add message when the rmw implementation import failure goes unhandled

rclpy/rclpy/__init__.py

@@ -19,6 +19,7 @@
 from rclpy.exceptions import InvalidRCLPYImplementation
 from rclpy.impl import implementation_singleton
 from rclpy.impl import rmw_implementation_tools
+from rclpy.impl.rmw_implementation_tools import RCLPY_IMPLEMENTATION_ENV_NAME
 from rclpy.node import Node
 from rclpy.impl import excepthook
 
@@ -29,7 +30,7 @@
 
 
 def init(args=None):
-    rclpy_rmw_env = os.getenv('RCLPY_IMPLEMENTATION', None)
+    rclpy_rmw_env = os.getenv(RCLPY_IMPLEMENTATION_ENV_NAME, None)
     if rclpy_rmw_env is not None:
         available_rmw_implementations = rmw_implementation_tools.get_rmw_implementations()
         if rclpy_rmw_env not in available_rmw_implementations:

rclpy/rclpy/impl/rmw_implementation_tools.py

@@ -19,11 +19,14 @@
 
 from rclpy.exceptions import ImplementationAlreadyImportedException
 from rclpy.exceptions import InvalidRCLPYImplementation
+from rclpy.impl.excepthook import add_unhandled_exception_addendum
 
 __rmw_implementations = None
 __selected_rmw_implementation = None
 __rmw_implementation_module = None
 
+RCLPY_IMPLEMENTATION_ENV_NAME = 'RCLPY_IMPLEMENTATION'
+
 
 def reload_rmw_implementations():
     """(Re)Load the available rmw implementations by inspecting the ament index."""
@@ -77,13 +80,32 @@ def import_rmw_implementation():
     global __rmw_implementation_module
     if __rmw_implementation_module is not None:
         return __rmw_implementation_module
+    available_implementations = get_rmw_implementations()
     if __selected_rmw_implementation is None:
         logger = logging.getLogger('rclpy')
-        select_rmw_implementation(get_rmw_implementations()[0])  # select the first one
+        select_rmw_implementation(available_implementations[0])  # select the first one
         logger.debug("Implicitly selecting the '{0}' rmw implementation."
                      .format(__selected_rmw_implementation))
     module_name = '._rclpy__{rmw_implementation}'.format(
         rmw_implementation=__selected_rmw_implementation,
     )
-    __rmw_implementation_module = importlib.import_module(module_name, package='rclpy')
+    try:
+        __rmw_implementation_module = importlib.import_module(module_name, package='rclpy')
+    except ImportError as exc:
+        if "No module named 'rclpy._rclpy__" in str(exc):
+            log_args = [
+                __selected_rmw_implementation,
+                RCLPY_IMPLEMENTATION_ENV_NAME,
+                '\n'.join(['  - {0}'.format(x) for x in available_implementations]),
+            ]
+            # This message will only be printed if this exception goes unhandled.
+            add_unhandled_exception_addendum(
+                exc,
+                "\n"
+                "Failed to import the Python extension for the '{0}' rmw implementation.\n"
+                "A different rmw implementation can be selected using the '{1}' env variable.\n"
+                "These are the available rmw implementations:\n"
+                "{2}\n".format(*log_args)
+            )
+        raise
     return __rmw_implementation_module

rclpy/test/test_select_rmw_implementation.py

@@ -46,7 +46,7 @@ def func_import_each_available_rmw_implementation(rmw_implementation):
     try:
         rclpy.init([])
     except ImportError as exc:
-        if "No module named 'rclpy._rclpy__" in '{0}'.format(exc):
+        if "No module named 'rclpy._rclpy__" in str(exc):
             # Not all rmw implementations generate a Python implementation.
             # TODO(wjwwood): prune get_rmw_implementations by
             #   implementations that have generated a Python implementation
@@ -96,7 +96,7 @@ def func_select_rmw_implementation_by_environment(rmw_implementation):
     try:
         rclpy.init([])
     except ImportError as exc:
-        if "No module named 'rclpy._rclpy__" in '{0}'.format(exc):
+        if "No module named 'rclpy._rclpy__" in str(exc):
             # Not all rmw implementations generate a Python implementation.
             # TODO(wjwwood): prune get_rmw_implementations by
             #   implementations that have generated a Python implementation