python · esadomer · May 22, 2026
@@ -36,6 +36,7 @@
 import threading
 import traceback
 
+from bisect import bisect_left
 from socketserver import ThreadingTCPServer, StreamRequestHandler
 
 
@@ -197,6 +198,21 @@ def _handle_existing_loggers(existing, child_loggers, disable_existing):
         else:
             logger.disabled = disable_existing
 
+def _discard_existing_logger(name, existing, existing_set, child_loggers):
+    """Discard a configured logger and record its existing children."""
+    if name in existing_set:
+        prefixed = name + "."
+        i = bisect_left(existing, prefixed)
+        num_existing = len(existing)
+        while i < num_existing:
+            child = existing[i]
+            if not child.startswith(prefixed):
+                break
+            if child in existing_set:
+                child_loggers.add(child)
+            i += 1
+        existing_set.remove(name)
+
 def _install_loggers(cp, handlers, disable_existing):
     """Create and install loggers"""
 
@@ -235,25 +251,17 @@ def _install_loggers(cp, handlers, disable_existing):
     #named loggers. With a sorted list it is easier
     #to find the child loggers.
     existing.sort()
+    existing_set = set(existing)
     #We'll keep the list of existing loggers
     #which are children of named loggers here...
-    child_loggers = []
+    child_loggers = set()
     #now set up the new ones...
     for log in llist:
         section = cp["logger_%s" % log]
         qn = section["qualname"]
         propagate = section.getint("propagate", fallback=1)
         logger = logging.getLogger(qn)
-        if qn in existing:
-            i = existing.index(qn) + 1 # start with the entry after qn
-            prefixed = qn + "."
-            pflen = len(prefixed)
-            num_existing = len(existing)
-            while i < num_existing:
-                if existing[i][:pflen] == prefixed:
-                    child_loggers.append(existing[i])
-                i += 1
-            existing.remove(qn)
+        _discard_existing_logger(qn, existing, existing_set, child_loggers)
         if "level" in section:
             level = section["level"]
             logger.setLevel(level)
@@ -281,6 +289,7 @@ def _install_loggers(cp, handlers, disable_existing):
     #        logger.propagate = 1
     #    elif disable_existing_loggers:
     #        logger.disabled = 1
+    existing = [name for name in existing if name in existing_set]
     _handle_existing_loggers(existing, child_loggers, disable_existing)
 
 
@@ -638,22 +647,15 @@ def configure(self):
                 #named loggers. With a sorted list it is easier
                 #to find the child loggers.
                 existing.sort()
+                existing_set = set(existing)
                 #We'll keep the list of existing loggers
                 #which are children of named loggers here...
-                child_loggers = []
+                child_loggers = set()
                 #now set up the new ones...
                 loggers = config.get('loggers', EMPTY_DICT)
                 for name in loggers:
-                    if name in existing:
-                        i = existing.index(name) + 1 # look after name
-                        prefixed = name + "."
-                        pflen = len(prefixed)
-                        num_existing = len(existing)
-                        while i < num_existing:
-                            if existing[i][:pflen] == prefixed:
-                                child_loggers.append(existing[i])
-                            i += 1
-                        existing.remove(name)
+                    _discard_existing_logger(name, existing, existing_set,
+                                             child_loggers)
                     try:
                         self.configure_logger(name, loggers[name])
                     except Exception as e:
@@ -673,6 +675,7 @@ def configure(self):
                 #        logger.propagate = True
                 #    elif disable_existing:
                 #        logger.disabled = True
+                existing = [name for name in existing if name in existing_set]
                 _handle_existing_loggers(existing, child_loggers,
                                          disable_existing)
 

@@ -4173,6 +4173,26 @@ def test_90195(self):
         # Logger should be enabled, since explicitly mentioned
         self.assertFalse(logger.disabled)
 
+    def test_disable_existing_loggers_preserves_children(self):
+        parent = logging.getLogger('many')
+        child = logging.getLogger('many.child')
+        cousin = logging.getLogger('many-child')
+        for i in range(20):
+            logging.getLogger(f'many-sibling-{i}')
+
+        self.apply_config({
+            'version': 1,
+            'loggers': {
+                'many': {
+                    'level': 'INFO',
+                },
+            },
+        })
+
+        self.assertFalse(parent.disabled)
+        self.assertFalse(child.disabled)
+        self.assertTrue(cousin.disabled)
+
     def test_111615(self):
         # See gh-111615
         import_helper.import_module('_multiprocessing')  # see gh-113692

diff --git a/Misc/NEWS.d/next/Library/2026-05-22-15-30-00.gh-issue-132372.YP4a6x.rst b/Misc/NEWS.d/next/Library/2026-05-22-15-30-00.gh-issue-132372.YP4a6x.rst
@@ -0,0 +1,2 @@
+Speed up :func:`logging.config.fileConfig` and
+:func:`logging.config.dictConfig` when handling many existing loggers.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Speed up :func:`logging.config.fileConfig` and
		:func:`logging.config.dictConfig` when handling many existing loggers.