How to get output from subprocess.Popen(). proc.stdout.readline() blocks, no data prints out

You obviously can use subprocess.communicate but I think you are looking for real time input and output.

readline was blocked because the process is probably waiting on your input. You can read character by character to overcome this like the following:

import subprocessimport sysprocess = subprocess.Popen(    cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)while True:    out = process.stdout.read(1)    if out == '' and process.poll() != None:        break    if out != '':        sys.stdout.write(out)        sys.stdout.flush()

Nadia's snippet does work but calling read with a 1 byte buffer is highly unrecommended. The better way to do this would be to set the stdout file descriptor to nonblocking using fcntl

fcntl.fcntl(    proc.stdout.fileno(),    fcntl.F_SETFL,    fcntl.fcntl(proc.stdout.fileno(), fcntl.F_GETFL) | os.O_NONBLOCK,)

and then using select to test if the data is ready

while proc.poll() == None:    readx = select.select([proc.stdout.fileno()], [], [])[0]    if readx:        chunk = proc.stdout.read()        print chunk

She was correct in that your problem must be different from what you posted as Caller.py and Test_Pipe.py do work as provided.

• https://derrickpetzold.com/p/capturing-output-from-ffmpeg-python/

Test_Pipe.py buffers its stdout by default so proc in Caller.py doesn't see any output until the child's buffer is full (if the buffer size is 8KB then it takes around a minute to fill Test_Pipe.py's stdout buffer).

To make the output unbuffered (line-buffered for text streams) you could pass -u flag to the child Python script. It allows to read subprocess' output line by line in "real-time":

import sysfrom subprocess import Popen, PIPEproc = Popen([sys.executable, "-u", "Test_Pipe.py"], stdout=PIPE, bufsize=1)for line in iter(proc.stdout.readline, b''):    print line,proc.communicate()

See links in Python: read streaming input from subprocess.communicate() on how to solve the block-buffering issue for non-Python child processes.