asyncio: __del__() keep reference to warnings.warn (GH-11491)

* asyncio: __del__() keep reference to warnings.warn

The __del__() methods of asyncio classes now keep a strong reference
to the warnings.warn() to be able to display the ResourceWarning
warning in more cases. Ensure that the function remains available if
instances are destroyed late during Python shutdown (while module
symbols are cleared).

* Rename warn parameter to _warn

"_warn" name is a hint that it's not the regular warnings.warn()
function.

1 files changed, 2 insertions, 3 deletions

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index 60a189b..cec47ce 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -622,10 +622,9 @@ class BaseEventLoop(events.AbstractEventLoop):
         """Returns True if the event loop was closed."""
         return self._closed
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if not self.is_closed():
-            warnings.warn(f"unclosed event loop {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed event loop {self!r}", ResourceWarning, source=self)
             if not self.is_running():
                 self.close()