CODEWITHSUNDEEP
pythonsubprocesspython-multiprocessing
Таня Воробець
Таня Воробець

Reputation: 3

subprocess.Popen does not work in an infinite loop

subprocess.Popen calls the calculator.py only when the loop ends. I need the code to run the calculator at once.

from subprocess import Popen
import calculator
while True:
    a='op'
    c='io'
    b=input()
    if a==b:
        #calculator
        Popen(['python', 'calculator.py'])
    elif c==b:
        print('hello')
    else:
        print("hi")
        break

Upvotes: 0

Views: 856

Answers (2)

thisisrandy
thisisrandy

Reputation: 3075

Per the docs, "The recommended approach to invoking subprocesses is to use the run() function for all use cases it can handle. For more advanced use cases, the underlying Popen interface can be used directly." This is a very simple use case, so using run() is preferred. The problem here is that Popen isn't blocking, meaning that your program isn't waiting for it to finish executing, which I think is what you want. run(), by contrast, "Wait(s) for (the) command to complete, then return(s) a CompletedProcess instance."

For clarity's sake, here's some code to explain what's going on:

# instead of Popen, we're going to import run. CompletedProcess is used below
# for illustration purposes, but you don't need it and can remove it from your
# code
from subprocess import CompletedProcess, run

# there's no reason to import calculator, since you don't use it. you should
# remove this line
# import calculator

while True:
    a = "op"
    c = "io"
    b = input()
    if a == b:
        # after the specified command has completed, run returns an object of
        # type CompletedProcess[bytes], which I've indicated below using
        # python's type annotation syntax, i.e. variable: Type. this is for
        # illustration only. Since you're not using the return value, you can
        # simply say:
        # run(['python', 'calculator.py'])
        _completed_process: CompletedProcess[bytes] = run(["python", "calculator.py"])
    elif c == b:
        print("hello")
    else:
        print("hi")
        break

Upvotes: 2

JojOatXGME
JojOatXGME

Reputation: 3296

subprocess.Popen starts the process, but does not wait for its completion. You might use subprocess.run instead.

Upvotes: 0

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