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I have a windows executable that I want to run over and over. The problem is that sometimes there's an error about 1 second in, but the program doesn't exit. So what I would like to do is to be able to grab the contents of stdout, recognize there is an error, and then kill the subprocess and start it over.

When I run this executable, stuff prints to the screen just fine. But when I wrap it in a subprocess from python then the stdout stuff doesn't show up until the program terminates.

I've tried basically everything posted here with no luck: Constantly print Subprocess output while process is running

Here's my current code, I replaced the executable with a second python program just to remove any other weird variables:

parent_program.py:

import subprocess, os, sys
program = "python "+os.path.dirname(os.path.abspath(__file__)) + "/child_program.py"

with subprocess.Popen(program, shell=True, stdout=subprocess.PIPE, bufsize=1, universal_newlines=True) as p:
    for line in p.stdout:
        print(line, end='')

child_program.py:

from time import sleep
for i in range(0,10):
    print(i)
    sleep(1)

What I would expect is that I would see 1,2,3,4... printed one second at a time, as if I had just run python child_program.py, but instead I get nothing for 10 seconds and then get all the output at once.

I also thought about trying to run the program from the CMD prompt and piping the stdout to a file python child_program.py 2>&1 > output.txt and then having python read that file, but it's the same problem, the file doesn't get written until the program terminates.

Is there any way to fix this on windows?

Chase Roberts
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  • In your child, don't print to STDOUT because it is buffered (line buffered when writing to interactive output and fully buffered when writing to non interactive output). Instead, use STDERR which isn't buffered. I don't know python so I can't tell you how - Perhaps this? https://stackoverflow.com/questions/5574702/how-to-print-to-stderr-in-python – Jerry Jeremiah Aug 24 '22 at 23:52
  • But the child is just a placeholder for an exe that I want to run. The .exe that I am running prints to stdout and not stderr. – Chase Roberts Aug 25 '22 at 14:53

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