asyncio.set_event_loop_policy(asyncio.DefaultEventLoopPolicy())
+            asyncio._set_event_loop_policy(asyncio.DefaultEventLoopPolicy())
             loop = asyncio.new_event_loop()
             self.addCleanup(loop.close)
 
@@ -2923,7 +2927,7 @@ async def func():
                 asyncio.get_event_loop()
 
         finally:
-            asyncio.set_event_loop_policy(old_policy)
+            asyncio._set_event_loop_policy(old_policy)
             if loop is not None:
                 loop.close()
 

Lib/test/test_asyncio/test_futures.py

@@ -17,7 +17,7 @@
 
 
 def tearDownModule():
-    asyncio.set_event_loop_policy(None)
+    asyncio._set_event_loop_policy(None)
 
 
 def _fakefunc(f):