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How to keep ssh session open after logging in using subprocess.popen?

I am new to Python.
I am trying to SSH to a server to perform some operations. However, before performing the operations, i need to load a profile, which takes 60-90 seconds. After loading the profile, is there a way to keep the SSH session open so that i can perform the operations later?
p = subprocess.Popen("ssh abc#xyz'./profile'", stdout=subprocess.PIPE, shell=True)
result = p.communicate()[0]
print result
return result
This loads the profile and exits. Is there a way to keep the above ssh session open and run some commands?
Example:
p = subprocess.Popen("ssh abc#xyz'./profile'", stdout=subprocess.PIPE, shell=True)
<More Python Code>
<More Python Code>
<More Python Code>
<Run some scripts/commands on xyz server non-interactively>
After loading the profile, I want to run some scripts/commands on the remote server, which I am able to do by simply doing below:
p = subprocess.Popen("ssh abc#xyz './profile;**<./a.py;etc>**'", stdout=subprocess.PIPE, shell=True)
However, once done, it exists and the next time I want to execute some script on the above server, I need to load the profile again (which takes 60-90 seconds). I am trying to figure out a way where we can create some sort of tunnel (or any other way) where the ssh connection remains open after loading the profile, so that the users don't have to wait 60-90 seconds whenever anything is to be executed.
I don't have access to strip down the profile.

Try an ssh library like asyncssh or spur. Keeping the connection object should keep the session open.
You could send a dummy command like date to prevent the timeout as well.

You have to construct a ssh command like this ['ssh', '-T', 'host_user_name#host_address'] then follow below code.
Code:
from subprocess import Popen, PIPE
ssh_conn = ['ssh', '-T', 'host_user_name#host_address']
# if you have to add port then ssh_conn should be as following
# ssh_conn = ['ssh', '-T', 'host_user_name#host_address', '-p', 'port']
commands = """
cd Documents/
ls -l
cat test.txt
"""
with Popen(ssh_conn, stdin=PIPE, stdout=PIPE, stderr=PIPE, universal_newlines=True) as p:
output, error = p.communicate(commands)
print(output)
print(error)
print(p.returncode)
# or can do following things
p.stdin.write('command_1')
# add as many command as you want
p.stdin.write('command_n')
Terminal Output:
Please let me know if you need further explanations.
N.B: You can add command in commands string as many as you want.

What you want to do is write/read to the process's stdin/stdout.
from subprocess import Popen, PIPE
import shlex
shell_command = "ssh user#address"
proc = Popen(shlex.split(shell_command), stdin=PIPE, universal_newlines=True)
# Do python stuff here
proc.stdin.write("cd Desktop\n")
proc.stdin.write("mkdir Example\n")
# And so on
proc.stdin.write("exit\n")
You must include the trailing newline for each command. If you prefer, print() (as of Python 3.x, where it is a function) takes a keyword argument file, which allows you to forget about that newline (and also gain all the benefits of print()).
print("rm Example", file=proc.stdin)
Additionally, if you need to see the output of your command, you can pass stdout=PIPE and then read via proc.stdout.read() (same for stderr).
You may also want to but the exit command in a try/finally block, to ensure you exit the ssh session gracefully.
Note that a) read is blocking, so if there's no output, it'll block forever and b) it will only return what was available to read from the stdout at that time- so you may need to read repeatedly, sleep for a short time, or poll for additional data. See the fnctl and select stdlib modules for changing blocking -> nonblocking read and polling for events, respectively.

Hello Koshur!
I think that what you are trying to achieve looks like what I've tried in the past when trying to make my terminal accessible from a private website:
I would open a bash instance, keep it open and would listen for commands through a WebSocket connection.
What I did to achieve this was using the O_NONBLOCK flag on STDOUT.
Example
import fcntl
import os
import shlex
import subprocess
current_process = subprocess.Popen(shlex.split("/bin/sh"), stdin=subprocess.PIPE,
stdout=subprocess.PIPE, stderr=subprocess.STDOUT) # Open a shell prompt
fcntl.fcntl(current_process.stdout.fileno(), fcntl.F_SETFL,
os.O_NONBLOCK) # Non blocking stdout and stderr reading
What I would have after this is a loop checking for new output in another thread:
from time import sleep
from threading import Thread
def check_output(process):
"""
Checks the output of stdout and stderr to send it to the WebSocket client
"""
while process.poll() is None: # while the process isn't exited
try:
output = process.stdout.read() # Read the stdout PIPE (which contains stdout and stderr)
except Exception:
output = None
if output:
print(output)
sleep(.1)
# from here, we are outside the loop: the process exited
print("Process exited with return code: {code}".format(code=process.returncode))
Thread(target=check_output, args=(current_process,), daemon=True).start() # Start checking for new text in stdout and stderr
So you would need to implement your logic to SSH when starting the process:
current_process = subprocess.Popen(shlex.split("ssh abc#xyz'./profile'"), stdin=subprocess.PIPE,
stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
And send commands like so:
def send_command(process, cmd):
process.stdin.write(str(cmd + "\n").encode("utf-8")) # Write the input to STDIN
process.stdin.flush() # Run the command
send_command(current_process, "echo Hello")
EDIT
I tried to see the minimum Python requirements for the given examples and found out that Thread(daemon) might not work on Python 2.7, which you asked in the tags.
If you are sure to exit the Thread before exiting, you can ignore daemon and use Thread() which works on 2.7. (You could for example use atexit and terminate the process)
References
fcntl(2) man page
https://man7.org/linux/man-pages/man2/fcntl.2.html
fcntl Python 3 Documentation
https://docs.python.org/3/library/fcntl.html
fcntl Python 2.7 Documentation
https://docs.python.org/2.7/library/fcntl.html
O_NONBLOCK Python 3 Documentation
https://docs.python.org/3/library/os.html#os.O_NONBLOCK
O_NONBLOCK Python 2.7 Documentation
https://docs.python.org/2.7/library/os.html#os.O_NONBLOCK

Reading stdout from a subprocess in real time

Given this code snippet:
from subprocess import Popen, PIPE, CalledProcessError
def execute(cmd):
with Popen(cmd, shell=True, stdout=PIPE, bufsize=1, universal_newlines=True) as p:
for line in p.stdout:
print(line, end='')
if p.returncode != 0:
raise CalledProcessError(p.returncode, p.args)
base_cmd = [
"cmd", "/c", "d:\\virtual_envs\\py362_32\\Scripts\\activate",
"&&"
]
cmd1 = " ".join(base_cmd + ['python -c "import sys; print(sys.version)"'])
cmd2 = " ".join(base_cmd + ["python -m http.server"])
If I run execute(cmd1) the output will be printed without any problems.
However, If I run execute(cmd2) instead nothing will be printed, why is that and how can I fix it so I could see the http.server's output in real time.
Also, how for line in p.stdout is been evaluated internally? is it some sort of endless loop till reaches stdout eof or something?
This topic has already been addressed few times here in SO but I haven't found a windows solution. The above snippet is code from this answer and I'm running http.server from a virtualenv (python3.6.2-32bits on win7)

If you want to read continuously from a running subprocess, you have to make that process' output unbuffered. Your subprocess being a Python program, this can be done by passing -u to the interpreter:
python -u -m http.server
This is how it looks on a Windows box.

