Don't configure logging for trait notification handler exceptions (#1161

)

* Don't configure logging

* Add changelog entry

* Add test to check that logs are created

* Add note about log configuration to the 'Notes on upgrading' section

* Wording tweak

* Update changelog wording

* More wording tweaks

* Fix another typo

CHANGES.rst

@@ -62,6 +62,22 @@ However, there are a few things to be aware of when upgrading.
   TraitsUI views, and an error will be raised if you attempt to create a
   TraitsUI view.
 
+* Traits now does no logging configuration at all, leaving all such
+  configuration to the application.
+
+  In more detail: trait notification handlers should not raise exceptions in
+  normal use, so an exception is logged whenever a trait notification handler
+  raises. This part of the behaviour has not changed. What *has* changed is the
+  way that logged exception is handled under default exception handling.
+
+  Previously, Traits added a :class:`~logging.StreamHandler` to the
+  top-level ``"traits"`` logger, so that trait notification exceptions would
+  always be visible. Traits also added a :class:`~logging.NullHandler` to that
+  logger. Both of those handlers have now been removed. We now rely on
+  Python's "handler of last resort", which will continue to make notification
+  exceptions to the user visible in the absence of any application-level
+  log configuration.
+
 * When listening for changes to the items of a :class:`.List` trait, an index
   or slice set operation no longer performs an equality check between the
   replaced elements and the replacement elements when deciding whether to issue
@@ -169,6 +185,9 @@ Changes
   result in content that compares equally to the old values. (#1026)
 * ``TraitListEvent.index`` reported by mutations to a list is now normalized.
   (#1009)
+* The default notification error handler for Traits no longer configures
+  logging, and the top-level ``NullHandler`` log handler has been removed.
+  (#1161)
 
 Fixes
 ~~~~~

traits/__init__.py

@@ -16,10 +16,3 @@
     # hasn't been built, so this isn't a viable Traits installation. OTOH, it
     # can be useful if a simple "import traits" doesn't actually fail.
     __version__ = "unknown"
-
-# Add a NullHandler so 'traits' loggers don't complain when they get used.
-import logging
-
-logging.getLogger(__name__).addHandler(logging.NullHandler())
-
-del logging

traits/tests/test_listeners.py

@@ -16,6 +16,7 @@
 
 import contextlib
 import io
+import logging
 import sys
 import threading
 import time
@@ -152,6 +153,28 @@ def _handle_exception(obj, name, old, new):
         finally:
             trait_notifiers.pop_exception_handler()
 
+    def test_exceptions_logged(self):
+        # Check that default exception handling logs the exception.
+        ge = GenerateFailingEvents()
+
+        traits_logger = logging.getLogger("traits")
+
+        with self.assertLogs(
+                logger=traits_logger, level=logging.ERROR) as log_watcher:
+            ge.name = "Terry Jones"
+
+        self.assertEqual(len(log_watcher.records), 1)
+        log_record = log_watcher.records[0]
+
+        self.assertIn(
+            "Exception occurred in traits notification handler",
+            log_record.message,
+        )
+
+        _, exc_value, exc_traceback = log_record.exc_info
+        self.assertIsInstance(exc_value, RuntimeError)
+        self.assertIsNotNone(exc_traceback)
+
 
 class A(HasTraits):
     exception = Any

traits/trait_notifiers.py

@@ -12,6 +12,7 @@
 """
 
 import contextlib
+import logging
 import threading
 from threading import local as thread_local
 from threading import Thread
@@ -211,16 +212,7 @@ def _log_exception(self, object, trait_name, old, new):
 
         logger = self.traits_logger
         if logger is None:
-            import logging
-
             self.traits_logger = logger = logging.getLogger("traits")
-            handler = logging.StreamHandler()
-            handler.setFormatter(logging.Formatter("%(message)s"))
-            logger.addHandler(handler)
-            print(
-                "Exception occurred in traits notification handler.\n"
-                "Please check the log file for details."
-            )
 
         try:
             logger.exception(