python code to run an external command with a timeout and return the output

cmdTimeout.py

import fcntl
import subprocess
import sys
import shlex
from threading import Timer
import time
import os
'''
  python3 code to allow one to run an external command and return the output
'''
def nonBlockRead(output):
    try:
        fd = output.fileno()
        fl = fcntl.fcntl(fd, fcntl.F_GETFL)
        fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
        out= output.read()
        if out !=None:
            return out
        else:
            return b''
    except:
        return b''
        
def kill_proc(proc):
    try:
        proc.stdout.close()  # If they are not closed the fds will hang around until
        proc.stderr.close()  # os.fdlimit is exceeded and cause a nasty exception
        proc.kill()     # Important to close the fds prior to terminating the process!
    except:
        pass
        
        
def cmdTimeout(cmd, timeout_sec):
    #run a command for awhile, timeout if it doesn't complete in the time alloted.
    proc = subprocess.Popen(shlex.split(cmd), stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    timer = Timer(timeout_sec, kill_proc, [proc])
    timer.start()
    stdout = ''
    stderr = ''
    while proc.poll() is None : # Monitor process
        time.sleep(0.1) # Wait a little

        # p.std* blocks on read(), which messes up the timeout timer.
        # To fix this, we use a nonblocking read()
        # Note: Not sure if this is Windows compatible
        stdout+= str(nonBlockRead(proc.stdout).decode("utf-8"))
        stderr+= str(nonBlockRead(proc.stderr).decode("utf-8"))
    timer.cancel()
    returncode  = proc.returncode
    return (returncode, stdout, stderr)


if __name__ == '__main__':
    #run a command that finishes
    rcode,sout,serr=cmdTimeout("ping -c1 google.com",3)
    print(rcode,sout,serr)
    
    #one that will timeout
    rcode,sout,serr=cmdTimeout("ping google.com",3)
    print(rcode,sout,serr)