I am trying to make a simple python script that starts a subprocess and monitors its standard output. Here is a snippet from the code:
process = subprocess.Popen([path_to_exe, os.path.join(temp_dir,temp_file)], stdout=subprocess.PIPE) while True: output=process.stdout.readline() print "test"
The problem is that the script hangs on output=process.stdout.readline()
and that the line print "test"
only executes after the subprocess is terminated.
Is there a way to read standard output and print it without having to wait for the subprocess to terminate?
The subprocess which I am starting is a Windows binary for which I do not have the source code.
I have found several similar questions, but the answers are only applicable on Linux or in case I have the source of the suprocess I am starting.
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Answer
Check select module
import subprocess import select import time x=subprocess.Popen(['/bin/bash','-c',"while true; do sleep 5; echo yes; done"],stdout=subprocess.PIPE) y=select.poll() y.register(x.stdout,select.POLLIN) while True: if y.poll(1): print x.stdout.readline() else: print "nothing here" time.sleep(1)
EDIT:
Threaded Solution for non posix systems:
import subprocess from threading import Thread import time linebuffer=[] x=subprocess.Popen(['/bin/bash','-c',"while true; do sleep 5; echo yes; done"],stdout=subprocess.PIPE) def reader(f,buffer): while True: line=f.readline() if line: buffer.append(line) else: break t=Thread(target=reader,args=(x.stdout,linebuffer)) t.daemon=True t.start() while True: if linebuffer: print linebuffer.pop(0) else: print "nothing here" time.sleep(1)