Creating NodeJS child processes asynchronously - node.js

Question

I am creating child processes in NodeJS in a function called "pythonGraphTools" in a for loop after generating some variables that need to be passed in. This for loop may run 50 times.
Then I am writing to the stdin of the spawned process. However, sometimes I am getting a "Error:EPIPE writing to closed socket" error for this line py.stdin.write(JSON.stringify(dotfilepath));
I suspect it is because the child process has not yet completed spawning and am attempting to write to it when it is not ready. I have seen asynchronous spawning such as that on https://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options but these only seem to have asynch events for the flow of data/messages from child to parent.
Any insight in how I might make sure that the child is fully spawned before I call py.stdin.write()
function pythonGraphTools(dotfilepath,allGraphsPerTrans,graphtools_color,graphtools_label,transHashArray){
var spawn = require('child_process').spawn,
py = spawn('python', ['python_module.py']);
//write file to disk temporarily.
console.log("dotfilepath is "+dotfilepath)
fs.writeFile(dotfilepath,allGraphsPerTrans, function(err){ //must create a file first //2nd param was res_str_dot_no_lbl
if(err){
console.log("there was an error writing to file" + err);
}
//now send this dot file path to the python module which will make the graph
console.log("now writing to python module!"+py.pid)
console.log("nodejs colorarray length for debuging "+ graphtools_color.length)
py.stdin.write(JSON.stringify(dotfilepath)); //sending data to the python process!
py.stdin.write("\n")
py.stdin.write(JSON.stringify(graphtools_color)); // sending colours
py.stdin.write("\n")
py.stdin.write(JSON.stringify(graphtools_label));//sending opcodes
py.stdin.write("\n");
py.stdin.write(JSON.stringify(transHashArray));//sending opcodes
py.stdin.write("\n");
py.stdin.end();
});
var dataString=""; //variable to store return from python module
py.stdout.on('data', function(data){ // listen for data coming back from python!
dataString += data.toString();
});
py.stdout.on('end', function(){ //pythons stdout has finished - now do stuff
console.log(dataString); // print out everything collected from python stdout
//now delete temp dot file (with all dot files in it)
fs.stat(dotfilepath, function (err, stats) { //check first if there is a dot file
console.log(stats);//here we got all information of file in stats variable
if (err) {
return console.error(err);
}
fs.unlink(dotfilepath,function(err){ //actually deleting comment this functiont to not delete
if(err) return console.log(err);
console.log('file deleted successfully');
});//end unlink
});//end file stat
py.stdout.end();
}); // on python 'finish'
py.on('exit', function (code, signal) { //which process? add pid ?
console.log('child process '+py.pid +' exited with ' +
`code ${code} and signal ${signal}`);
});
}