Improved rclpy import failed message

[wjwwood]

I achieved this by adding a helpful message which prints when the import fails, but only if it goes unhandled.

I wanted to avoid printing it always incase someone is explicitly trying each of the implementations and wants to avoid the logging output. To accomplish this I added a custom excepthook for rclpy to override the built-in sys.excepthook. You can also add "addendum's" for uncaught exceptions, which will printed before the traceback. In the custom excepthook, it looks for a matching addendum, prints it if it finds one, and then passes along the unhandled exception to the original excepthook.

This is the observed behavior:

• In the normal case you get something like:

% python3 -c "import talker_py; talker_py.main()"

Failed to import the Python extension for the 'rmw_fastrtps_cpp' rmw implementation.
A different rmw implementation can be selected using the 'RCLPY_IMPLEMENTATION' env variable.
These are the available rmw implementations:
  - rmw_fastrtps_cpp
  - rmw_opensplice_cpp

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/william/ros2_ws/build_isolated/rclpy_examples/talker_py.py", line 27, in main
    rclpy.init(args)
  File "/Users/william/ros2_ws/install_isolated/rclpy/lib/python3.5/site-packages/rclpy/__init__.py", line 47, in init
    rmw_implementation_tools.import_rmw_implementation()
  File "/Users/william/ros2_ws/install_isolated/rclpy/lib/python3.5/site-packages/rclpy/impl/rmw_implementation_tools.py", line 90, in import_rmw_implementation
    __rmw_implementation_module = importlib.import_module(module_name, package='rclpy')
  File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 956, in _find_and_load_unlocked
ImportError: No module named 'rclpy._rclpy__rmw_fastrtps_cpp'

• In the case that you catch it, you get:

% python3 -c "
import talker_py
try:
    talker_py.main()
except ImportError:
    print('failed to import')
"
failed to import

[wjwwood]

I also addressed the style problems from #10 (comment).

[wjwwood]

Actually, scratch that. I remove the commit that cleans up #10 so I can put it in a different pr.

[jacquelinekay]

I wonder if you could do slightly better than hardcoding the module prefix here. If the module name changed from rclpy._rclpy__rmw_.... to something else then this logic would break. Wouldn't something like "No module named 'rclpy.{0}".format(module_name) work?

[wjwwood]

CI:

• http://ci.ros2.org/job/ci_linux/1174/
• http://ci.ros2.org/job/ci_osx/949/
• http://ci.ros2.org/job/ci_windows/1229/

[wjwwood]

I fixed http://ci.ros2.org/job/ci_linux/1174/testReport/junit/projectroot.home.rosbuild.ci_scripts.ws.src.ros2.rclpy/rclpy/pep257/ in b994e52, it shouldn't need a retest.

[wjwwood]

I believe all the CI is satisfactory (with my pep257 fix), looking for final reviews before squash and merge.

[wjwwood]

I believe all the CI is satisfactory (with my pep257 fix), looking for final reviews before squash and merge.

bump

[jacquelinekay]

+1

[dirk-thomas]

I would suggest the opposite order:

if not available_implementations:
    raise NoImplementationAvailableException()
select_rmw_implementation(available_implementations[0])  # select the first one

[wjwwood]

I don't think it makes a difference, but if you want to change it I'm fine with that.