Stop a running process started from Python

Question

I have a ROS code rostopic pub toggle_led std_msgs/Empty that basically starts once and keeps running until CTRL+C is pressed.
Now, I would like to automate this command from Python. I checked Calling an external command in Python but it only shows how to start the command.
How would I start and stop running this process as and when I want?

Answer 1

How would I start and stop running this process as and when I want?

Well, you already know how to start it, as you said in the previous sentence.

How do you stop it? If you want to stop it exactly like a Ctrl-C,* you do that by calling send_signal on it, using CTRL_C_EVENT on Windows, or SIGTERM on Unix.** So:

import signal
import subprocess

try:
    sig = signal.CTRL_C_EVENT
except NameError:
    sig = signal.SIGTERM

p = subprocess.Popen(['/path/to/prog', '-opt', '42', 'arg'])
# ... later
p.send_signal(sig)

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If you only care about Linux (or *nix in general), you can make this even simpler: terminate is guaranteed to do the same thing as send_signal(SIGTERM). So:

import subprocess
p = subprocess.Popen(['/path/to/prog', '-opt', '42', 'arg'])
# ... later
p.terminate()

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Since you asked in a comment "Could you please explain the various parameters to subprocess.Popen()": Well, there are a whole lot of them (see Popen Constructor and Frequently Used Arguments in the docs, but I'm only using one, the args parameter.

Normally, you pass a list to args, with the name of the program as the first element in the list, and each separate command-line argument as a separate element. But if you want to use the shell, you pass a string for args, and add a shell=True as another argument.

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_{* Note that "exactly like a Ctrl-C" may not actually be what you want on Windows, unless the program has a console and is a process group owner. This may mean you'll need to add creationflags=subprocess.CREATE_NEW_PROCESS_GROUP to the Popen call. Or it may not—e.g.., if you use shell=True.}

_{** In Python, you can usually ignore the platform differences between CTRL_C_EVENT and SIGTERM and always use the latter, but subprocess.send_signal is one of the few places you can't. On Windows, send_signal(SIGTERM) will call terminate instead of sending a Ctrl-C. If you don't actually care exactly how the process gets stopped, just that it gets stopped somehow, then of course you can use SIGTERM… but in that case, you might as well just call terminate.}