With this code, you can`t see the real-time output because of buffering:
for line in p.stdout:
print(line, end='')
But if you use p.stdout.readline() it should work:
while True:
line = p.stdout.readline()
if not line: break
print(line, end='')
See corresponding python bug discussion for details
UPD: here you can find almost the same problem with various solutions on stackoverflow.

I think the main problem is that http.server somehow is logging the output to stderr, here I have an example with asyncio, reading the data either from stdout or stderr.
My first attempt was to use asyncio, a nice API, which exists in since Python 3.4. Later I found a simpler solution, so you can choose, both of em should work.
asyncio as solution
In the background asyncio is using IOCP - a windows API to async stuff.
# inspired by https://pymotw.com/3/asyncio/subprocesses.html
import asyncio
import sys
import time
if sys.platform == 'win32':
loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)
async def run_webserver():
buffer = bytearray()
# start the webserver without buffering (-u) and stderr and stdin as the arguments
print('launching process')
proc = await asyncio.create_subprocess_exec(
sys.executable, '-u', '-mhttp.server',
stdout=asyncio.subprocess.PIPE,
stderr=asyncio.subprocess.PIPE
)
print('process started {}'.format(proc.pid))
while 1:
# wait either for stderr or stdout and loop over the results
for line in asyncio.as_completed([proc.stderr.readline(), proc.stdout.readline()]):
print('read {!r}'.format(await line))
event_loop = asyncio.get_event_loop()
try:
event_loop.run_until_complete(run_df())
finally:
event_loop.close()
redirecting the from stdout
based on your example this is a really simple solution. It just redirects the stderr to stdout and only stdout is read.
from subprocess import Popen, PIPE, CalledProcessError, run, STDOUT import os
def execute(cmd):
with Popen(cmd, stdout=PIPE, stderr=STDOUT, bufsize=1) as p:
while 1:
print('waiting for a line')
print(p.stdout.readline())
cmd2 = ["python", "-u", "-m", "http.server"]
execute(cmd2)

How for line in p.stdout is been evaluated internally? is it some sort of endless loop till reaches stdout eof or something?
p.stdout is a buffer (blocking). When you are reading from an empty buffer, you are blocked until something is written to that buffer. Once something is in it, you get the data and execute the inner part.
Think of how tail -f works on linux: it waits until something is written to the file, and when it does it echo's the new data to the screen. What happens when there is no data? it waits. So when your program gets to this line, it waits for data and process it.
As your code works, but when run as a model not, it has to be related to this somehow. The http.server module probably buffers the output. Try adding -u parameter to Python to run the process as unbuffered:
-u : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x
see man page for details on internal buffering relating to '-u'
Also, you might want to try change your loop to for line in iter(lambda: p.stdout.read(1), ''):, as this reads 1 byte at a time before processing.
Update: The full loop code is
for line in iter(lambda: p.stdout.read(1), ''):
sys.stdout.write(line)
sys.stdout.flush()
Also, you pass your command as a string. Try passing it as a list, with each element in its own slot:
cmd = ['python', '-m', 'http.server', ..]

You could implement the no-buffer behavior at the OS level.
In Linux, you could wrap your existing command line with stdbuf :
stdbuf -i0 -o0 -e0 YOURCOMMAND
Or in Windows, you could wrap your existing command line with winpty:
winpty.exe -Xallow-non-tty -Xplain YOURCOMMAND
I'm not aware of OS-neutral tools for this.

Run command and get its stdout, stderr separately in near real time like in a terminal

I am trying to find a way in Python to run other programs in such a way that:
The stdout and stderr of the program being run can be logged
separately.
The stdout and stderr of the program being run can be
viewed in near-real time, such that if the child process hangs, the
user can see. (i.e. we do not wait for execution to complete before
printing the stdout/stderr to the user)
Bonus criteria: The
program being run does not know it is being run via python, and thus
will not do unexpected things (like chunk its output instead of
printing it in real-time, or exit because it demands a terminal to
view its output). This small criteria pretty much means we will need
to use a pty I think.
Here is what i've got so far...
Method 1:
def method1(command):
## subprocess.communicate() will give us the stdout and stderr sepurately,
## but we will have to wait until the end of command execution to print anything.
## This means if the child process hangs, we will never know....
proc=subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True, executable='/bin/bash')
stdout, stderr = proc.communicate() # record both, but no way to print stdout/stderr in real-time
print ' ######### REAL-TIME ######### '
######## Not Possible
print ' ########## RESULTS ########## '
print 'STDOUT:'
print stdout
print 'STDOUT:'
print stderr
Method 2
def method2(command):
## Using pexpect to run our command in a pty, we can see the child's stdout in real-time,
## however we cannot see the stderr from "curl google.com", presumably because it is not connected to a pty?
## Furthermore, I do not know how to log it beyond writing out to a file (p.logfile). I need the stdout and stderr
## as strings, not files on disk! On the upside, pexpect would give alot of extra functionality (if it worked!)
proc = pexpect.spawn('/bin/bash', ['-c', command])
print ' ######### REAL-TIME ######### '
proc.interact()
print ' ########## RESULTS ########## '
######## Not Possible
Method 3:
def method3(command):
## This method is very much like method1, and would work exactly as desired
## if only proc.xxx.read(1) wouldn't block waiting for something. Which it does. So this is useless.
proc=subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True, executable='/bin/bash')
print ' ######### REAL-TIME ######### '
out,err,outbuf,errbuf = '','','',''
firstToSpeak = None
while proc.poll() == None:
stdout = proc.stdout.read(1) # blocks
stderr = proc.stderr.read(1) # also blocks
if firstToSpeak == None:
if stdout != '': firstToSpeak = 'stdout'; outbuf,errbuf = stdout,stderr
elif stderr != '': firstToSpeak = 'stderr'; outbuf,errbuf = stdout,stderr
else:
if (stdout != '') or (stderr != ''): outbuf += stdout; errbuf += stderr
else:
out += outbuf; err += errbuf;
if firstToSpeak == 'stdout': sys.stdout.write(outbuf+errbuf);sys.stdout.flush()
else: sys.stdout.write(errbuf+outbuf);sys.stdout.flush()
firstToSpeak = None
print ''
print ' ########## RESULTS ########## '
print 'STDOUT:'
print out
print 'STDERR:'
print err
To try these methods out, you will need to import sys,subprocess,pexpect
pexpect is pure-python and can be had with
sudo pip install pexpect
I think the solution will involve python's pty module - which is somewhat of a black art that I cannot find anyone who knows how to use. Perhaps SO knows :)
As a heads-up, i recommend you use 'curl www.google.com' as a test command, because it prints its status out on stderr for some reason :D
UPDATE-1:
OK so the pty library is not fit for human consumption. The docs, essentially, are the source code.
Any presented solution that is blocking and not async is not going to work here. The Threads/Queue method by Padraic Cunningham works great, although adding pty support is not possible - and it's 'dirty' (to quote Freenode's #python).
It seems like the only solution fit for production-standard code is using the Twisted framework, which even supports pty as a boolean switch to run processes exactly as if they were invoked from the shell.
But adding Twisted into a project requires a total rewrite of all the code. This is a total bummer :/
UPDATE-2:
Two answers were provided, one of which addresses the first two
criteria and will work well where you just need both the stdout and
stderr using Threads and Queue. The other answer uses select, a
non-blocking method for reading file descriptors, and pty, a method to
"trick" the spawned process into believing it is running in a real
terminal just as if it was run from Bash directly - but may or may not
have side-effects. I wish I could accept both answers, because the
"correct" method really depends on the situation and why you are
subprocessing in the first place, but alas, I could only accept one.

