Constantly print Subprocess output while process is running

Question

To launch programs from my Python-scripts, I\'m using the following method:

def execute(command):
    process = subprocess.Popen(command, shell=True, stdout=s         


        

Answer 1

For anyone trying the answers to this question to get the stdout from a Python script note that Python buffers its stdout, and therefore it may take a while to see the stdout.

This can be rectified by adding the following after each stdout write in the target script:

sys.stdout.flush()

Answer 2

@tokland

tried your code and corrected it for 3.4 and windows dir.cmd is a simple dir command, saved as cmd-file

import subprocess
c = "dir.cmd"

def execute(command):
    popen = subprocess.Popen(command, stdout=subprocess.PIPE,bufsize=1)
    lines_iterator = iter(popen.stdout.readline, b"")
    while popen.poll() is None:
        for line in lines_iterator:
            nline = line.rstrip()
            print(nline.decode("latin"), end = "\r\n",flush =True) # yield line

execute(c)

Answer 3

To print subprocess' output line-by-line as soon as its stdout buffer is flushed in Python 3:

from subprocess import Popen, PIPE, CalledProcessError

with Popen(cmd, stdout=PIPE, bufsize=1, universal_newlines=True) as p:
    for line in p.stdout:
        print(line, end='') # process line here

if p.returncode != 0:
    raise CalledProcessError(p.returncode, p.args)

Notice: you do not need p.poll() -- the loop ends when eof is reached. And you do not need iter(p.stdout.readline, '') -- the read-ahead bug is fixed in Python 3.

See also, Python: read streaming input from subprocess.communicate().

Answer 4

This PoC constantly reads the output from a process and can be accessed when needed. Only the last result is kept, all other output is discarded, hence prevents the PIPE from growing out of memory:

import subprocess
import time
import threading
import Queue


class FlushPipe(object):
    def __init__(self):
        self.command = ['python', './print_date.py']
        self.process = None
        self.process_output = Queue.LifoQueue(0)
        self.capture_output = threading.Thread(target=self.output_reader)

    def output_reader(self):
        for line in iter(self.process.stdout.readline, b''):
            self.process_output.put_nowait(line)

    def start_process(self):
        self.process = subprocess.Popen(self.command,
                                        stdout=subprocess.PIPE)
        self.capture_output.start()

    def get_output_for_processing(self):
        line = self.process_output.get()
        print ">>>" + line


if __name__ == "__main__":
    flush_pipe = FlushPipe()
    flush_pipe.start_process()

    now = time.time()
    while time.time() - now < 10:
        flush_pipe.get_output_for_processing()
        time.sleep(2.5)

    flush_pipe.capture_output.join(timeout=0.001)
    flush_pipe.process.kill()

print_date.py

#!/usr/bin/env python
import time

if __name__ == "__main__":
    while True:
        print str(time.time())
        time.sleep(0.01)

output: You can clearly see that there is only output from ~2.5s interval nothing in between.

>>>1520535158.51
>>>1520535161.01
>>>1520535163.51
>>>1520535166.01

Answer 5

None of the answers here addressed all of my needs.

1. No threads for stdout (no Queues, etc, either)
2. Non-blocking as I need to check for other things going on
3. Use PIPE as I needed to do multiple things, e.g. stream output, write to a log file and return a string copy of the output.

A little background: I am using a ThreadPoolExecutor to manage a pool of threads, each launching a subprocess and running them concurrency. (In Python2.7, but this should work in newer 3.x as well). I don't want to use threads just for output gathering as I want as many available as possible for other things (a pool of 20 processes would be using 40 threads just to run; 1 for the process thread and 1 for stdout...and more if you want stderr I guess)

I'm stripping back a lot of exception and such here so this is based on code that works in production. Hopefully I didn't ruin it in the copy and paste. Also, feedback very much welcome!

import time
import fcntl
import subprocess
import time

proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

# Make stdout non-blocking when using read/readline
proc_stdout = proc.stdout
fl = fcntl.fcntl(proc_stdout, fcntl.F_GETFL)
fcntl.fcntl(proc_stdout, fcntl.F_SETFL, fl | os.O_NONBLOCK)

def handle_stdout(proc_stream, my_buffer, echo_streams=True, log_file=None):
    """A little inline function to handle the stdout business. """
    # fcntl makes readline non-blocking so it raises an IOError when empty
    try:
        for s in iter(proc_stream.readline, ''):   # replace '' with b'' for Python 3
            my_buffer.append(s)

            if echo_streams:
                sys.stdout.write(s)

            if log_file:
                log_file.write(s)
    except IOError:
        pass

# The main loop while subprocess is running
stdout_parts = []
while proc.poll() is None:
    handle_stdout(proc_stdout, stdout_parts)

    # ...Check for other things here...
    # For example, check a multiprocessor.Value('b') to proc.kill()

    time.sleep(0.01)

# Not sure if this is needed, but run it again just to be sure we got it all?
handle_stdout(proc_stdout, stdout_parts)

stdout_str = "".join(stdout_parts)  # Just to demo

I'm sure there is overhead being added here but it is not a concern in my case. Functionally it does what I need. The only thing I haven't solved is why this works perfectly for log messages but I see some print messages show up later and all at once.

Answer 6

Ok i managed to solve it without threads (any suggestions why using threads would be better are appreciated) by using a snippet from this question Intercepting stdout of a subprocess while it is running

def execute(command):
    process = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

    # Poll process for new output until finished
    while True:
        nextline = process.stdout.readline()
        if nextline == '' and process.poll() is not None:
            break
        sys.stdout.write(nextline)
        sys.stdout.flush()

    output = process.communicate()[0]
    exitCode = process.returncode

    if (exitCode == 0):
        return output
    else:
        raise ProcessException(command, exitCode, output)