How to invoke external scripts/programs from node.js

Question

I have a C++ program and a Python script that I want to incorporate into my node.js web app.

I want to use them to parse the files that are uploaded to my site; it may take a few seconds to process, so I would avoid to block the app as well.

How can I just accept the file then just run the C++ program and script in a sub-process from a node.js controller?

Is your C++ program being called from within Python or are these two separate calls you will be making? 1) To Python and 2) to C++ app? Or does Python code load up and call your C++ app? — Steve, Jan 07 '14 at 13:29
Node's `child_process` stuff will run processes async. If all you want to do is launch a program from within node, that will do it. — Joe, Jan 07 '14 at 13:44
@StevenLeggett The python script and C++ app do not interact at all; I want to call them both separately (they don't need to be in any particular order either). — alh, Jan 07 '14 at 14:27
if you can execute the c++ app from the command line then my solution should work... you may want to look at the [async](https://github.com/caolan/async) library to help with control flow e.g. calling python then c++ in series — Plato, Jan 07 '14 at 14:38

score 40 · Accepted Answer · edited Feb 24 '16 at 20:30

40

see child_process. here is an example using spawn, which allows you to write to stdin and read from stderr/stdout as data is output. If you have no need to write to stdin and you can handle all output when the process completes, child_process.exec offers a slightly shorter syntax to execute a command.

// with express 3.x
var express = require('express'); 
var app = express();
app.use(express.logger('dev'));
app.use(express.bodyParser());
app.use(app.router);
app.post('/upload', function(req, res){
   if(req.files.myUpload){
     var python = require('child_process').spawn(
     'python',
     // second argument is array of parameters, e.g.:
     ["/home/me/pythonScript.py"
     , req.files.myUpload.path
     , req.files.myUpload.type]
     );
     var output = "";
     python.stdout.on('data', function(data){ output += data });
     python.on('close', function(code){ 
       if (code !== 0) {  
           return res.send(500, code); 
       }
       return res.send(200, output);
     });
   } else { res.send(500, 'No file found') }
});

require('http').createServer(app).listen(3000, function(){
  console.log('Listening on 3000');
});

edited Feb 24 '16 at 20:30

Ganesh Iyer

411
5
14

answered Jan 07 '14 at 13:37

Plato

10,812
2
41
61

How are types handled with this format? If I send 0.123 as a parameter, how will python treat it since I think it is coming in as a string? – Spencer Jun 12 '15 at 16:59
1

@spencer AFAIK URL components, as well as unix/windows shell commands, always use strings for everything so I consider it the python app's responsibility to cast it to a number. how would you tell python it's a number in a normal command line invocation? – Plato Jun 12 '15 at 17:05
1

you're right and that makes a lot of sense. I should have thought about it some more haha. Thanks! – Spencer Jun 12 '15 at 17:25
1

@spencer One thing that might help is to have the node app create a JSON object with your data, that will distinguish number vs. string. Then send it to the python app's stdin and parse it – Plato Jun 12 '15 at 20:20

score 1 · Answer 2 · answered Sep 23 '18 at 14:54

Might be a old question but some of these references will provide more details and different ways of including python in NodeJS.

There are multiple ways of doing this.

first way is by doing npm install python-shell

and here's the code

var PythonShell = require('python-shell');
//you can use error handling to see if there are any errors
PythonShell.run('my_script.py', options, function (err, results) { 
//your code

you can send a message to python shell using pyshell.send('hello');

you can find the API reference here- https://github.com/extrabacon/python-shell

second way - another package you can refer to is node python , you have to do npm install node-python
third way - you can refer to this question where you can find an example of using a child process- How to invoke external scripts/programs from node.js

a few more references - https://www.npmjs.com/package/python

if you want to use service-oriented architecture - http://ianhinsdale.com/code/2013/12/08/communicating-between-nodejs-and-python/

this works if I can spawn a python process from node, I already have a node server running and a python script that runs in daemon thread mode, I want to access a variable in python from node, how would ya do that — PirateApp, Feb 18 '19 at 12:18

How to invoke external scripts/programs from node.js

2 Answers2

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