Read streaming input from subprocess.communicate()

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())?

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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:
        break
    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.

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

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

import subprocess
proc = subprocess.Popen('ls',
                       shell=True,
                       stdout=subprocess.PIPE,
                       )
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,