Communication between node and python

I have a node script :

//start.js
var spawn = require('child_process').spawn,
    py    = spawn('python', ['compute_input.py']),
    data = [1,2,3,4,5,6,7,8,9],
    dataString = '';

py.stdout.on('data', function(data){
  dataString += data.toString();
});
py.stdout.on('end', function(){
  console.log('Sum of numbers=',dataString);
});
py.stdin.write(JSON.stringify(data));
py.stdin.end();

and a python script :

## compute_input.py

import sys, json, numpy as np

#Read data from stdin
def read_in():
    lines = sys.stdin.readlines()
    #Since our input would only be having one line, parse our JSON data from that
    return json.loads(lines[0])

def main():
    #get our data as an array from read_in()
    lines = read_in()

    #create a numpy array
    np_lines = np.array(lines)

    #use numpys sum method to find sum of all elements in the array
    lines_sum = np.sum(np_lines)

    #return the sum to the output stream
    print lines_sum

#start process
if __name__ == '__main__':
    main()

These two scripts are in a folder my_folder/

If I'm inside my_folder and run the command node start.js, I get Sum of number=45, the scripts is working.

If I'm outside the folder and run the command node my_folder/start.js, I get Sum of number=, the script is not working.

Why ??

via Rodolphe LAMPE