How to run a Python script in Node.js synchronously?

Question

I am running the following Python script in Node.js through python-shell:

import sys
import time
 x=0
 completeData = "";
 while x<800:
  crgb =  ""+x;
  print crgb
  completeData = completeData + crgb + "@";
  time.sleep(.0001)
  x = x+1
  file = open("sensorData.txt", "w")
  file.write(completeData)
  file.close()
  sys.stdout.flush()
else:
 print "Device not found\n"

And my corresponding Node.js code is:

var PythonShell = require('python-shell');

PythonShell.run('sensor.py', function (err) {
    if (err) throw err;
    console.log('finished');
});
console.log ("Now reading data");

Output is:

Now reading data
finished

But expected output is:

finished 
Now reading data

Node.js can not execute my Python script synchronously, it executes first all the code following the PythonShell.run function then executes PythonShell.run. How can I execute first PythonShell.run then the following code? Any help will be mostly appreciated... It is an emergency please!

The main design goal of node.js is to run task asynchronously. Can you please elaborate why it has to be asynchronously? — Klaus D.
– Klaus D., Commented Aug 2, 2016 at 19:54
Actually, I am using a color detector sensor. The color detection is done by the python script. And based on the detected color I have to do some other calculations which are done using node.js. I am working with a big project and the color detection is the part of my tasks and it is very much easy to detect the color using python but other calculations need to be done using node.js — Ramzan
– Ramzan, Commented Aug 2, 2016 at 20:06

Diarex · Accepted Answer · 2021-01-30 09:50:17Z

It work for me:

let {PythonShell} = require('python-shell');

var options = {
    mode:           'text',
    pythonPath:     'python',
    pythonOptions:  [],
    scriptPath:     '',
    args:           []
};

async function runTest()
{
    const { success, err = '', results } = await new Promise(
        (resolve, reject) =>
        {
            PythonShell.run('hello.py', options,
                function (err, results)
                {
                    if (err)
                    {
                        reject({ success: false, err });
                    }

                    console.log('PythonShell results: %j', results);

                    resolve({ success: true, results });
                }
            );
        }
    );

    console.log("python call ends");

    if (! success)
    {
        console.log("Test Error: " + err);
        return;
    }

    console.log("The result is: " + results);

    // My code here

    console.log("end runTest()");
}

console.log('start ...');

runTest();

console.log('... end main');

The result is:

start ...
... end main
PythonShell results: ["Hello World!"]
python call ends
The result is: Hello World!
end runTest()

David Buck · Accepted Answer · 2020-09-27 14:35:12Z

1

I had the same problem and resolved it by making a promise with the PythonShell.run method inside it. Then, you only have to await that promise and your code runs synchronously.

The promisify method:

RunPythonScript: function(scriptPath, args, pythonFile){
  let options = {
    mode: 'text',
    pythonPath: 'python',
    pythonOptions: [], 
    scriptPath: scriptPath,
    args: args,
  };

  return new Promise((resolve,reject) =>{
    try{
      PythonShell.run(pythonFile, options, function(err, results) {
        if (err) {console.log(err);}
        // results is an array consisting of messages collected during execution
        console.log('results', results);
        resolve();          
      }); 
    }
    catch{
      console.log('error running python code')
      reject();
    }
  })
},

Then, you await the promise:

await RunPythonScript(YOURscriptPath,YOURargsScript,YOURpythonFile);

edited Sep 27, 2020 at 14:35

David Buck

3,87840 gold badges54 silver badges73 bronze badges

answered Sep 27, 2020 at 14:17

Enrique Ruiz

112 bronze badges

1 Comment

jboxxx Over a year ago

this is great. util.promisify could condense this a bit too!

Jean-François Fabre · Accepted Answer · 2016-08-02 20:08:48Z

0

As this is asynchronous, add an end callback (found in the documentation) instead of instructions at top level

// end the input stream and allow the process to exit 
pyshell.end(function (err) {
  if (err) throw err;
  console.log ("Now reading data");
});

answered Aug 2, 2016 at 20:08

Jean-François Fabre♦

141k24 gold badges179 silver badges246 bronze badges

2 Comments

Ramzan Over a year ago

I have tried this : If I have anymore code following 'pyshell.end(){}' function then the program execute first the following code then execute 'pyshell.end(){}' . But I need that 'pyshell.end(){}' need to be executed first then the following code

Jean-François Fabre Over a year ago

I think you misunderstood what Klaus D. stated at first: this is an asynchronous system. Your "following code" must be in the pyshell.end callback. Nothing must block the main program. It's just when you add a button in a graphical interface. You have to connect a callback to respond when someone clicks on it, but the main loop remains free.

Tarzi · Accepted Answer · 2020-05-25 22:00:23Z

-1

export default {
    data () {
            return {
            }
        },
        method: {
            callfunction () {
              console.log ("Now reading data");
            },
            pyth (){
              var PythonShell = require('python-shell');
              var vm = this; //save the object (this) in variable vm to use inside pythonshell.run
              PythonShell.run('sensor.py', function (err) {
                  if (err) throw err;
                  console.log('finished');
                  vm.callfunction();
              });
              
            }
        }
}

answered May 25, 2020 at 22:00

Tarzi

11 bronze badge

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