How can I print and display subprocess stdout and stderr output without distortion?

Make the pipes non-blocking by using fcntl.fcntl, and use select.select to wait for data to become available in either pipe. For example:

# Helper function to add the O_NONBLOCK flag to a file descriptor
def make_async(fd):
    fcntl.fcntl(fd, fcntl.F_SETFL, fcntl.fcntl(fd, fcntl.F_GETFL) | os.O_NONBLOCK)

# Helper function to read some data from a file descriptor, ignoring EAGAIN errors
def read_async(fd):
    try:
        return fd.read()
    except IOError, e:
        if e.errno != errno.EAGAIN:
            raise e
        else:
            return ''

process = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
make_async(process.stdout)
make_async(process.stderr)

stdout = str()
stderr = str()
returnCode = None

while True:
    # Wait for data to become available 
    select.select([process.stdout, process.stderr], [], [])

    # Try reading some data from each
    stdoutPiece = read_async(process.stdout)
    stderrPiece = read_async(process.stderr)

    if stdoutPiece:
        print stdoutPiece,
    if stderrPiece:
        print stderrPiece,

    stdout += stdoutPiece
    stderr += stderrPiece
    returnCode = process.poll()

    if returnCode != None:
        return (returnCode, stdout, stderr)

Note that fcntl is only available on Unix-like platforms, including Cygwin.

If you need it to work on Windows without Cygwin, it’s doable, but it’s much, much tougher. You’ll have to:

• Use the pywin32 library to call through to the native Win32 API
• Use SetNamedPipeHandleState with PIPE_NOWAIT to make the stdout and stderr pipes non-blocking
• Use WaitForMultipleObjects instead of select to wait for data to become available
• Use ReadFile to read the data