The stdout and stderr of the program being run can be logged separately.
You can't use pexpect because both stdout and stderr go to the same pty and there is no way to separate them after that.
The stdout and stderr of the program being run can be viewed in near-real time, such that if the child process hangs, the user can see. (i.e. we do not wait for execution to complete before printing the stdout/stderr to the user)
If the output of a subprocess is not a tty then it is likely that it uses a block buffering and therefore if it doesn't produce much output then it won't be "real time" e.g., if the buffer is 4K then your parent Python process won't see anything until the child process prints 4K chars and the buffer overflows or it is flushed explicitly (inside the subprocess). This buffer is inside the child process and there are no standard ways to manage it from outside. Here's picture that shows stdio buffers and the pipe buffer for command 1 | command2 shell pipeline:
The program being run does not know it is being run via python, and thus will not do unexpected things (like chunk its output instead of printing it in real-time, or exit because it demands a terminal to view its output).
It seems, you meant the opposite i.e., it is likely that your child process chunks its output instead of flushing each output line as soon as possible if the output is redirected to a pipe (when you use stdout=PIPE in Python). It means that the default threading or asyncio solutions won't work as is in your case.
There are several options to workaround it:
the command may accept a command-line argument such as grep --line-buffered or python -u, to disable block buffering.
stdbuf works for some programs i.e., you could run ['stdbuf', '-oL', '-eL'] + command using the threading or asyncio solution above and you should get stdout, stderr separately and lines should appear in near-real time:
#!/usr/bin/env python3
import os
import sys
from select import select
from subprocess import Popen, PIPE
with Popen(['stdbuf', '-oL', '-e0', 'curl', 'www.google.com'],
stdout=PIPE, stderr=PIPE) as p:
readable = {
p.stdout.fileno(): sys.stdout.buffer, # log separately
p.stderr.fileno(): sys.stderr.buffer,
}
while readable:
for fd in select(readable, [], [])[0]:
data = os.read(fd, 1024) # read available
if not data: # EOF
del readable[fd]
else:
readable[fd].write(data)
readable[fd].flush()
finally, you could try pty + select solution with two ptys:
#!/usr/bin/env python3
import errno
import os
import pty
import sys
from select import select
from subprocess import Popen
masters, slaves = zip(pty.openpty(), pty.openpty())
with Popen([sys.executable, '-c', r'''import sys, time
print('stdout', 1) # no explicit flush
time.sleep(.5)
print('stderr', 2, file=sys.stderr)
time.sleep(.5)
print('stdout', 3)
time.sleep(.5)
print('stderr', 4, file=sys.stderr)
'''],
stdin=slaves[0], stdout=slaves[0], stderr=slaves[1]):
for fd in slaves:
os.close(fd) # no input
readable = {
masters[0]: sys.stdout.buffer, # log separately
masters[1]: sys.stderr.buffer,
}
while readable:
for fd in select(readable, [], [])[0]:
try:
data = os.read(fd, 1024) # read available
except OSError as e:
if e.errno != errno.EIO:
raise #XXX cleanup
del readable[fd] # EIO means EOF on some systems
else:
if not data: # EOF
del readable[fd]
else:
readable[fd].write(data)
readable[fd].flush()
for fd in masters:
os.close(fd)
I don't know what are the side-effects of using different ptys for stdout, stderr. You could try whether a single pty is enough in your case e.g., set stderr=PIPE and use p.stderr.fileno() instead of masters[1]. Comment in sh source suggests that there are issues if stderr not in {STDOUT, pipe}

If you want to read from stderr and stdout and get the output separately, you can use a Thread with a Queue, not overly tested but something like the following:
import threading
import queue
def run(fd, q):
for line in iter(fd.readline, ''):
q.put(line)
q.put(None)
def create(fd):
q = queue.Queue()
t = threading.Thread(target=run, args=(fd, q))
t.daemon = True
t.start()
return q, t
process = Popen(["curl","www.google.com"], stdout=PIPE, stderr=PIPE,
universal_newlines=True)
std_q, std_out = create(process.stdout)
err_q, err_read = create(process.stderr)
while std_out.is_alive() or err_read.is_alive():
for line in iter(std_q.get, None):
print(line)
for line in iter(err_q.get, None):
print(line)

