bpo-34075: Deprecate non-ThreadPoolExecutor in loop.set_default_executor() (GH-8533)

Various asyncio internals expect that the default executor is a
`ThreadPoolExecutor`, so deprecate passing anything else to
`loop.set_default_executor()`.

2 files changed, 19 insertions, 1 deletions

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index dc0ca3f..75989a7 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -741,6 +741,12 @@ class BaseEventLoop(events.AbstractEventLoop):
             executor.submit(func, *args), loop=self)
 
     def set_default_executor(self, executor):
+        if not isinstance(executor, concurrent.futures.ThreadPoolExecutor):
+            warnings.warn(
+                'Using the default executor that is not an instance of '
+                'ThreadPoolExecutor is deprecated and will be prohibited '
+                'in Python 3.9',
+                DeprecationWarning, 2)
         self._default_executor = executor
 
     def _getaddrinfo_debug(self, host, port, family, type, proto, flags):
diff --git a/Lib/test/test_asyncio/test_base_events.py b/Lib/test/test_asyncio/test_base_events.py
index bda8cc6..f3ae140 100644
--- a/Lib/test/test_asyncio/test_base_events.py
+++ b/Lib/test/test_asyncio/test_base_events.py
@@ -1,5 +1,6 @@
 """Tests for base_events.py"""
 
+import concurrent.futures
 import errno
 import logging
 import math
@@ -211,10 +212,21 @@ class BaseEventLoopTests(test_utils.TestCase):
         self.assertFalse(self.loop._ready)
 
     def test_set_default_executor(self):
-        executor = mock.Mock()
+        class DummyExecutor(concurrent.futures.ThreadPoolExecutor):
+            def submit(self, fn, *args, **kwargs):
+                raise NotImplementedError(
+                    'cannot submit into a dummy executor')
+
+        executor = DummyExecutor()
         self.loop.set_default_executor(executor)
         self.assertIs(executor, self.loop._default_executor)
 
+    def test_set_default_executor_deprecation_warnings(self):
+        executor = mock.Mock()
+
+        with self.assertWarns(DeprecationWarning):
+            self.loop.set_default_executor(executor)
+
     def test_call_soon(self):
         def cb():
             pass