ChildProcess: stdin not getting read by Python readline

Question

I am trying to interact with a Python script from Node via stdin/out using child_process like so:

var p = require('child_process').spawn('python', ['test_io.py']);

p.stdout.on('data', function(data) {
  console.log(data.toString());
});

p.stdin.write('thing');

and this is the relevant Python portion:

import io
import sys

_input = io.open(sys.stdin.fileno())
_output = io.open(sys.stdout.fileno(), 'w')

while True:
  _output.write(_input.readline())

However, right now it seems the Python script is not reading "thing" passed in via stdin.write . Shouldn't these writes be buffering? What am I doing wrong here.

Thanks in advance.

Answer 1

try this

var p = require('child_process').spawn('python', ['test_io.py'], { stdio: 'pipe'});

ChildProcess has some idiosyncracies -- from the docs for ChildProcess.stdin:

If the child was not spawned with stdio[0] set to 'pipe', then this will not be set.

https://nodejs.org/api/child_process.html#child_process_child_stdin

Answer 2

In addition to @lispHK01 answer You need to terminate the stdin - particularly when sending small amounts of data as it will be sitting in a buffer.

Try this:

var p = require('child_process').spawn('python', ['test_io.py'], { stdio: 'pipe'});

p.stdin.write('thing');
p.stdin.end();