While J.F. Sebastian's answer certainly solves the heart of the problem, i'm running python 2.7 (which wasn't in the original criteria) so im just throwing this out there to any other weary travellers who just want to cut/paste some code.
I havent tested this throughly yet, but on all the commands i have tried it seems to work perfectly :)
you may want to change .decode('ascii') to .decode('utf-8') - im still testing that bit out.
#!/usr/bin/env python2.7
import errno
import os
import pty
import sys
from select import select
import subprocess
stdout = ''
stderr = ''
command = 'curl google.com ; sleep 5 ; echo "hey"'
masters, slaves = zip(pty.openpty(), pty.openpty())
p = subprocess.Popen(command, stdin=slaves[0], stdout=slaves[0], stderr=slaves[1], shell=True, executable='/bin/bash')
for fd in slaves: os.close(fd)
readable = { masters[0]: sys.stdout, masters[1]: sys.stderr }
try:
print ' ######### REAL-TIME ######### '
while readable:
for fd in select(readable, [], [])[0]:
try: data = os.read(fd, 1024)
except OSError as e:
if e.errno != errno.EIO: raise
del readable[fd]
finally:
if not data: del readable[fd]
else:
if fd == masters[0]: stdout += data.decode('ascii')
else: stderr += data.decode('ascii')
readable[fd].write(data)
readable[fd].flush()
except:
print "Unexpected error:", sys.exc_info()[0]
raise
finally:
p.wait()
for fd in masters: os.close(fd)
print ''
print ' ########## RESULTS ########## '
print 'STDOUT:'
print stdout
print 'STDERR:'
print stderr

live output from subprocess command

I'm using a python script as a driver for a hydrodynamics code. When it comes time to run the simulation, I use subprocess.Popen to run the code, collect the output from stdout and stderr into a subprocess.PIPE --- then I can print (and save to a log-file) the output information, and check for any errors. The problem is, I have no idea how the code is progressing. If I run it directly from the command line, it gives me output about what iteration its at, what time, what the next time-step is, etc.
Is there a way to both store the output (for logging and error checking), and also produce a live-streaming output?
The relevant section of my code:
ret_val = subprocess.Popen( run_command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True )
output, errors = ret_val.communicate()
log_file.write(output)
print output
if( ret_val.returncode ):
print "RUN failed\n\n%s\n\n" % (errors)
success = False
if( errors ): log_file.write("\n\n%s\n\n" % errors)
Originally I was piping the run_command through tee so that a copy went directly to the log-file, and the stream still output directly to the terminal -- but that way I can't store any errors (to my knowlege).
My temporary solution so far:
ret_val = subprocess.Popen( run_command, stdout=log_file, stderr=subprocess.PIPE, shell=True )
while not ret_val.poll():
log_file.flush()
then, in another terminal, run tail -f log.txt (s.t. log_file = 'log.txt').

TLDR for Python 3:
import subprocess
import sys
with open("test.log", "wb") as f:
process = subprocess.Popen(your_command, stdout=subprocess.PIPE)
for c in iter(lambda: process.stdout.read(1), b""):
sys.stdout.buffer.write(c)
f.buffer.write(c)
You have two ways of doing this, either by creating an iterator from the read or readline functions and do:
import subprocess
import sys
# replace "w" with "wb" for Python 3
with open("test.log", "w") as f:
process = subprocess.Popen(your_command, stdout=subprocess.PIPE)
# replace "" with b'' for Python 3
for c in iter(lambda: process.stdout.read(1), ""):
sys.stdout.write(c)
f.write(c)
or
import subprocess
import sys
# replace "w" with "wb" for Python 3
with open("test.log", "w") as f:
process = subprocess.Popen(your_command, stdout=subprocess.PIPE)
# replace "" with b"" for Python 3
for line in iter(process.stdout.readline, ""):
sys.stdout.write(line)
f.write(line)
Or you can create a reader and a writer file. Pass the writer to the Popen and read from the reader
import io
import time
import subprocess
import sys
filename = "test.log"
with io.open(filename, "wb") as writer, io.open(filename, "rb", 1) as reader:
process = subprocess.Popen(command, stdout=writer)
while process.poll() is None:
sys.stdout.write(reader.read())
time.sleep(0.5)
# Read the remaining
sys.stdout.write(reader.read())
This way you will have the data written in the test.log as well as on the standard output.
The only advantage of the file approach is that your code doesn't block. So you can do whatever you want in the meantime and read whenever you want from the reader in a non-blocking way. When you use PIPE, read and readline functions will block until either one character is written to the pipe or a line is written to the pipe respectively.

