Reading stdout of child process unbuffered

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?

from Reading stdout of child process unbuffered