Executing a python script from NodeJS

Question

I'm trying to execute a machine learning script from a node.js application, using the child-process core module as explained here

However, I can't get a simple output from script.stdout.on.

I'm using Node v12.5.0 and python 3.7.3

I made a sample "hello world" script and JS function to test (shown below), and ran the script from command line just fine.

These are my scripts:

test.py

import sys

print('hello world')
sys.stdout.flush()

JavaScript test function

  const { spawn } = require("child_process");
  const pytest = spawn("python", ["./path/to/test.py"]);
  console.log("firstPrint");

  pytest.stdout.on("data", data => {
    console.log("secondPrint");
    console.log(data);
  });

  pytest.stdout.on("close", code => {
    console.log(`script closed: ${code}`);
  });

My console output is:

firstPrint
script closed: false

Although it should be:

firstPrint
secondPrint
hello world
script closed: {code}

Chances are the script errored out for some reasons (e.g., wrong PATH). I'f add an on('close') call to pytest` and double check the script executed properly. — Mureinik
– Mureinik, Commented Jul 21, 2019 at 9:04
Add pytest.stderr.on('data', (data) => { console.log(`stderr: ${data}`); }); to see if there are errors. — dfsq
– dfsq, Commented Jul 21, 2019 at 9:09
@dfsq Can't open file or directory error. I tried moving the file to the same directory and using "./test.py", "/test.py" and "test.py", still can't open it. — J_mesh
– J_mesh, Commented Jul 21, 2019 at 9:13
Check files permissions, what uses and group owns the files? — dfsq
– dfsq, Commented Jul 21, 2019 at 9:18

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

What ended up being the problem for me:

As suggested by @dfsq I added a pytest.stderr.on callback and received a "can't open file" error.

Node always executes the "spawn" command from the root folder of the project, so in my case I had to manually add "./src" to the path even though the script executing the "spawn" command was inside /src.

The script which executes JS function is inside /src

Old path (didn't work): "./test.py"

New path (works): "./src/test.py"

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 21, 2019 at 9:54

J_mesh

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