Force Node.js to flush writes to child processes

Question

I spawn a child process like this:

var child = require('child_process');
var proc = child.spawn('python', ['my_script.py', '-p', 'example']);

I also set some data handling:

proc.stdin.setEncoding('utf8');
proc.stdout.setEncoding('utf8');
proc.stderr.setEncoding('utf8');

proc.stdout.on('data', function (data) {
  console.log('out: ' + data);
});

proc.stderr.on('data', function (data) {
  console.log('err: ' + data);
});

proc.on('close', function (code) {
  console.log('subprocess exited with status ' + code);
  proc.stdin.end();
});

My Python script reads lines from stdin and for each line does some operations and prints to stdout. It works fine in the shell (I write a line and I get the output immediately) but when I do this in Node:

for (var i = 0; i < 10; i++) {
  proc.stdin.write('THIS IS A TEST\n');
}

I get nothing.

I got to fix it calling proc.stdin.end() but that also terminates the child process (which I want to stay in background, streaming data).

I also triggered a flush filling the buffer with lots of writes, but that's not really an option.

Is there any way to manually flush the stream?

Answer 1

You are not flushing the output from Python after print statement. I had similar problem and @Alfe answered my question. Take a look at this:

Stream child process output in flowing mode