showing progress while spawning and running subprocess

Question

I need to show some progress bar or something while spawning and running subprocess. How can I do that with python?

import subprocess

cmd = ['python','wait.py']
p = subprocess.Popen(cmd, bufsize=1024,stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
p.stdin.close()
outputmessage = p.stdout.read() #This will print the standard output from the spawned process
message = p.stderr.read()

I could spawn subprocess with this code, but I need to print out something when each second is passing.

Answer 1

Since the subprocess call is blocking, one way to print something out while waiting would be to use multithreading. Here's an example using threading._Timer:

import threading
import subprocess

class RepeatingTimer(threading._Timer):
    def run(self):
        while True:
            self.finished.wait(self.interval)
            if self.finished.is_set():
                return
            else:
                self.function(*self.args, **self.kwargs)


def status():
    print "I'm alive"
timer = RepeatingTimer(1.0, status)
timer.daemon = True # Allows program to exit if only the thread is alive
timer.start()

proc = subprocess.Popen([ '/bin/sleep', "5" ])
proc.wait()

timer.cancel()

On an unrelated note, calling stdout.read() while using multiple pipes can lead to deadlock. The subprocess.communicate() function should be used instead.

Answer 2

As far as I see it all you need to do is put those reads in a loop with a delay and a print - does it have to be precisely a second or around about a second?