How to replicate tee behavior in Python when using subprocess?

Question

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 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.