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

Question

I want output from execute Test_Pipe.py, I tried following code on Linux but it did not work.

Test_Pipe.py

import time
while True :         


        

Answer 1

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 sys
from subprocess import Popen, PIPE

proc = 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.