printing stdout in realtime from a subprocess that requires stdin

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In order to grab stdout from the subprocess in real time you need to decide exactly what behavior you want; specifically, you need to decide whether you want to deal with the output line-by-line or character-by-character, and whether you want to block while waiting for output or be able to do something else while waiting.

It looks like it will probably suffice for your case to read the output in line-buffered fashion, blocking until each complete line comes in, which means the convenience functions provided by subprocess are good enough:

p = subprocess.Popen(some_cmd, stdout=subprocess.PIPE)
# Grab stdout line by line as it becomes available.  This will loop until 
# p terminates.
while p.poll() is None:
    l = p.stdout.readline() # This blocks until it receives a newline.
    print l
# When the subprocess terminates there might be unconsumed output 
# that still needs to be processed.
print p.stdout.read()

If you need to write to the stdin of the process, just use another pipe:

p = subprocess.Popen(some_cmd, stdout=subprocess.PIPE, stdin=subprocess.PIPE)
# Send input to p.
p.stdin.write("some input\n")
p.stdin.flush()
# Now start grabbing output.
while p.poll() is None:
    l = p.stdout.readline()
    print l
print p.stdout.read()

Pace the other answer, there’s no need to indirect through a file in order to pass input to the subprocess.

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