add a custom excepthook for rclpy

this excepthook allows you to register something
to be printed for an exception, but only if it
goes unhandled.

rclpy/rclpy/__init__.py

@@ -20,9 +20,13 @@
 from rclpy.impl import implementation_singleton
 from rclpy.impl import rmw_implementation_tools
 from rclpy.node import Node
+from rclpy.impl import excepthook
 
 from rclpy.impl.implementation_singleton import rclpy_implementation as _rclpy
 
+# install the excepthook
+excepthook.install_rclpy_excepthook()
+
 
 def init(args=None):
     rclpy_rmw_env = os.getenv('RCLPY_IMPLEMENTATION', None)

rclpy/rclpy/impl/excepthook.py

@@ -0,0 +1,60 @@
+# Copyright 2016 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+import sys
+
+__original_excepthook = None
+__unhandled_exception_addendums = {}
+
+
+def rclpy_excepthook(exc_type, value, traceback):
+    """rclpy's custom except hook for unhandled exceptions.
+
+    This excepthook will check for any unhandled exception addendum's,
+    log them if they exist, and then call the original excepthook.
+    The original excepthook is what ever was in ``sys.excepthook`` when
+    :py:func:`install_rclpy_excepthook` was called.
+
+    Any addendum found during this excepthook will be removed after logging.
+    """
+    if value in __unhandled_exception_addendums:
+        logger = logging.getLogger('rclpy')
+        logger.error(__unhandled_exception_addendums[value])
+        del __unhandled_exception_addendums[value]
+    return __original_excepthook(exc_type, value, traceback)
+
+
+def install_rclpy_excepthook():
+    """Replace the system excepthook with the custom rclpy excepthook.
+
+    Repeated calls to this function will return without any side effects.
+    """
+    global __original_excepthook
+    if __original_excepthook is not None:
+        # The rclpy excepthook has already been installed.
+        return
+    __original_excepthook = sys.excepthook
+    sys.excepthook = rclpy_excepthook
+
+
+def add_unhandled_exception_addendum(exception, message):
+    """Add an addendum for the given exception.
+
+    If the exception is never handled, then the addendum will be logged before
+    passing the exception to the original excepthook.
+    This only works if the custom rclpy excepthook has been installed with
+    :py:func:`install_rclpy_excepthook`.
+    """
+    __unhandled_exception_addendums[exception] = message