Popen stdout reading pipe, deadlock using sleep

Question

Well, I have two scripts. The a.py which prints the output of the b.py script as follows:

#a.py from subprocess import Popen, PIPE, STDOUT p = Popen(['/Users/damian/Desktop/b.py'], shell=False, stdout=PIPE, stderr=STDOUT) while p.poll() is None: print p.stdout.readline() #b.py #!/usr/bin/env python import time while 1: print 'some output' #time.sleep(1)

This works.But, Why do my scripts deadlock when I uncomment the time.sleep() line?

Are you sure that b.py will not deadlock itself if executed separately? (check indentations if they are same for both lines in while 1: ! — przemo_li
– przemo_li, Commented Oct 1, 2012 at 19:37

Martijn Pieters · Accepted Answer · 2012-10-01 19:37:45Z

5

Your output is probably buffered. Add a .flush() for stdout to clear it:

import sys import time while 1: print 'someoutput' sys.stdout.flush() time.sleep(1)

answered Oct 1, 2012 at 19:37

Martijn Pieters

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1 Comment

ScotchAndSoda Over a year ago

Indeed, flush does it. Thanks

Community · Accepted Answer · 2017-05-23 12:18:49Z

If you add -u to the call in a.py (make the output unbuffered) then you don't need to modify b.py script:

import sys from subprocess import Popen, PIPE, STDOUT p = Popen([sys.executable, '-u', '/Users/damian/Desktop/b.py'], stdout=PIPE, stderr=STDOUT, close_fds=True) for line in iter(p.stdout.readline, ''): print line, p.stdout.close() if p.wait() != 0: raise RuntimeError("%r failed, exit status: %d" % (cmd, p.returncode))

See more ways to get output from a subprocess.

Nice. Heh, also thanks for the output examples. One more proof of Python's power.

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