how to run a python script from node js

Question

I have written a server in node.js on Raspberry Pi. I want to run a python script from it but it does not execute the line. On previous instances, I have run files but coming to this instance of socket.io I can't run the file! Though the console shows accurate output passed to it from the client but it fails to read the file. The socket is responsible for movement.

/*jshint esversion: 6 */
var express = require('express');
// library for executing system calls
const spawn = require('child_process').spawn;
var XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;
app = express();
server = require('http').createServer(app);
io = require('socket.io').listen(server);
var colour = require('colors');
var ip = require('ip');
var PortNumber = 3000;
var xPos = 0, yPos = 0;
server.listen(PortNumber);
app.use(express.static('public'));
io.sockets.on('connection', function (socket) {
 socket.emit('ip', {value: 'http://' + ip.address() + ':8081'});
 // otp generation socket
 socket.on('userEmail', function (data) {
 var userEmail = data.value;
 // system call to generate OTP
 const OTPgeneration = spawn('python', [__dirname + '/python/OTPgeneration.py', userEmail]);
 OTPgeneration.stdout.on('data', (data) => {
 console.log(`${data}`);
 });
 });
 // otp submit button socket
 socket.on('userOtp', function (data) {
 var userOtp = data.value;
 console.log(userOtp);
 // system call to generate OTP
 const otpValidate = spawn('python', [__dirname + '/python/OTPvalidation.py', userOtp]);
 otpValidate.stdout.on('data', (data) => {
 console.log(`${data}`);
 // Get last output from python file
 var lastChar = `${data}`.slice(-2);
 if (lastChar === '0\n') {
 console.log('Wrong OTP');
 }
 else io.sockets.emit('otpConformation', {value: 'confirm'}); //sends the confirmation to all connected clients
 });
 });
 // x y values socket
 socket.on('servoPosition', function (data) {
 servoPosition = data.value;
 // servoPosition[0] is x and servoPosition[1] is y
 console.log('Reveived X: ' + servoPosition[0] + ' Y: ' + servoPosition[1]);
 // system call for i2c comminication
 const i2cData = spawn('python', [__dirname + '/python/sendI2C.py', servoPosition[0], servoPosition[1]]);
 i2cData.stdout.on('data', (data) => {
 console.log(`${data}`);
 });
 });
 // movement socket
 socket.on('movement', function (data) {
 var m = data.value;
 console.log("Movement :" + m);
 const submitMove = spawn('python', [__dirname + '/python/move.py', m]);
 submitMove.stdout.on('data', (data) => {
 console.log(`${data}`);
 });
 });
});
console.log('Server Running @'.green + ip.address().green + ":3000".green);
function readTextFile(file) {
 var rawFile = new XMLHttpRequest();
 //var rawFile = new XMLHttpRequest();
 rawFile.open("GET", file, false);
 rawFile.onreadystatechange = function () {
 if(rawFile.readyState === 4) {
 if(rawFile.status === 200 || rawFile.status === 0) {
 allText = rawFile.responseText;
 //alert(allText);
 }
 }
 };
 rawFile.send(null);
}

• Matt: What do stderr and the exit code contain when the command fails?
• Gireesh Punathil: right - please add these sections as well, along with stdout callback: OTPgeneration.stderr.on('data', (data) => { console.log(${data}); }); OTPgeneration.on('close', (code) => { console.log(${code}); })
• pradumna: the above code works fine , as per requirement . the socket module doesnt work as expected.the node never excutes the python move.py script.even the console here at the movement sockets works fine..

Answer 1

While using Node's native "child_process" module is the most performant option, it can be tedious in getting it to work. A quick solution is to use the node-cmd module. Run npm i node-cmd --save to install it. Import it via var cmd = require('node-cmd'); at the top of your server file. Then,

var pyProcess = cmd.get('python YOURPYTHONSCRIPT.py',
 function(data, err, stderr) {
 if (!err) {
 console.log("data from python script " + data)
 } else {
 console.log("python script cmd error: " + err)
 }
 }
 );

Editing as needed. node-cmd has 2 methods which invoke it, get and run. run simply runs the provided CLI command whereas get provides the use of a callback allowing for better error handling and makes working between stdin, stdout, stderr much easier.

Answer 2

Consider using a package instead of dealing with spawning processes by your self. you can check this package on npm that i created- native-python

it provides a very simple and powerful way to run python functions from node might solve your problem.

import { runFunction } from '@guydev/native-python'
const example = async () => {
 const input = [1,[1,2,3],{'foo':'bar'}]
 const { error, data } = await runFunction('/path/to/file.py','hello_world', '/path/to/python', input)
 // error will be null if no error occured.
 if (error) {
 console.log('Error: ', error)
 }
 else {
 console.log('Success: ', data)
 // prints data or null if function has no return value
 }
}

python module

# module: file.py
def hello_world(a,b,c):
 print( type(a), a) 
 # <class 'int'>, 1
 print(type(b),b)
 # <class 'list'>, [1,2,3]
 print(type(c),c)
 # <class 'dict'>, {'foo':'bar'}

• starball: It looks like you are promoting your own work. Please see the instructions on self-promotion: "you must disclose your affiliation in your post."