[subprocess] Simple way to handle error in Popen()?

[Winfried]

Hello,

What is a simple and reliable way to handle an error when calling and monitoring a CLI application through subprocess.Popen()?

Is this good enough?

p = subprocess.Popen("blah", text=True, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
if p.stderr:
	self.statusbar.SetStatusText("Popen error.")
	exit()
else	
	while (line := p.stdout.readline()) != "":
		self.statusbar.SetStatusText(line.strip())
output = f"End of output.  Return code: {p.wait()}"
self.statusbar.SetStatusText(output)

Thank you.

[snippsat]

This shows how to stream lines as they arrive(and avoid deadlocks) by merging stderr into stdout.
Can add other check eg timeout, exception depends on application you monitor.

import subprocess
import shlex

def stream_cmd(cmd: str, on_line, on_done):
    args = shlex.split(cmd)  # safer than shell=True
    p = subprocess.Popen(
        args,
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT, # merge stderr into stdout
        text=True,
        bufsize=1,
    )

    # Stream until process exits
    for line in p.stdout:
        on_line(line.rstrip())
    rc = p.wait()
    on_done(rc)

# Example usage
if __name__ == "__main__":
    def print_line(s):
        print(s)
    def done(rc):
        print(f"== DONE, exit code {rc} ==")

    # Monitor a CLI stream
    stream_cmd("ping -n 50 127.0.0.1", print_line, done) 

So if i close the ping process before finished,it just exit with error code 1

Output:
.....
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
== DONE, exit code 1 ==