Python-将参数传递给已经运行的进程

Question

first of all, i´m pretty new to Python. I also searched for a solution, but i guess the usual approach (subprocess.popen) won´t work in my case.

I have to pass arguments to a listener in an already running python script without starting the script over and over again. There is an example how to pass a message to a lcd-screen:

function printMsgText(message_text)
local f = io.popen("home/test/show_message.py '" .. message_text .. "'")
end

This lua-script above defines the process which gets called everytime a message is recieved. The called process (show_message.py) looks like that:

import sys
from sense_hat import SenseHat
sense = SenseHat()
sense.clear()
sense.show_message(sys.argv[1])

I need something similar, except that there is another script running in the backround, so show_message.py is not the final process but needs to pass the argument/message to another, already running script. My idea was to just let show_message.py print the message to the console and use sys.argv in the main process aswell but i´ma little afraid that it could get messy.

Is there any easy way to do this? Kind regards

Edit:

The main script is controlling a stepper-motor. Based on the user input, the motor drives a pre-defined number of steps. The part of the script waiting for the user-input looks like this:

while wait:
            user_position = input("Where do you wanna go? (0, 1, 2, back): ")
            wait = False
            #  Console output
            print("Position: " + str(user_position))

            if user_position == "0":
                stepper.set_target_position(position_zero)
                wait = True
            elif user_position == "1":
                stepper.set_target_position(position_one
                wait = True
            elif user_position == "2":
                stepper.set_target_position(position_two)
                wait = True
            elif user_position == "back":
                break

Now i need to pass the desired position via a web-application which is designed the way i described above (eg calling a lua-script every time a variable/argument is passed) and not via the console.

Answer 1

Once a process is running it won't re-evaluate its command line arguments. You need other ways to communicate with it. This is colloquially called inter-process communication (IPC), and there are several ways to achieve it. Here are a few:

• Files
• Pipes (on platforms that support them)
• Shared memory
• Socket communication
• Message passing
• RPC

Probably the most approachable way is standard streams (STDIN, STDOUT) as provided by eg subprocess.popen() which you mentioned. But this requires a parent-child relation between the communicating processes. Maybe you can go this route.

Another way for Python, if you want to avoid parent-child relations, which I have made good experiences with is Pyro . It was easy to use and worked well, albeit with some performance penalty. It is also a very "infrastructure-ish" way to go about it, both processes have to be coded to use it.

Answer 2

You can use some sort of messaging library that will allow you to communicate between processes. ZeroMQ is a good option, and has python bindings.