How to replicate tee behavior in Python when using subprocess?

Question

I'm looking for a Python solution that will allow me to save the output of a command in a file without hiding it from the console.

FYI: I'm asking about tee (as the Unix command line utility) and not the function with the same name from Python intertools module.

Details

• Python solution (not calling tee, it is not available under Windows)
• I do not need to provide any input to stdin for called process
• I have no control over the called program. All I know is that it will output something to stdout and stderr and return with an exit code.
• To work when calling external programs (subprocess)
• To work for both stderr and stdout
• Being able to differentiate between stdout and stderr because I may want to display only one of the to the console or I could try to output stderr using a different color - this means that stderr = subprocess.STDOUT will not work.
• Live output (progressive) - the process can run for a long time, and I'm not able to wait for it to finish.
• Python 3 compatible code (important)

References

Here are some incomplete solutions I found so far:

• http://devlishgenius.blogspot.com/2008/10/logging-in-real-time-in-python.html (mkfifo works only on Unix)
• http://blog.kagesenshi.org/2008/02/teeing-python-subprocesspopen-output.html (doesn't work at all)

Diagram http://blog.i18n.ro/wp-content/uploads/2010/06/Drawing_tee_py.png

Current code (second try)

#!/usr/bin/python
from __future__ import print_function

import sys, os, time, subprocess, io, threading
cmd = "python -E test_output.py"

from threading import Thread
class StreamThread ( Thread ):
    def __init__(self, buffer):
        Thread.__init__(self)
        self.buffer = buffer
    def run ( self ):
        while 1:
            line = self.buffer.readline()
            print(line,end="")
            sys.stdout.flush()
            if line == '':
                break

proc = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdoutThread = StreamThread(io.TextIOWrapper(proc.stdout))
stderrThread = StreamThread(io.TextIOWrapper(proc.stderr))
stdoutThread.start()
stderrThread.start()
proc.communicate()
stdoutThread.join()
stderrThread.join()

print("--done--")

#### test_output.py ####

#!/usr/bin/python
from __future__ import print_function
import sys, os, time

for i in range(0, 10):
    if i%2:
        print("stderr %s" % i, file=sys.stderr)
    else:
        print("stdout %s" % i, file=sys.stdout)
    time.sleep(0.1)

Real output

stderr 1
stdout 0
stderr 3
stdout 2
stderr 5
stdout 4
stderr 7
stdout 6
stderr 9
stdout 8
--done--

Expected output was to have the lines ordered. Remark, modifying the Popen to use only one PIPE is not allowed because in the real life I will want to do different things with stderr and stdout.

Also even in the second case I was not able to obtain real-time like out, in fact all the results were received when the process finished. By default, Popen should use no buffers (bufsize=0).

Answer 1

I see that this is a rather old post but just in case someone is still searching for a way to do this:

proc = subprocess.Popen(["ping", "localhost"], 
                        stdout=subprocess.PIPE, 
                        stderr=subprocess.PIPE)

with open("logfile.txt", "w") as log_file:
  while proc.poll() is None:
     line = proc.stderr.readline()
     if line:
        print "err: " + line.strip()
        log_file.write(line)
     line = proc.stdout.readline()
     if line:
        print "out: " + line.strip()
        log_file.write(line)