nodejs: Using child process in aws lambda to run python script not working

Question

Using the serverless framework and this post as guide I would like to print out some info from a python script, I have got this relevant code in my handler.js

const path = await require('path')
const { spawn } = await require('child_process')

/**
 * Run python script, pass in `-u` to not buffer console output 
 * @return {ChildProcess}
 */
runScript() {
    return spawn('python', [
        "-u",
        path.join(__dirname, 'script.py'),
        "--foo", "some value for foo",
    ]);
}
const subprocess = await runScript()

// print output of script
await subprocess.stdout.on('data', (data) => {
    console.log(`data:${data}`);
});
await subprocess.stderr.on('data', (data) => {
    console.log(`error:${data}`);
});
await subprocess.stderr.on('close', () => {
    console.log("Closed");
});

and I also have script.py

#!/usr/bin/python
import sys, getopt, time

def main(argv):
    argument = ''
    usage = 'usage: script.py -f <sometext>'

    # parse incoming arguments
    try:
        opts, args = getopt.getopt(argv,"hf:",["foo="])
    except getopt.GetoptError:
        print(usage)
        sys.exit(2)
    for opt, arg in opts:
        if opt == '-h':
            print(usage)
            sys.exit()
        elif opt in ("-f", "--foo"):
            argument = arg

    # print output
    print("Start : %s" % time.ctime())
    time.sleep( 5 )
    print('Foo is')
    time.sleep( 5 )
    print(argument)
    print("End : %s" % time.ctime())

if __name__ == "__main__":
    main(sys.argv[1:])

I am expecting my console to show output or an error when I invoke handler function but it prints nothing

Erez · Accepted Answer · 2019-08-21 16:19:16Z

2

This is the way to do it:

const path = require("path");
const { spawn } = require("child_process");

const runScript = () => {
  return spawn("python", [
    "-u",
    path.join(__dirname, "script.py"),
    "--foo",
    "some value for foo"
  ]);
};

const promise = new Promise((resolve, reject) => {
  const subprocess = runScript();

  subprocess.stdout.on("data", data => {
    console.log(`data:${data}`);
  });

  subprocess.stderr.on("data", data => {
    console.log(`error:${data}`);
  });

  subprocess.on("close", code => {
    if (code !== 0) {
      reject(code);
    } else {
      resolve(code);
    }
  });
});

await promise;

No need to use await for synchronous functions, and by wrapping subprocess with a promise you can await on it.

answered Aug 21, 2019 at 16:19

Erez

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2 Comments

imlearningcode Over a year ago

Thanks that worked great! but I've tried returning data after 'subprocess.stdout.on("data", data => {' but instead it returns '0'

Erez Over a year ago

Can you please post your new code? Maybe in a new question. If you use return data inside the stdout.on callback it returns the data to the caller which is the spawn internal code. You can do

let allData = '';  subprocess.stdout.on("data", data => { allData = allData + data.toString();  }); resolve({ code, allData });

or something similar

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