Executive Summary (or "tl;dr" version): it's easy when there's at most one subprocess.PIPE, otherwise it's hard.
It may be time to explain a bit about how subprocess.Popen does its thing.
(Caveat: this is for Python 2.x, although 3.x is similar; and I'm quite fuzzy on the Windows variant. I understand the POSIX stuff much better.)
The Popen function needs to deal with zero-to-three I/O streams, somewhat simultaneously. These are denoted stdin, stdout, and stderr as usual.
You can provide:
None, indicating that you don't want to redirect the stream. It will inherit these as usual instead. Note that on POSIX systems, at least, this does not mean it will use Python's sys.stdout, just Python's actual stdout; see demo at end.
An int value. This is a "raw" file descriptor (in POSIX at least). (Side note: PIPE and STDOUT are actually ints internally, but are "impossible" descriptors, -1 and -2.)
A stream—really, any object with a fileno method. Popen will find the descriptor for that stream, using stream.fileno(), and then proceed as for an int value.
subprocess.PIPE, indicating that Python should create a pipe.
subprocess.STDOUT (for stderr only): tell Python to use the same descriptor as for stdout. This only makes sense if you provided a (non-None) value for stdout, and even then, it is only needed if you set stdout=subprocess.PIPE. (Otherwise you can just provide the same argument you provided for stdout, e.g., Popen(..., stdout=stream, stderr=stream).)
The easiest cases (no pipes)
If you redirect nothing (leave all three as the default None value or supply explicit None), Pipe has it quite easy. It just needs to spin off the subprocess and let it run. Or, if you redirect to a non-PIPE—an int or a stream's fileno()—it's still easy, as the OS does all the work. Python just needs to spin off the subprocess, connecting its stdin, stdout, and/or stderr to the provided file descriptors.
The still-easy case: one pipe
If you redirect only one stream, Pipe still has things pretty easy. Let's pick one stream at a time and watch.
Suppose you want to supply some stdin, but let stdout and stderr go un-redirected, or go to a file descriptor. As the parent process, your Python program simply needs to use write() to send data down the pipe. You can do this yourself, e.g.:
proc = subprocess.Popen(cmd, stdin=subprocess.PIPE)
proc.stdin.write('here, have some data\n') # etc
or you can pass the stdin data to proc.communicate(), which then does the stdin.write shown above. There is no output coming back so communicate() has only one other real job: it also closes the pipe for you. (If you don't call proc.communicate() you must call proc.stdin.close() to close the pipe, so that the subprocess knows there is no more data coming through.)
Suppose you want to capture stdout but leave stdin and stderr alone. Again, it's easy: just call proc.stdout.read() (or equivalent) until there is no more output. Since proc.stdout() is a normal Python I/O stream you can use all the normal constructs on it, like:
for line in proc.stdout:
or, again, you can use proc.communicate(), which simply does the read() for you.
If you want to capture only stderr, it works the same as with stdout.
There's one more trick before things get hard. Suppose you want to capture stdout, and also capture stderr but on the same pipe as stdout:
proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
In this case, subprocess "cheats"! Well, it has to do this, so it's not really cheating: it starts the subprocess with both its stdout and its stderr directed into the (single) pipe-descriptor that feeds back to its parent (Python) process. On the parent side, there's again only a single pipe-descriptor for reading the output. All the "stderr" output shows up in proc.stdout, and if you call proc.communicate(), the stderr result (second value in the tuple) will be None, not a string.
The hard cases: two or more pipes
The problems all come about when you want to use at least two pipes. In fact, the subprocess code itself has this bit:
def communicate(self, input=None):
...
# Optimization: If we are only using one pipe, or no pipe at
# all, using select() or threads is unnecessary.
if [self.stdin, self.stdout, self.stderr].count(None) >= 2:
But, alas, here we've made at least two, and maybe three, different pipes, so the count(None) returns either 1 or 0. We must do things the hard way.
On Windows, this uses threading.Thread to accumulate results for self.stdout and self.stderr, and has the parent thread deliver self.stdin input data (and then close the pipe).
On POSIX, this uses poll if available, otherwise select, to accumulate output and deliver stdin input. All this runs in the (single) parent process/thread.
Threads or poll/select are needed here to avoid deadlock. Suppose, for instance, that we've redirected all three streams to three separate pipes. Suppose further that there's a small limit on how much data can be stuffed into to a pipe before the writing process is suspended, waiting for the reading process to "clean out" the pipe from the other end. Let's set that small limit to a single byte, just for illustration. (This is in fact how things work, except that the limit is much bigger than one byte.)
If the parent (Python) process tries to write several bytes—say, 'go\n'to proc.stdin, the first byte goes in and then the second causes the Python process to suspend, waiting for the subprocess to read the first byte, emptying the pipe.
Meanwhile, suppose the subprocess decides to print a friendly "Hello! Don't Panic!" greeting. The H goes into its stdout pipe, but the e causes it to suspend, waiting for its parent to read that H, emptying the stdout pipe.
Now we're stuck: the Python process is asleep, waiting to finish saying "go", and the subprocess is also asleep, waiting to finish saying "Hello! Don't Panic!".
The subprocess.Popen code avoids this problem with threading-or-select/poll. When bytes can go over the pipes, they go. When they can't, only a thread (not the whole process) has to sleep—or, in the case of select/poll, the Python process waits simultaneously for "can write" or "data available", writes to the process's stdin only when there is room, and reads its stdout and/or stderr only when data are ready. The proc.communicate() code (actually _communicate where the hairy cases are handled) returns once all stdin data (if any) have been sent and all stdout and/or stderr data have been accumulated.
If you want to read both stdout and stderr on two different pipes (regardless of any stdin redirection), you will need to avoid deadlock too. The deadlock scenario here is different—it occurs when the subprocess writes something long to stderr while you're pulling data from stdout, or vice versa—but it's still there.
The Demo
I promised to demonstrate that, un-redirected, Python subprocesses write to the underlying stdout, not sys.stdout. So, here is some code:
from cStringIO import StringIO
import os
import subprocess
import sys
def show1():
print 'start show1'
save = sys.stdout
sys.stdout = StringIO()
print 'sys.stdout being buffered'
proc = subprocess.Popen(['echo', 'hello'])
proc.wait()
in_stdout = sys.stdout.getvalue()
sys.stdout = save
print 'in buffer:', in_stdout
def show2():
print 'start show2'
save = sys.stdout
sys.stdout = open(os.devnull, 'w')
print 'after redirect sys.stdout'
proc = subprocess.Popen(['echo', 'hello'])
proc.wait()
sys.stdout = save
show1()
show2()
When run:
$ python out.py
start show1
hello
in buffer: sys.stdout being buffered
start show2
hello
Note that the first routine will fail if you add stdout=sys.stdout, as a StringIO object has no fileno. The second will omit the hello if you add stdout=sys.stdout since sys.stdout has been redirected to os.devnull.
(If you redirect Python's file-descriptor-1, the subprocess will follow that redirection. The open(os.devnull, 'w') call produces a stream whose fileno() is greater than 2.)

We can also use the default file iterator for reading stdout instead of using iter construct with readline().
import subprocess
import sys
process = subprocess.Popen(
your_command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT
)
for line in process.stdout:
sys.stdout.write(line)

In addition to all these answer, one simple approach could also be as follows:
process = subprocess.Popen(your_command, stdout=subprocess.PIPE)
while process.stdout.readable():
line = process.stdout.readline()
if not line:
break
print(line.strip())
Loop through the readable stream as long as it's readable and if it gets an empty result, stop.
The key here is that readline() returns a line (with \n at the end) as long as there's an output and empty if it's really at the end.
Hope this helps someone.

If you're able to use third-party libraries, You might be able to use something like sarge (disclosure: I'm its maintainer). This library allows non-blocking access to output streams from subprocesses - it's layered over the subprocess module.

If all you need is that the output will be visible on the console the easiest solution for me was to pass the following arguments to Popen
with Popen(cmd, stdout=sys.stdout, stderr=sys.stderr) as proc:
which will use your python scripts stdio file handles

Solution 1: Log stdout AND stderr concurrently in realtime
A simple solution which logs both stdout AND stderr concurrently, line-by-line in realtime into a log file.
import subprocess as sp
from concurrent.futures import ThreadPoolExecutor
def log_popen_pipe(p, stdfile):
with open("mylog.txt", "w") as f:
while p.poll() is None:
f.write(stdfile.readline())
f.flush()
# Write the rest from the buffer
f.write(stdfile.read())
with sp.Popen(["ls"], stdout=sp.PIPE, stderr=sp.PIPE, text=True) as p:
with ThreadPoolExecutor(2) as pool:
r1 = pool.submit(log_popen_pipe, p, p.stdout)
r2 = pool.submit(log_popen_pipe, p, p.stderr)
r1.result()
r2.result()
Solution 2: A function read_popen_pipes() that allows you to iterate over both pipes (stdout/stderr), concurrently in realtime
import subprocess as sp
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()
err_line = q_stderr.get_nowait()
except Empty:
pass
yield (out_line, err_line)
# The function in use:
with sp.Popen(["ls"], stdout=sp.PIPE, stderr=sp.PIPE, text=True) as p:
for out_line, err_line in read_popen_pipes(p):
print(out_line, end='')
print(err_line, end='')
p.poll()

Similar to previous answers but the following solution worked for me on windows using Python3 to provide a common method to print and log in realtime (source)
def print_and_log(command, logFile):
with open(logFile, 'wb') as f:
command = subprocess.Popen(command, stdout=subprocess.PIPE, shell=True)
while True:
output = command.stdout.readline()
if not output and command.poll() is not None:
f.close()
break
if output:
f.write(output)
print(str(output.strip(), 'utf-8'), flush=True)
return command.poll()

