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Python: read streaming input from subprocess.communicate()

Posted by: admin November 29, 2017 Leave a comment


I’m using Python’s subprocess.communicate() to read stdout from a process that runs for about a minute.

How can I print out each line of that process’s stdout in a streaming fashion, so that I can see the output as it’s generated, but still block on the process terminating before continuing?

subprocess.communicate() appears to give all the output at once.


Please note, I think J.F. Sebastian’s method (below) is better.

Here is an simple example (with no checking for errors):

import subprocess
proc = subprocess.Popen('ls',
while proc.poll() is None:
    output = proc.stdout.readline()
    print output,

If ls ends too fast, then the while loop may end before you’ve read all the data.

You can catch the remainder in stdout this way:

output = proc.communicate()[0]
print output,


To get subprocess’ output line by line as soon as the subprocess flushes its stdout buffer:

#!/usr/bin/env python2
from subprocess import Popen, PIPE

p = Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1)
with p.stdout:
    for line in iter(p.stdout.readline, b''):
        print line,
p.wait() # wait for the subprocess to exit

iter() is used to read lines as soon as they are written to workaround the read-ahead bug in Python 2.

If subprocess’ stdout uses a block buffering instead of a line buffering in non-interactive mode (that leads to a delay in the output until the child’s buffer is full or flushed explicitly by the child) then you could try to force an unbuffered output using pexpect, pty modules or unbuffer, stdbuf, script utilities, see Q: Why not just use a pipe (popen())?

Here’s Python 3 code:

#!/usr/bin/env python3
from subprocess import Popen, PIPE

with Popen(["cmd", "arg1"], stdout=PIPE, bufsize=1, universal_newlines=True) as p:
    for line in p.stdout:
        print(line, end='')

Note: Unlike Python 2 that outputs subprocess’ bytestrings as is; Python 3 uses text mode (cmd’s output is decoded using locale.getpreferredencoding(False) encoding).


I believe the simplest way to collect output from a process in a streaming fashion is like this:

import sys
from subprocess import *
proc = Popen('ls', shell=True, stdout=PIPE)
while True:
    data = proc.stdout.readline()   # Alternatively proc.stdout.read(1024)
    if len(data) == 0:
    sys.stdout.write(data)   # sys.stdout.buffer.write(data) on Python 3.x

The readline() or read() function should only return an empty string on EOF, after the process has terminated – otherwise it will block if there is nothing to read (readline() includes the newline, so on empty lines, it returns “\n”). This avoids the need for an awkward final communicate() call after the loop.

On files with very long lines read() may be preferable to reduce maximum memory usage – the number passed to it is arbitrary, but excluding it results in reading the entire pipe output at once which is probably not desirable.


If you want a non-blocking approach, don’t use process.communicate(). If you set the subprocess.Popen() argument stdout to PIPE, you can read from process.stdout and check if the process still runs using process.poll().


If you’re simply trying to pass the output through in realtime, it’s hard to get simpler than this:

import subprocess

# This will raise a CalledProcessError if the program return a nonzero code.
# You can use call() instead if you don't care about that case.
subprocess.check_call(['ls', '-l'])

See the docs for subprocess.check_call().

If you need to process the output, sure, loop on it. But if you don’t, just keep it simple.

Edit: J.F. Sebastian points out both that the defaults for the stdout and stderr parameters pass through to sys.stdout and sys.stderr, and that this will fail if sys.stdout and sys.stderr have been replaced (say, for capturing output in tests).