gh-160936: Clear all cached loggers unconditionally in _clear_cache

Skipping loggers that are not in loggerDict kept them in the set forever, and per-item discard left the set's table large after a bulk clear, so later setLevel() calls stayed slower than on main. Clear every tracked logger and empty the set instead. This also fixes a stale isEnabledFor cache on loggers that are not registered with the manager.

Author: Ar-maan05

Lib/logging/__init__.py

@@ -1475,10 +1475,9 @@ def _clear_cache(self):
         with _lock:
             # Only loggers that have populated a cache are visited, so
             # setLevel() costs O(loggers with cached results), not O(loggers).
-            for logger in list(self._cached_loggers):
-                if self.loggerDict.get(logger.name) is logger:
-                    logger._cache.clear()
-                    self._cached_loggers.discard(logger)
+            for logger in self._cached_loggers:
+                logger._cache.clear()
+            self._cached_loggers.clear()
             self.root._cache.clear()
 
 #---------------------------------------------------------------------------

Lib/test/test_logging.py

@@ -6398,6 +6398,25 @@ def test_pickling(self):
                 unpickled = pickle.loads(s)
                 self.assertIs(unpickled, logger)
 
+    def test_caching_unregistered_logger(self):
+        # A logger that is not in loggerDict must not keep a stale cache.
+        logger = logging.Logger("unregistered")
+        logger.setLevel(logging.DEBUG)
+        self.assertTrue(logger.isEnabledFor(logging.DEBUG))
+        logger.setLevel(logging.ERROR)
+        self.assertEqual(logger._cache, {})
+        self.assertFalse(logger.isEnabledFor(logging.DEBUG))
+
+    def test_clear_cache_empties_cached_loggers(self):
+        manager = logging.root.manager
+        registered = logging.getLogger("cached.registered")
+        unregistered = logging.Logger("cached.unregistered")
+        for logger in (registered, unregistered):
+            logger.isEnabledFor(logging.DEBUG)
+            self.assertIn(logger, manager._cached_loggers)
+        registered.setLevel(logging.INFO)
+        self.assertEqual(len(manager._cached_loggers), 0)
+
     def test_caching(self):
         root = self.root_logger
         logger1 = logging.getLogger("abc")

Misc/NEWS.d/next/Library/2026-10-29-20-30-00.gh-issue-160936.lgCach.rst

@@ -1 +1,4 @@
-Speed up :meth:`logging.Logger.setLevel` and :func:`logging.disable` when there are many loggers: only loggers that have cached results are now cleared, instead of every logger.
+Speed up :meth:`logging.Logger.setLevel` and :func:`logging.disable` when
+there are many loggers: only loggers that have cached results are now cleared,
+instead of every logger. This also clears the cache of loggers that are not
+registered with the manager, which could previously keep a stale result.