Fix string-valued launch argument substitution

[okvik]

Consider a launch file that declares an argument meant to be used as a string,
in this example an IP address.

from simple_launch import SimpleLauncher

sl = SimpleLauncher()

sl.declare_arg('robot_ip', '10.0.30.112')


def launch():
    print(sl.arg('robot_ip'))
    return sl.launch_description()


generate_launch_description = sl.launch_description(opaque_function=launch)

Performing the substitution as above inside the opaque function context results
in an SyntaxError being thrown since the given literal value 10.0.30.112 --
as seen by ast.literal_eval -- cannot be interpreted as valid Python.

My suggested fix is simply catching this exception and returning any such
"invalid" inputs as strings.

It might be worth noting that one can bodge a fix on the user side as follows:

sl.declare_arg('robot_ip', '"10.0.30.112"')

ros2 launch ... robot_ip:=\"10.0.30.112\"

Note the additional quotes to force interpretation as a Python string.

P.S. Thank you for making this wonderful library. With it, I no longer feel existential
dread when faced with launching off a few ROS 2 nodes...

[oKermorgant]

Hi,

Good catch!

As you have guessed, the whole point of simple_launch is to focus on robotics so yeah, it is much better to hide this complexity inside the try_perform function than going into details on adding quotes around IPs as launch arguments.

Thanks for the PR, I'll merge it and prepare a new release.