Displaying subprocess output to stdout and redirecting it

To save subprocess’ stdout to a variable for further processing and to display it while the child process is running as it arrives:

#!/usr/bin/env python3
from io import StringIO
from subprocess import Popen, PIPE

with Popen('/path/to/script', stdout=PIPE, bufsize=1,
           universal_newlines=True) as p, StringIO() as buf:
    for line in p.stdout:
        print(line, end='')
        buf.write(line)
    output = buf.getvalue()
rc = p.returncode

To save both subprocess’s stdout and stderr is more complex because you should consume both streams concurrently to avoid a deadlock:

stdout_buf, stderr_buf = StringIO(), StringIO()
rc =  teed_call('/path/to/script', stdout=stdout_buf, stderr=stderr_buf,
                universal_newlines=True)
output = stdout_buf.getvalue()
...

where teed_call() is define here.

---

Update: here’s a simpler asyncio version.

---

^{Old version:}

Here’s a single-threaded solution based on child_process.py example from tulip:

import asyncio
import sys
from asyncio.subprocess import PIPE

@asyncio.coroutine
def read_and_display(*cmd):
    """Read cmd's stdout, stderr while displaying them as they arrive."""
    # start process
    process = yield from asyncio.create_subprocess_exec(*cmd,
            stdout=PIPE, stderr=PIPE)

    # read child's stdout/stderr concurrently
    stdout, stderr = [], [] # stderr, stdout buffers
    tasks = {
        asyncio.Task(process.stdout.readline()): (
            stdout, process.stdout, sys.stdout.buffer),
        asyncio.Task(process.stderr.readline()): (
            stderr, process.stderr, sys.stderr.buffer)}
    while tasks:
        done, pending = yield from asyncio.wait(tasks,
                return_when=asyncio.FIRST_COMPLETED)
        assert done
        for future in done:
            buf, stream, display = tasks.pop(future)
            line = future.result()
            if line: # not EOF
                buf.append(line)    # save for later
                display.write(line) # display in terminal
                # schedule to read the next line
                tasks[asyncio.Task(stream.readline())] = buf, stream, display

    # wait for the process to exit
    rc = yield from process.wait()
    return rc, b''.join(stdout), b''.join(stderr)

The script runs '/path/to/script command and reads line by line both its stdout&stderr concurrently. The lines are printed to parent’s stdout/stderr correspondingly and saved as bytestrings for future processing. To run the read_and_display() coroutine, we need an event loop:

import os

if os.name == 'nt':
    loop = asyncio.ProactorEventLoop() # for subprocess' pipes on Windows
    asyncio.set_event_loop(loop)
else:
    loop = asyncio.get_event_loop()
try:
    rc, *output = loop.run_until_complete(read_and_display("/path/to/script"))
    if rc:
        sys.exit("child failed with '{}' exit code".format(rc))
finally:
    loop.close()