A good but "heavyweight" solution is to use Twisted - see the bottom.
If you're willing to live with only stdout something along those lines should work:
import subprocess
import sys
popenobj = subprocess.Popen(["ls", "-Rl"], stdout=subprocess.PIPE)
while not popenobj.poll():
stdoutdata = popenobj.stdout.readline()
if stdoutdata:
sys.stdout.write(stdoutdata)
else:
break
print "Return code", popenobj.returncode
(If you use read() it tries to read the entire "file" which isn't useful, what we really could use here is something that reads all the data that's in the pipe right now)
One might also try to approach this with threading, e.g.:
import subprocess
import sys
import threading
popenobj = subprocess.Popen("ls", stdout=subprocess.PIPE, shell=True)
def stdoutprocess(o):
while True:
stdoutdata = o.stdout.readline()
if stdoutdata:
sys.stdout.write(stdoutdata)
else:
break
t = threading.Thread(target=stdoutprocess, args=(popenobj,))
t.start()
popenobj.wait()
t.join()
print "Return code", popenobj.returncode
Now we could potentially add stderr as well by having two threads.
Note however the subprocess docs discourage using these files directly and recommends to use communicate() (mostly concerned with deadlocks which I think isn't an issue above) and the solutions are a little klunky so it really seems like the subprocess module isn't quite up to the job (also see: http://www.python.org/dev/peps/pep-3145/ ) and we need to look at something else.
A more involved solution is to use Twisted as shown here: https://twistedmatrix.com/documents/11.1.0/core/howto/process.html
The way you do this with Twisted is to create your process using reactor.spawnprocess() and providing a ProcessProtocol that then processes output asynchronously. The Twisted sample Python code is here: https://twistedmatrix.com/documents/11.1.0/core/howto/listings/process/process.py

Based on all the above I suggest a slightly modified version (python3):
while loop calling readline (The iter solution suggested seemed to block forever for me - Python 3, Windows 7)
structered so handling of read data does not need to be duplicated after poll returns not-None
stderr piped into stdout so both output outputs are read
Added code to get exit value of cmd.
Code:
import subprocess
proc = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE,
stderr=subprocess.STDOUT, universal_newlines=True)
while True:
rd = proc.stdout.readline()
print(rd, end='') # and whatever you want to do...
if not rd: # EOF
returncode = proc.poll()
if returncode is not None:
break
time.sleep(0.1) # cmd closed stdout, but not exited yet
# You may want to check on ReturnCode here

I found a simple solution to a much complicated problem.
Both stdout and stderr need to be streamed.
Both of them need to be non-blocking: when there is no output and when there are too much output.
Do not want to use Threading or multiprocessing, also not willing to use pexpect.
This solution uses a gist I found here
import subprocess as sbp
import fcntl
import os
def non_block_read(output):
fd = output.fileno()
fl = fcntl.fcntl(fd, fcntl.F_GETFL)
fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
try:
return output.readline()
except:
return ""
with sbp.Popen('find / -name fdsfjdlsjf',
shell=True,
universal_newlines=True,
encoding='utf-8',
bufsize=1,
stdout=sbp.PIPE,
stderr=sbp.PIPE) as p:
while True:
out = non_block_read(p.stdout)
err = non_block_read(p.stderr)
if out:
print(out, end='')
if err:
print('E: ' + err, end='')
if p.poll() is not None:
break

It looks like line-buffered output will work for you, in which case something like the following might suit. (Caveat: it's untested.) This will only give the subprocess's stdout in real time. If you want to have both stderr and stdout in real time, you'll have to do something more complex with select.
proc = subprocess.Popen(run_command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
while proc.poll() is None:
line = proc.stdout.readline()
print line
log_file.write(line + '\n')
# Might still be data on stdout at this point. Grab any
# remainder.
for line in proc.stdout.read().split('\n'):
print line
log_file.write(line + '\n')
# Do whatever you want with proc.stderr here...

Why not set stdout directly to sys.stdout? And if you need to output to a log as well, then you can simply override the write method of f.
import sys
import subprocess
class SuperFile(open.__class__):
def write(self, data):
sys.stdout.write(data)
super(SuperFile, self).write(data)
f = SuperFile("log.txt","w+")
process = subprocess.Popen(command, stdout=f, stderr=f)

All of the above solutions I tried failed either to separate stderr and stdout output, (multiple pipes) or blocked forever when the OS pipe buffer was full which happens when the command you are running outputs too fast (there is a warning for this on python poll() manual of subprocess). The only reliable way I found was through select, but this is a posix-only solution:
import subprocess
import sys
import os
import select
# returns command exit status, stdout text, stderr text
# rtoutput: show realtime output while running
def run_script(cmd,rtoutput=0):
p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
poller = select.poll()
poller.register(p.stdout, select.POLLIN)
poller.register(p.stderr, select.POLLIN)
coutput=''
cerror=''
fdhup={}
fdhup[p.stdout.fileno()]=0
fdhup[p.stderr.fileno()]=0
while sum(fdhup.values()) < len(fdhup):
try:
r = poller.poll(1)
except select.error, err:
if err.args[0] != EINTR:
raise
r=[]
for fd, flags in r:
if flags & (select.POLLIN | select.POLLPRI):
c = os.read(fd, 1024)
if rtoutput:
sys.stdout.write(c)
sys.stdout.flush()
if fd == p.stderr.fileno():
cerror+=c
else:
coutput+=c
else:
fdhup[fd]=1
return p.poll(), coutput.strip(), cerror.strip()

None of the Pythonic solutions worked for me.
It turned out that proc.stdout.read() or similar may block forever.
Therefore, I use tee like this:
subprocess.run('./my_long_running_binary 2>&1 | tee -a my_log_file.txt && exit ${PIPESTATUS}', shell=True, check=True, executable='/bin/bash')
This solution is convenient if you are already using shell=True.
${PIPESTATUS} captures the success status of the entire command chain (only available in Bash).
If I omitted the && exit ${PIPESTATUS}, then this would always return zero since tee never fails.
unbuffer might be necessary for printing each line immediately into the terminal, instead of waiting way too long until the "pipe buffer" gets filled.
However, unbuffer swallows the exit status of assert (SIG Abort)...
2>&1 also logs stderror to the file.

