Reading stdout of child process unbuffered

Question

I'm trying to read the output of a Python script launched by Node.js as it arrives. However, I only get access to the data once the process has finished.

var proc, args;

args = [
    './bin/build_map.py',
    '--min_lon',
    opts.sw.lng,
    '--max_lon',
    opts.ne.lng,
    '--min_lat',
    opts.sw.lat,
    '--max_lat',
    opts.ne.lat,
    '--city',
    opts.city
];

proc = spawn('python', args);

proc.stdout.on('data', function (buf) {
    console.log(buf.toString());
    socket.emit('map-creation-response', buf.toString());
});

If I launch the process with { stdio : 'inherit' } I can see the output as it happens directly in the console. But doing something like process.stdout.on('data', ...) will not work.

How do I make sure I can read the output from the child process as it arrives and direct it somewhere else?

Answer 1

The process doing the buffering, because it knows the terminal was redirected and not really going to the terminal, is python. You can easily tell Python not to do this buffering: Just run "python -u" instead of "python". Should be easy as that.