How to replicate tee behavior in Python when using subprocess?

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)

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).

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)

If requiring python 3.6 isn't an issue there is now a way of doing this using asyncio. This method allows you to capture stdout and stderr separately but still have both stream to the tty without using threads. Here's a rough outline:

class RunOutput:
    def __init__(self, returncode, stdout, stderr):
        self.returncode = returncode
        self.stdout = stdout
        self.stderr = stderr


async def _read_stream(stream, callback):
    while True:
        line = await stream.readline()
        if line:
            callback(line)
        else:
            break


async def _stream_subprocess(cmd, stdin=None, quiet=False, echo=False) -> RunOutput:
    if isWindows():
        platform_settings = {"env": os.environ}
    else:
        platform_settings = {"executable": "/bin/bash"}
    if echo:
        print(cmd)
    p = await asyncio.create_subprocess_shell(
        cmd,
        stdin=stdin,
        stdout=asyncio.subprocess.PIPE,
        stderr=asyncio.subprocess.PIPE,
        **platform_settings
    )
    out = []
    err = []

    def tee(line, sink, pipe, label=""):
        line = line.decode("utf-8").rstrip()
        sink.append(line)
        if not quiet:
            print(label, line, file=pipe)

    await asyncio.wait(
        [
            _read_stream(p.stdout, lambda l: tee(l, out, sys.stdout)),
            _read_stream(p.stderr, lambda l: tee(l, err, sys.stderr, label="ERR:")),
        ]
    )

    return RunOutput(await p.wait(), out, err)


def run(cmd, stdin=None, quiet=False, echo=False) -> RunOutput:
    loop = asyncio.get_event_loop()
    result = loop.run_until_complete(
        _stream_subprocess(cmd, stdin=stdin, quiet=quiet, echo=echo)
    )

    return result

The code above was based on this blog post: https://kevinmccarthy.org/2016/07/25/streaming-subprocess-stdin-and-stdout-with-asyncio-in-python/

This is a straightforward port of tee(1) to Python.

import sys

sinks = sys.argv[1:]
sinks = [open(sink, "w") for sink in sinks]
sinks.append(sys.stderr)
while True:
    input = sys.stdin.read(1024)
    if input:
        for sink in sinks:
            sink.write(input)
    else:
        break

I'm running on Linux right now but this ought to work on most platforms.

---

Now for the subprocess part, I don't know how you want to 'wire' the subprocess's stdin, stdout and stderr to your stdin, stdout, stderr and file sinks, but I know you can do this:

import subprocess

callee = subprocess.Popen(
    ["python", "-i"],
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
)

Now you can access callee.stdin, callee.stdout and callee.stderr like normal files, enabling the above "solution" to work. If you want to get the callee.returncode, you'll need to make an extra call to callee.poll().

Be careful with writing to callee.stdin: if the process has exited when you do that, an error may be rised (on Linux, I get IOError: [Errno 32] Broken pipe).

This is how it can be done

import sys
from subprocess import Popen, PIPE

with open('log.log', 'w') as log:
    proc = Popen(["ping", "google.com"], stdout=PIPE, encoding='utf-8')
    while proc.poll() is None:
        text = proc.stdout.readline() 
        log.write(text)
        sys.stdout.write(text)

If you don't want to interact with the process you can use the subprocess module just fine.

Example:

tester.py

import os
import sys

for file in os.listdir('.'):
    print file

sys.stderr.write("Oh noes, a shrubbery!")
sys.stderr.flush()
sys.stderr.close()

testing.py

import subprocess

p = subprocess.Popen(['python', 'tester.py'], stdout=subprocess.PIPE,
                     stdin=subprocess.PIPE, stderr=subprocess.PIPE)

stdout, stderr = p.communicate()
print stdout, stderr

In your situation you can simply write stdout/stderr to a file first. You can send arguments to your process with communicate as well, though I wasn't able to figure out how to continually interact with the subprocess.

My solution isn't elegant, but it works.

You can use powershell to gain access to "tee" under WinOS.

import subprocess
import sys

cmd = ['powershell', 'ping', 'google.com', '|', 'tee', '-a', 'log.txt']

if 'darwin' in sys.platform:
    cmd.remove('powershell')

p = subprocess.Popen(cmd)
p.wait()