I think that the subprocess.communicate method is a bit misleading: it actually fills the stdout and stderr that you specify in the subprocess.Popen.
Yet, reading from the subprocess.PIPE that you can provide to the subprocess.Popen's stdout and stderr parameters will eventually fill up OS pipe buffers and deadlock your app (especially if you've multiple processes/threads that must use subprocess).
My proposed solution is to provide the stdout and stderr with files - and read the files' content instead of reading from the deadlocking PIPE. These files can be tempfile.NamedTemporaryFile() - which can also be accessed for reading while they're being written into by subprocess.communicate.
Below is a sample usage:
try:
with ProcessRunner(
("python", "task.py"), env=os.environ.copy(), seconds_to_wait=0.01
) as process_runner:
for out in process_runner:
print(out)
except ProcessError as e:
print(e.error_message)
raise
And this is the source code which is ready to be used with as many comments as I could provide to explain what it does:
If you're using python 2, please make sure to first install the latest version of the subprocess32 package from pypi.
import os
import sys
import threading
import time
import tempfile
import logging
if os.name == 'posix' and sys.version_info[0] < 3:
# Support python 2
import subprocess32 as subprocess
else:
# Get latest and greatest from python 3
import subprocess
logger = logging.getLogger(__name__)
class ProcessError(Exception):
"""Base exception for errors related to running the process"""
class ProcessTimeout(ProcessError):
"""Error that will be raised when the process execution will exceed a timeout"""
class ProcessRunner(object):
def __init__(self, args, env=None, timeout=None, bufsize=-1, seconds_to_wait=0.25, **kwargs):
"""
Constructor facade to subprocess.Popen that receives parameters which are more specifically required for the
Process Runner. This is a class that should be used as a context manager - and that provides an iterator
for reading captured output from subprocess.communicate in near realtime.
Example usage:
try:
with ProcessRunner(('python', task_file_path), env=os.environ.copy(), seconds_to_wait=0.01) as process_runner:
for out in process_runner:
print(out)
except ProcessError as e:
print(e.error_message)
raise
:param args: same as subprocess.Popen
:param env: same as subprocess.Popen
:param timeout: same as subprocess.communicate
:param bufsize: same as subprocess.Popen
:param seconds_to_wait: time to wait between each readline from the temporary file
:param kwargs: same as subprocess.Popen
"""
self._seconds_to_wait = seconds_to_wait
self._process_has_timed_out = False
self._timeout = timeout
self._process_done = False
self._std_file_handle = tempfile.NamedTemporaryFile()
self._process = subprocess.Popen(args, env=env, bufsize=bufsize,
stdout=self._std_file_handle, stderr=self._std_file_handle, **kwargs)
self._thread = threading.Thread(target=self._run_process)
self._thread.daemon = True
def __enter__(self):
self._thread.start()
return self
def __exit__(self, exc_type, exc_val, exc_tb):
self._thread.join()
self._std_file_handle.close()
def __iter__(self):
# read all output from stdout file that subprocess.communicate fills
with open(self._std_file_handle.name, 'r') as stdout:
# while process is alive, keep reading data
while not self._process_done:
out = stdout.readline()
out_without_trailing_whitespaces = out.rstrip()
if out_without_trailing_whitespaces:
# yield stdout data without trailing \n
yield out_without_trailing_whitespaces
else:
# if there is nothing to read, then please wait a tiny little bit
time.sleep(self._seconds_to_wait)
# this is a hack: terraform seems to write to buffer after process has finished
out = stdout.read()
if out:
yield out
if self._process_has_timed_out:
raise ProcessTimeout('Process has timed out')
if self._process.returncode != 0:
raise ProcessError('Process has failed')
def _run_process(self):
try:
# Start gathering information (stdout and stderr) from the opened process
self._process.communicate(timeout=self._timeout)
# Graceful termination of the opened process
self._process.terminate()
except subprocess.TimeoutExpired:
self._process_has_timed_out = True
# Force termination of the opened process
self._process.kill()
self._process_done = True
#property
def return_code(self):
return self._process.returncode

Here is a class which I'm using in one of my projects. It redirects output of a subprocess to the log. At first I tried simply overwriting the write-method but that doesn't work as the subprocess will never call it (redirection happens on filedescriptor level). So I'm using my own pipe, similar to how it's done in the subprocess-module. This has the advantage of encapsulating all logging/printing logic in the adapter and you can simply pass instances of the logger to Popen: subprocess.Popen("/path/to/binary", stderr = LogAdapter("foo"))
class LogAdapter(threading.Thread):
def __init__(self, logname, level = logging.INFO):
super().__init__()
self.log = logging.getLogger(logname)
self.readpipe, self.writepipe = os.pipe()
logFunctions = {
logging.DEBUG: self.log.debug,
logging.INFO: self.log.info,
logging.WARN: self.log.warn,
logging.ERROR: self.log.warn,
}
try:
self.logFunction = logFunctions[level]
except KeyError:
self.logFunction = self.log.info
def fileno(self):
#when fileno is called this indicates the subprocess is about to fork => start thread
self.start()
return self.writepipe
def finished(self):
"""If the write-filedescriptor is not closed this thread will
prevent the whole program from exiting. You can use this method
to clean up after the subprocess has terminated."""
os.close(self.writepipe)
def run(self):
inputFile = os.fdopen(self.readpipe)
while True:
line = inputFile.readline()
if len(line) == 0:
#no new data was added
break
self.logFunction(line.strip())
If you don't need logging but simply want to use print() you can obviously remove large portions of the code and keep the class shorter. You could also expand it by an __enter__ and __exit__ method and call finished in __exit__ so that you could easily use it as context.

import os
def execute(cmd, callback):
for line in iter(os.popen(cmd).readline, ''):
callback(line[:-1])
execute('ls -a', print)

Had the same problem and worked out a simple and clean solution using process.sdtout.read1() which works perfectly for my needs in python3.
Here is a demo using the ping command (requires internet connection):
from subprocess import Popen, PIPE
cmd = "ping 8.8.8.8"
proc = Popen([cmd], shell=True, stdout=PIPE)
while True:
print(proc.stdout.read1())
Every second or so a new line is printed in the python console as the ping command reports its data in real time.

In my view "live output from subprocess command" means that both stdout and stderr should be live. And stdin should also be delivered to the subprocess.
The fragment below produces live output on stdout and stderr and also captures them as bytes in outcome.{stdout,stderr}.
The trick involves proper use of select and poll.
Works well for me on Python 3.9.
if self.log == 1:
print(f"** cmnd= {fullCmndStr}")
self.outcome.stdcmnd = fullCmndStr
try:
process = subprocess.Popen(
fullCmndStr,
shell=True,
encoding='utf8',
executable="/bin/bash",
stdin=subprocess.PIPE,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE,
)
except OSError:
self.outcome.error = OSError
else:
process.stdin.write(stdin)
process.stdin.close() # type: ignore
stdoutStrFile = io.StringIO("")
stderrStrFile = io.StringIO("")
pollStdout = select.poll()
pollStderr = select.poll()
pollStdout.register(process.stdout, select.POLLIN)
pollStderr.register(process.stderr, select.POLLIN)
stdoutEOF = False
stderrEOF = False
while True:
stdoutActivity = pollStdout.poll(0)
if stdoutActivity:
c= process.stdout.read(1)
if c:
stdoutStrFile.write(c)
if self.log == 1:
sys.stdout.write(c)
else:
stdoutEOF = True
stderrActivity = pollStderr.poll(0)
if stderrActivity:
c= process.stderr.read(1)
if c:
stderrStrFile.write(c)
if self.log == 1:
sys.stderr.write(c)
else:
stderrEOF = True
if stdoutEOF and stderrEOF:
break
if self.log == 1:
print(f"** cmnd={fullCmndStr}")
process.wait() # type: ignore
self.outcome.stdout = stdoutStrFile.getvalue()
self.outcome.stderr = stderrStrFile.getvalue()
self.outcome.error = process.returncode # type: ignore

