How to execute python scripts in their source folder when using colcon?

edit

I've just started using colcon to build a bunch of ROS packages, mixed python/C++.

colcon moves the built packages to the install folder, including the python files. After sourcing install/setup.bash I can roslaunch my python nodes. However, the python files being executed are the ones copied to the install folder, so if I want to attach a debbuger to the running python code I have to do it with the files copied to the install folder. This is a big problem for me, because I'm constantly changing the python code while debugging and I want to do that in the source folder, not in the install folder.

I have tried to use colcon build with --symlink-install but it still copies the python files.

Is there any way to execute the python files in the source folder?

UPDATE:

I haven't got to work --symlink-install as I would expect yet. So I'll give a bit more of detail on what I've tried so far:

first: I use Ubuntu 18.04 with ROS Melodic. the colcon version is:

colcon version-check
colcon-argcomplete 0.3.3: up-to-date
colcon-bash 0.4.2: up-to-date
colcon-cd 0.1.1: up-to-date
colcon-cmake 0.2.18: up-to-date
colcon-core 0.5.3: up-to-date
colcon-defaults 0.2.3: up-to-date
colcon-devtools 0.2.2: up-to-date
colcon-library-path 0.2.1: up-to-date
colcon-metadata 0.2.3: up-to-date
colcon-notification 0.2.12: up-to-date
colcon-output 0.2.7: up-to-date
colcon-package-information 0.3.1: up-to-date
colcon-package-selection 0.2.5: up-to-date
colcon-parallel-executor 0.2.4: up-to-date
colcon-pkg-config 0.1.0: up-to-date
colcon-powershell 0.3.6: up-to-date
colcon-python-setup-py 0.2.3: up-to-date
colcon-recursive-crawl 0.2.1: up-to-date
colcon-ros 0.3.15: up-to-date
colcon-test-result 0.3.8: up-to-date
colcon-zsh 0.4.0: up-to-date

As my python node is very short I initially tried to install it as a script: My directory structure is:

workspace
    src
        gazebo_utils
            launch
                node.launch
           meshes
           urdf
           src
                python_node.py
           CMakeList.txt
           package.xml

My CMakeLists.txt is:

cmake_minimum_required(VERSION 3.5)
project(gazebo_utils)

find_package(catkin REQUIRED)
catkin_package()
catkin_install_python(PROGRAMS
    src/python_node.py
    DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)

install( 
    DIRECTORY launch urdf meshes 
    DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
)

The package.xml is minimal with only catkin as buildtool_depend

I build with: colcon build --symlink-install --packages-select gazebo_utils

I source . install/setup.bash and I can roslaunch gazebo_utils node.launch successfully and the node works. However, what has been installed and is executed is:

workspace
   install
       gazebo_utils
          lib
             gazebo_utils
                python_node.py  (this is a copy not a symlink)

and my python code is a copy no a symlink.

Then I tried to put the code into a python package: I use this directory structure:

workspace
    src
        gazebo_utils
            launch
                node.launch
           meshes
           urdf
           src
                gazebo_utils
                    __init__.py
                    python_node.py
           CMakeList.txt
           package.xml   
           setup.py

__init__.py is empty

The CMakeLists.txt now is:

cmake_minimum_required(VERSION 3.5)
project(gazebo_utils)

find_package(catkin REQUIRED)
catkin_package()
catkin_python_setup()

install( 
    DIRECTORY launch urdf meshes 
    DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
)

and setup.py is:

from distutils.core import setup
from catkin_pkg.python_setup import generate_distutils_setup

# fetch values ...