Importing python files/functions from the same directory (as simple as it sounds!)

edit

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.