How to terminate the script started with subprocess.Popen() based on specific condition

Question

I am starting a Python script called test.py from the main script called main.py . In the test.py I am tracking some machine learning metrics. When these metrics reach a certain threshold, I want to terminate the subprocess in which the test.py was started.

Is there a possibility to achieve this in Python if I have started this script by using:
proc = subprocess.Popen("python test.py", shell=True)

I haven't found anything in the documentation which would allow me to trigger this event on my own.

Answer 1

You can use printing and reading from stdout and stdin . As an example, consider a simple test.py that calculates (in a very inefficient way) some primes:

test.py

import time

primes = [2, 3]

if __name__ == "__main__":
    for p in primes:
        print(p, flush=True)

    i = 5
    while True:
        for p in primes:
            if i % p == 0:
                break
        if i % p:
            primes.append(i)
            print(i, flush=True)
        i += 2
        time.sleep(.005)

You can read the output and choose to terminate the process when you achieve the desired output. As an example, I want to get primes up to 1000 .

import subprocess

proc = subprocess.Popen("python test.py",
                        stdout=subprocess.PIPE, stdin=subprocess.PIPE,
                        bufsize=1, universal_newlines=True,
                        shell=True, text=True)
must_stop = False
primes = []
while proc.poll() is None:
    line = proc.stdout.readline()
    if line:
        new_prime = int(line)
        primes.append(new_prime)
        if  new_prime > 1000:
            print("Threshold achieved", line)
            proc.terminate()
        else:
            print("new prime:", new_prime)
print(primes)

please notice that since there is a delay in the processing and communication, you might get one or two more primes than desired. If you want to avoid that, you'd need bi-directional communication and test.py would be more complicated.