ImportError: No module named my_package.msg

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Hi I have created a package, that contains custom messages. They seem to be compiled correctly. Because I can call

rosmsg package my_package

and get

my_package/Collision

which is correct. But when I try to run a python script that depends on this message like:

from my_package.msg import Collision

I get:

ImportError: No module named my_package.msg

Do you have any Ideas what I am doing wrong?

Some additional Info:

export | grep ROS
declare -x ROS
declare -x ROS_DISTRO="hydro"
declare -x ROS_ETC_DIR="/opt/ros/hydro/etc/ros"
declare -x ROS_MASTER_URI="http://localhost:11311"
declare -x ROS_PACKAGE_PATH="/home/aub-ch/ros/hydro_ws/src:/home/aub-ch/ros/hydro_mynavigationtest/src:/opt/ros/hydro/share:/opt/ros/hydro/stacks"
declare -x ROS_ROOT="/opt/ros/hydro/share/ros"
declare -x ROS_TEST_RESULTS_DIR="/home/aub-ch/ros/hydro_mynavigationtest/build/test_results"

and

echo $PYTHONPATH 
/home/aub-ch/ros/hydro_mynavigationtest/devel/lib/python2.7/dist-packages:/opt/ros/hydro/lib/python2.7/dist-packages