Python import: rosrun works well but roslaunch reports error

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Hi, I'm writing some rospy nodes recently and some import errors ocur when I utilize roslaunch to start my node.

a.py
1 import sys
2 sys.path.append("..")
3 from middle_abstraction.function_unit import FunctionUnit

ImportError: No module named middle_abstraction.function_unit

The file is organized as following

package
    scripts
       basic_support
          a.py
      middle_abstraction
          function_unit.py

When I cd to basic_support and run python a.py, everthing is working well as well as when I utilize rosrun pkg a.py. But when I utilize roslaunch, this error ocurs.

I have written one setup.py file in my pkg folder:

## ! 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=['multi_robot_patrol'],#my pkg name
    package_dir={'': 'src'},
    requires=['std_msgs', 'rospy', 'message_filters', 'gps_common', 'sensor_msgs']
)

setup(**setup_args)

I'm really confused that rosrun works pretty well but roslaunch reports import error. I'd appreciate it if anyone can help me.