• In python3, for a ros node consisting of multiple .py files, it looks like there are two approaches you can take:

  Path 1. No python module. Do NOT use catkin_install_python in the CMakeLists.txt, and set the executable bit on your main .py file. Importing another python file from the same directory will work. The downside of this simple approach is that the ros package can not be installed to /opt/ros/, but it's fine for development purposes.

  Path 2. Use a python module. Put your .py files in a python module, add a setup.py to top-level to install the python module, and use catkin_install_python() and catkin_python_setup() in CMakeLists.txt. Your imports must be changed to use the dot-prefix notation (e.g. from .sum import Sum)

• Do not put a __init__.py in your ros package's scripts/ directory. roslaunch and rosrun do not expect to find a python package there.

• You should not mix the output of catkin_make and catkin build - they are incompatible. You must use one or the other, and fully remove the directories catkin_ws/build/ and catkin_ws/devel/ if you switch between them.

• You are asking for trouble using a ros or python package name of "test". It's theoretically legal, but the probability of an undesired name collision is very high.

I appreciate Mike's help. Hopefully, this answer can help someone that stumbles upon the same importing issue.

I'm not quite sure what the underlying problem is, yet allow me to show the structure that removes the error:

CMakeLists.xtx:

cmake_minimum_required(VERSION 3.0.2)
project(test_package)

## Find catkin macros and libraries
## if COMPONENTS list like find_package(catkin REQUIRED COMPONENTS xyz)
## is used, also find other catkin packages
find_package(catkin REQUIRED COMPONENTS
  message_generation
  rospy
  std_msgs
)

catkin_package(
#  INCLUDE_DIRS include
#  LIBRARIES test
#  CATKIN_DEPENDS rospy std_msgs message_runtime
#  DEPENDS system_lib
)

## Specify additional locations of header files
## Your package locations should be listed before other locations
include_directories(
# include
  ${catkin_INCLUDE_DIRS}
)

## Mark executable scripts (Python etc.) for installation
## in contrast to setup.py, you can choose the destination
catkin_install_python(PROGRAMS
  src/talker.py 
  src/listener.py #src/test_package/sum.py
  DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)

package.xml

<?xml version="1.0"?>
<package format="2">
  <name>test_package</name>
  <version>0.0.0</version>
  <description>The test package</description>

  <maintainer email="anon@todo.todo">anon</maintainer>

  <license>TODO</license>

  <buildtool_depend>catkin</buildtool_depend>

  <build_depend>message_generation</build_depend>
  <build_depend>rospy</build_depend>
  <build_depend>std_msgs</build_depend>

  <build_export_depend>rospy</build_export_depend>
  <build_export_depend>std_msgs</build_export_depend>

  <exec_depend>rospy</exec_depend>
  <exec_depend>std_msgs</exec_depend>

  <export>

  </export>
</package>

setup.py

## ! DO NOT MANUALLY INVOKE THIS setup.py, USE CATKIN INSTEAD
from distutils.core import setup
from catkin_pkg.python_setup import generate_distutils_setup
# fetch values from package.xml
setup_args = generate_distutils_setup(
packages=['test_package'],
package_dir={'': 'src'},
)
setup(**setup_args)

talker.py

#!/usr/bin/env python

import rospy
from test_package.sum import Sum
from std_msgs.msg import String

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    print(Sum(1,2))
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(10) # 10hz
    while not rospy.is_shutdown():
        hello_str = "hello world %s" % rospy.get_time()
        rospy.loginfo(hello_str)
        pub.publish(hello_str)
        rate.sleep()

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

And most importantly, the file structure:

catkin_ws/src/test_package
    /src
        /test_package
            __init__.py
            sum.py
        talker.py
        listener.py
    CmakeLists.txt
    package.xml
    setup.py

where I build with catkin build, then source devel/setup.bash and rosrun test_package talker.py after running an instance of roscore. Given the location of the __init__.py and the setup.py file, I suspect something went wrong there initially.