Similar to subprocess.check_output, but returns the results of stdout and stderr separately.

cmd_out_err.py

def check_out_and_err(*args, **kwargs):
    """
    Pipe from BOTH stdout and stderr. Avoids deadlock by performing nonblocking
    reads. Why this isn't in the stdlib is beyond me.

    """
    if 'stdout' in kwargs or 'stderr' in kwargs:
        raise ValueError("stdout/stderr arguments not allowed.")

    with nested(pipe("r"), pipe("r")) as ((rout, wout), (rerr, werr)):
        process = subprocess.Popen(stdout=wout, stderr=werr, *args, **kwargs)

        output = {rout: [], rerr: []}
        prev = ""
        while process.poll() is None:
            ready, _, _ = select.select([rout, rerr], [], [], 0.2)
            for fd in ready:
                output[fd].append(os.read(fd, 1024))

        output = (''.join(output[rout]), ''.join(output[rerr]))

        code = process.poll()
        if code:
            cmd = kwargs.get("args")
            if cmd is None:
                cmd = args[0]
            raise subprocess.CalledProcessError(code, cmd, output=output)

        return output