Python2 and Python3 interoperability

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I believe catkin_virtualenv can run Python 3 venvs in a Python 2 environment, fully integrated as a "regular" ROS package.

Alternatively: use rospypi/simple instead of the regular rospy. That's fully Melodic and Python 3 compatible.

I'm no expert in py2-py3 (I'm only recently struggling in production system dev). That said, I think of a few ways. If OPT-1 works then that might be preferrable as it might be extendable at lower cost.

OPT-1. Call Python2 module/executable from Python3 code. I don't know if there's officially supported Pythonic way to do this.

I think finding Python interpreter with specific version like 2.7 like this stackoverflow.com#q27863832 should work (I haven't tried). Code copied from the link:

import execnet

def call_python_version(Version, Module, Function, ArgumentList):
    gw      = execnet.makegateway("popen//python=python%s" % Version)
    channel = gw.remote_exec("""
        from %s import %s as the_function
        channel.send(the_function(*channel.receive()))
    """ % (Module, Function))
    channel.send(ArgumentList)
    return channel.receive()

OPT-2. Separately run Py2 executables/modules and Py3. ROS being distributed system (wiki.ros.org/ROS/Introduction) allows each node/process to communicate over the network, meaning they can be started separately. So divide the processes that need to run in Py2 and Py3. A challenge is when you're currently using roslaunch to start all processes by batch. I thought of separating a .launch file into a launch for Py2 and a .launch for Py3. However, roslaunch starts with the Python version default on your environment, and AFAICT there's no good way to start roslaunch with a Python version that is not default on your environment (related #q340390). In this case, I'd try (I haven't tried. Also for readability, I set Py2 is default on your environment):

• From all-in-one .launch file, move Py3 portion out. Make Py3 portion executable without relying on ROS toolset (roslaunch/rosrun etc.)
• Start .launch file, and maybe wait for a second or so (to wait for ROS Master to be available by the roslaunch) separately start Py3 portion.