Nodejs child_process spawn calling python script w

Question

I'm testing on nodejs child_process module to read the stdout from my python script. However, I noticed when my python emit message between every second. The nodejs callback can only collect the stdout at the end of the python script ends.

Python Script

 import time
 for i in range(0,5):
   ····print i
   ····time.sleep(0.5)

and the nodejs script is

var cp = require('child_process');
var spw = cp.spawn('python', ['tql.py']),
    str = "";
spw.stdout.on('data', function (data) {
    str+= data;
    console.log(data);
});

spw.on('close', function (code) {
    console.log(str);
});

spw.stderr.on('data', function (data) {
    console.log('stderr: ' + data);
});

Instead of emit message each second, my program only calls the callback when the python script ends. Is there anything I need to know in order to achieve my goal?