How to get the result of a subprocess operation in node.js

Problem

I want to pass a command to a python child process and then get the result. I would use exec, but I want to keep the child process open so that I don't have to open it every time I do a new command. Here is my code that currently does nothing:

var connect = require('connect'),
    io = require("socket.io").listen(1032),
    util = require("util"),
    child = require('child_process'),
    python = child.spawn("python");

var app = connect()
    .use(connect.static(__dirname + '/www'))
    .use(connect.logger('dev'))
    .listen(3000);

io.sockets.on('connection', function (socket) {
    console.log("Socket " + socket.id + " opened");

    python.stdout.on('data', function (data) {
        console.log("computed", data.toString("utf-8"));
        socket.emit("python", { result : data.toString("utf-8") });
    });

    socket.on('python', function (data) {
        console.log("received data" + data.cmd);

        python.stdin.resume();

        python.stdin.write(data.cmd);

        python.stdin.end();
    });
});
Problem courtesy of: Adam

Solution

Does your python code contain any non-ascii characters?

This works fine:

var
    spawn = require('child_process').spawn,
    python  = spawn('python');

python.stdin.write('print ("a")');
python.stdin.end();

python.stdout.on('data', function (data) {
        console.log(data.toString());
});

But if I change the letter "a" to a russian letter "п" it stops working. Event not fired.

But it works perfect with node interpreter (with any utf8 characters).

var
    spawn = require('child_process').spawn,
    node  = spawn('node');

node.stdin.write('console.log("п");');
node.stdin.end();

node.stdout.on('data', function (data) {
        console.log(data.toString());
});

I think you need to ask about it in python section.

Solution courtesy of: Vadim Baryshev

Discussion

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