CODEWITHSUNDEEP
pythonpython-3.xsubprocesspopen
Artiom Kozyrev
Artiom Kozyrev

Reputation: 3836

How to pass value to method argument which is inside Popen.subprocess?

This is my main python script:

import time
import subprocess
def main():
   while(True):
       a=input("Please enter parameter to pass to subprocess:")
       subprocess.Popen(args="python child.py")
       print(f"{a} was started")
       time.sleep(5)
if __name__ == '__main__':
    main()

This is python child script named child.py:

def main(a):
    while(True):
        print(a)

if __name__ == '__main__':
    main(a)

How to pass value to argument a which is in the child subprocess?

Upvotes: 4

Views: 4152

Answers (3)

Aran-Fey
Aran-Fey

Reputation: 43166

The easiest way to pass arguments to a child process is to use command line parameters.

The first step is to rewrite child.py so that it accepts command line arguments. There is detailed information about parsing command line arguments in this question: How to read/process command line arguments? For this simple example though, we will simply access the command line arguments through sys.argv.

import sys

def main(a):
    while(True):
        print(a)

if __name__ == '__main__':
    # the first element in the sys.argv list is the name of our program ("child.py"), so
    # the argument the parent process sends to the child process will be the 2nd element
    a = sys.argv[1]
    main(a)

Now child.py can be started with an argument like python child.py foobar and the text "foobar" will be used as the value for the a variable.

With that out of the way, all that's left is to rewrite parent.py and make it pass an argument to child.py. The recommended way to pass arguments with subprocess.Popen is as a list of strings, so we'll do just that:

import time
import subprocess

def main():
   while(True):
       a = input("Please enter parameter to pass to subprocess:")
       subprocess.Popen(["python", "child.py", a])  # pass a as an argument
       print(f"{a} was started")
       time.sleep(5)

if __name__ == '__main__':
    main()

Upvotes: 2

ruohola
ruohola

Reputation: 24038

You need to use command line arguments, like this;

import time
import subprocess

def main():
   while(True):
       a=input("Please enter parameter to pass to subprocess:")
       subprocess.Popen(["python", "child.py", a])
       print(f"{a} was started")
       time.sleep(5)

if __name__ == '__main__':
    main()

child.py:

import sys

def main(a):
    while(True):
        print(a)

if __name__ == '__main__':
    a = sys.argv[1]
    main(a)

Upvotes: 2

user10325516
user10325516

Reputation:

You may use subprocess.PIPE to pass data between your main process and spawned subprocess.

Main script:

import subprocess


def main():
    for idx in range(3):
        a = input(
            'Please enter parameter to pass to subprocess ({}/{}): '
            .format(idx + 1, 3))
        print('Child in progress...')
        pipe = subprocess.Popen(
            args='python child.py',
            stdin=subprocess.PIPE,
            stdout=subprocess.PIPE)
        pipe.stdin.write(str(a).encode('UTF-8'))
        pipe.stdin.close()
        print('Child output is:')
        print(pipe.stdout.read().decode('UTF-8'))


if __name__ == '__main__':
    main()

Child script:

import sys
import time


def main(a):
    for dummy in range(3):
        time.sleep(.1)
        print(a)


if __name__ == '__main__':
    a = sys.stdin.read()
    main(a)

Output:

>>> python main.py
Please enter parameter to pass to subprocess (1/3): qwe
Child in progress...
Child output is:
qwe
qwe
qwe

Please enter parameter to pass to subprocess (2/3): qweqwe
Child in progress...
Child output is:
qweqwe
qweqwe
qweqwe

Please enter parameter to pass to subprocess (3/3): 123
Child in progress...
Child output is:
123
123
123

Upvotes: 2

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