ImportError : No module named xxxx.msg

edit

For those that don't like to watch videos and want an answer straightaway (this is taken right from the wiki):

Update your package.xml with:

<build_depend>message_generation</build_depend>
<exec_depend>message_runtime</exec_depend>

This will tell the build system about the new message definitions.

Next, modify your CMakeLists.txt file by:

adding the following at the end of find_package()

find_package(
    ...
    # any packages your messages depend on such as std_msgs
    message_generation
)

export the message runtime dependency

catkin_package(
  ...
  CATKIN_DEPENDS message_runtime ...
  ...)

add any message files that you have defined and want to compile

add_message_files(
  FILES
  my_file.msg
)

call the generate_messages() function

generate_messages(
  DEPENDENCIES
  std_msgs
  # and any other packages your messages depend on
)

Now, navigate to your workspace's root and compile using either

~/catkin_ws/$ catkin_make

or

~/catkin_ws/$ catkin build

and source your workspace

~/catkin_ws/$ source devel/setup.bash