The only way i've found how to read a subprocess' output in a streaming fashion (while also capturing it in a variable) in Python (for multiple output streams, i.e. both stdout and stderr) is by passing the subprocess a named temporary file to write to and then opening the same temporary file in a separate reading handle.
Note: this is for Python 3
stdout_write = tempfile.NamedTemporaryFile()
stdout_read = io.open(stdout_write.name, "r")
stderr_write = tempfile.NamedTemporaryFile()
stderr_read = io.open(stderr_write.name, "r")
stdout_captured = ""
stderr_captured = ""
proc = subprocess.Popen(["command"], stdout=stdout_write, stderr=stderr_write)
while True:
proc_done: bool = cli_process.poll() is not None
while True:
content = stdout_read.read(1024)
sys.stdout.write(content)
stdout_captured += content
if len(content) < 1024:
break
while True:
content = stderr_read.read(1024)
sys.stderr.write(content)
stdout_captured += content
if len(content) < 1024:
break
if proc_done:
break
time.sleep(0.1)
stdout_write.close()
stdout_read.close()
stderr_write.close()
stderr_read.close()
However, if you don't need to capture the output, then you can simply pass sys.stdout and sys.stderr streams from your Python script to the called subprocess, as xaav suggested in his answer :
subprocess.Popen(["command"], stdout=sys.stdout, stderr=sys.stderr)

How do I get 'real-time' information back from a subprocess.Popen in python (2.5)

I'd like to use the subprocess module in the following way:
create a new process that potentially takes a long time to execute.
capture stdout (or stderr, or potentially both, either together or separately)
Process data from the subprocess as it comes in, perhaps firing events on every line received (in wxPython say) or simply printing them out for now.
I've created processes with Popen, but if I use communicate() the data comes at me all at once, once the process has terminated.
If I create a separate thread that does a blocking readline() of myprocess.stdout (using stdout = subprocess.PIPE) I don't get any lines with this method either, until the process terminates. (no matter what I set as bufsize)
Is there a way to deal with this that isn't horrendous, and works well on multiple platforms?

Update with code that appears not to work (on windows anyway)
class ThreadWorker(threading.Thread):
def __init__(self, callable, *args, **kwargs):
super(ThreadWorker, self).__init__()
self.callable = callable
self.args = args
self.kwargs = kwargs
self.setDaemon(True)
def run(self):
try:
self.callable(*self.args, **self.kwargs)
except wx.PyDeadObjectError:
pass
except Exception, e:
print e
if __name__ == "__main__":
import os
from subprocess import Popen, PIPE
def worker(pipe):
while True:
line = pipe.readline()
if line == '': break
else: print line
proc = Popen("python subprocess_test.py", shell=True, stdin=PIPE, stdout=PIPE, stderr=PIPE)
stdout_worker = ThreadWorker(worker, proc.stdout)
stderr_worker = ThreadWorker(worker, proc.stderr)
stdout_worker.start()
stderr_worker.start()
while True: pass

stdout will be buffered - so you won't get anything till that buffer is filled, or the subprocess exits.
You can try flushing stdout from the sub-process, or using stderr, or changing stdout on non-buffered mode.

It sounds like the issue might be the use of buffered output by the subprocess - if a relatively small amount of output is created, it could be buffered until the subprocess exits. Some background can be found here:

Here's what worked for me:
cmd = ["./tester_script.bash"]
p = subprocess.Popen( cmd, shell=False, stdout=subprocess.PIPE, stderr=subprocess.PIPE )
while p.poll() is None:
out = p.stdout.readline()
do_something_with( out, err )
In your case you could try to pass a reference to the sub-process to your Worker Thread, and do the polling inside the thread. I don't know how it will behave when two threads poll (and interact with) the same subprocess, but it may work.
Also note thate the while p.poll() is None: is intended as is. Do not replace it with while not p.poll() as in python 0 (the returncode for successful termination) is also considered False.

I've been running into this problem as well. The problem occurs because you are trying to read stderr as well. If there are no errors, then trying to read from stderr would block.
On Windows, there is no easy way to poll() file descriptors (only Winsock sockets).
So a solution is not to try and read from stderr.

Using pexpect [http://www.noah.org/wiki/Pexpect] with non-blocking readlines will resolve this problem. It stems from the fact that pipes are buffered, and so your app's output is getting buffered by the pipe, therefore you can't get to that output until the buffer fills or the process dies.

This seems to be a well-known Python limitation, see
PEP 3145 and maybe others.

Read one character at a time: http://blog.thelinuxkid.com/2013/06/get-python-subprocess-output-without.html
import contextlib
import subprocess
# Unix, Windows and old Macintosh end-of-line
newlines = ['\n', '\r\n', '\r']
def unbuffered(proc, stream='stdout'):
stream = getattr(proc, stream)
with contextlib.closing(stream):
while True:
out = []
last = stream.read(1)
# Don't loop forever
if last == '' and proc.poll() is not None:
break
while last not in newlines:
# Don't loop forever
if last == '' and proc.poll() is not None:
break
out.append(last)
last = stream.read(1)
out = ''.join(out)
yield out
def example():
cmd = ['ls', '-l', '/']
proc = subprocess.Popen(
cmd,
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT,
# Make all end-of-lines '\n'
universal_newlines=True,
)
for line in unbuffered(proc):
print line
example()

Using subprocess.Popen, I can run the .exe of one of my C# projects and redirect the output to my Python file. I am able now to print() all the information being output to the C# console (using Console.WriteLine()) to the Python console.
Python code:
from subprocess import Popen, PIPE, STDOUT
p = Popen('ConsoleDataImporter.exe', stdout = PIPE, stderr = STDOUT, shell = True)
while True:
line = p.stdout.readline()
print(line)
if not line:
break
This gets the console output of my .NET project line by line as it is created and breaks out of the enclosing while loop upon the project's termination. I'd imagine this would work for two python files as well.

I've used the pexpect module for this, it seems to work ok. http://sourceforge.net/projects/pexpect/