Like check_output, but also tees the output to a file object specified by the stdout kwarg. Passing in stdout=sys.stdout allows you to run the process with normal output (like check_call()) but also get the output as the return value (like check_output()).

check_teed_output.py

from subprocess import Popen, PIPE, CalledProcessError


def check_teed_output(*popenargs, **kwargs):
    """
    Like check_output, but also tees the output to a file object specified
    by the stdout kwarg. Passing in stdout=sys.stdout allows you to run the
    process with normal output (like check_call()) but also get the output
    as the return value (like check_output()).
    """
    stdout = kwargs.pop('stdout', None)
    if stdout is None:
        raise ValueError('stdout argument is required')

    process = Popen(stdout=PIPE, *popenargs, **kwargs)
    if [process.stdin, process.stdout, process.stderr].count(None) < 2:
        # As currently implemented, if more than one pipe is present, the call
        # to readline below could deadlock (see warnings in the subprocess docs)
        # That use-case should be pretty rare, but if needed, this could be
        # modified to use select or poll
        raise ValueError("check_streamed() doesn't support more than one pipe")

    lines = []
    for line in iter(process.stdout.readline, ''):
        stdout.write(line)
        stdout.flush()
        lines.append(line)
    output = ''.join(lines)

    if process.wait():
        cmd = kwargs.get("args")
        if cmd is None:
            cmd = popenargs[0]
        raise CalledProcessError(process.returncode, cmd, output=output)
    return output