read subprocess stdout line by line

Question

It's been a long time since I last worked with Python, but I think the problem is with the statement for line in proc.stdout, which reads the entire input before iterating over it. The solution is to use readline() instead:

#filters output
import subprocess
proc = subprocess.Popen(['python','fake_utility.py'],stdout=subprocess.PIPE)
while True:
  line = proc.stdout.readline()
  if not line:
    break
  #the real code does filtering here
  print "test:", line.rstrip()

Of course you still have to deal with the subprocess' buffering.

Note: according to the documentation the solution with an iterator should be equivalent to using readline(), except for the read-ahead buffer, but (or exactly because of this) the proposed change did produce different results for me (Python 2.5 on Windows XP).

Bit late to the party, but was surprised not to see what I think is the simplest solution here:

import io
import subprocess

proc = subprocess.Popen(["prog", "arg"], stdout=subprocess.PIPE)
for line in io.TextIOWrapper(proc.stdout, encoding="utf-8"):  # or another encoding
    # do something with line

(This requires Python 3.)

Indeed, if you sorted out the iterator then buffering could now be your problem. You could tell the python in the sub-process not to buffer its output.

proc = subprocess.Popen(['python','fake_utility.py'],stdout=subprocess.PIPE)

becomes

proc = subprocess.Popen(['python','-u', 'fake_utility.py'],stdout=subprocess.PIPE)

I have needed this when calling python from within python.

You want to pass these extra parameters to subprocess.Popen:

bufsize=1, universal_newlines=True

Then you can iterate as in your example. (Tested with Python 3.5)

A function that allows iterating over both stdout and stderr concurrently, in realtime, line by line

In case you need to get the output stream for both stdout and stderr at the same time, you can use the following function.

The function uses Queues to merge both Popen pipes into a single iterator.

Here we create the function read_popen_pipes():

from queue import Queue, Empty
from concurrent.futures import ThreadPoolExecutor


def enqueue_output(file, queue):
    for line in iter(file.readline, ''):
        queue.put(line)
    file.close()


def read_popen_pipes(p):

    with ThreadPoolExecutor(2) as pool:
        q_stdout, q_stderr = Queue(), Queue()

        pool.submit(enqueue_output, p.stdout, q_stdout)
        pool.submit(enqueue_output, p.stderr, q_stderr)

        while True:

            if p.poll() is not None and q_stdout.empty() and q_stderr.empty():
                break

            out_line = err_line = ''

            try:
                out_line = q_stdout.get_nowait()
            except Empty:
                pass
            try:
                err_line = q_stderr.get_nowait()
            except Empty:
                pass

            yield (out_line, err_line)

read_popen_pipes() in use:

import subprocess as sp


with sp.Popen(my_cmd, stdout=sp.PIPE, stderr=sp.PIPE, text=True) as p:

    for out_line, err_line in read_popen_pipes(p):

        # Do stuff with each line, e.g.:
        print(out_line, end='')
        print(err_line, end='')

    return p.poll() # return status-code