executing python script through nodejs

Question

I am trying to execute this piece of node js code which execute a python script. Through this code works fine. but the response "running" and "finished" are displayed on front end immediately. "finshed" has to be displayed once the execution of python scripts gets completed.

app.post('/execute', function(request, response){
 response.write("running");
 console.log("executing")
 var pyshell = new PythonShell('./python_codes/test.py')
 pyshell.on('message', function (message) {console.log(message);});
 pyshell.end(function (err) {if (err){throw err;};console.log('finished');});
 response.write("finished");
 response.end();
});

• Simon Fraser: PythonShell uses child_process.spawn, which is asynchronous - you're saying here "Run this python thing, and tell me when it's done by using the callback I've given you with .end, and while you're doing that I will carry on with my own stuff". I suspect you'll need to either wait for pyshell.terminated to show the right value, or pass the response into the .end callback
• Fredrick Brennan: @SimonFraser The full code isn't shown so it's unknown if the HTTP library supports that - it's possible (and even likely) that once the app.post callback is done running response.end() will be called implicitly.
• Simon Fraser: @FredrickBrennan Fair point :) I'm more python than node, so I was trying to work things out looking at PythonShell

Answer 1

You should add your response inside the callback function

app.post('/execute', function(request, response){
 response.setHeader('Connection', 'Transfer-Encoding'); 
 response.setHeader('Content-Type', 'text/html; charset=utf-8');
 response.write("running");
 console.log("executing")
 var pyshell = new PythonShell('./python_codes/test.py')
 pyshell.on('message', function (message) {console.log(message);});
 pyshell.end(function (err) {
 if (err){
 throw err;
 };
 console.log('finished');
 response.write("finished");
 response.end();
 });
});

• nishant kumar: I tried but with approach it is also not sending the "running" msg. all the msg are sent after execution test.py
• Ezzat: Add response.setHeader('Connection', 'Transfer-Encoding'); response.setHeader('Content-Type', 'text/html; charset=utf-8'); To allow browser to show the content immediately.

Answer 2

It happens because the PythonShell class is asynchronous. What your code is doing is creating a PythonShell object, storing it in variable pyshell, and then adding a few events to the pyshell object. It then directly continues to write "finished".

Because writing "finished" is not part of the callback for the end() function, it happens right away. I see at least three things you can do:

1. If the HTTP library you are using supports it, just add the response.write("finished"); response.end(); code to the pyshell.end callback.
2. Call Python using a library which supports pausing the current execution thread (or use execSync). This is bad practice, because it defeats the purpose of using a concurrent framework like node.js, but would work.
3. Use WebSockets (or socket.io, which works even if WebSockets aren't available, such as through CloudFlare) to transmit the "finished" message.