Preserve logs that LoggingPanel would previously overwrite

debug_toolbar/panels/logging.py

@@ -45,6 +45,24 @@ def emit(self, record):
         self.collector.collect(record)
 
 
+# Preserve Python's fallback log mechanism before adding ThreadTrackingHandler.
+
+# If the root logger has no handlers attached then everything that reaches it goes
+# to the "handler of last resort".
+# So a Django app that doesn't explicitly configure the root logger actually logs
+# through logging.lastResort.
+# However, logging.lastResort is not used after ThreadTrackingHandler gets added to
+# the root logger below. This means that users who have LoggingPanel enabled might
+# find their logs are gone from their app as soon as they install DDT.
+# Explicitly adding logging.lastResort to logging.root's handler sidesteps this
+# potential confusion.
+# Note that if root has already been configured, or logging.lastResort has been
+# removed, then the configuration is unchanged, so users who configured their
+# logging aren't exposed to the opposite confusion of seeing extra log lines from
+# their app.
+if not logging.root.hasHandlers() and logging.lastResort is not None:
+    logging.root.addHandler(logging.lastResort)
+
 # We don't use enable/disable_instrumentation because logging is global.
 # We can't add thread-local logging handlers. Hopefully logging is cheap.
 

tests/panels/test_logging.py

@@ -1,4 +1,5 @@
 import logging
+from unittest.mock import patch
 
 from debug_toolbar.panels.logging import (
     MESSAGE_IF_STRING_REPRESENTATION_INVALID,
@@ -86,3 +87,10 @@ def view(request):
         self.assertEqual(
             MESSAGE_IF_STRING_REPRESENTATION_INVALID, records[0]["message"]
         )
+
+    @patch("sys.stderr")
+    def test_fallback_logging(self, mock_stderr):
+        # make sure the log reaches stderr even though logging set up
+        # its own handler during its import
+        self.logger.warning("hello")
+        mock_stderr.write.assert_called_once_with("hello\n")

tests/settings.py

@@ -11,7 +11,10 @@
 
 INTERNAL_IPS = ["127.0.0.1"]
 
-LOGGING_CONFIG = None  # avoids spurious output in tests
+LOGGING = {  # avoids spurious output in tests
+    "version": 1,
+    "disable_existing_loggers": True,
+}
 
 
 # Application definition