[FIXED] How to trigger a function after return statement in Flask

Issue

I have 2 functions.

1st function stores the data received in a list and 2nd function writes the data into a csv file.

I'm using Flask. Whenever a web service has been called it will store the data and send response to it, as soon as it sends response it triggers the 2nd function.

My Code:

from flask import Flask, flash, request, redirect, url_for, session
import json

app = Flask(__name__)

arr = []

@app.route("/test", methods=['GET','POST'])
def check():
    arr.append(request.form['a'])
    arr.append(request.form['b'])
    res = {'Status': True}
    return json.dumps(res)

def trigger():
    df = pd.DataFrame({'x': arr})
    df.to_csv("docs/xyz.csv", index=False)
    return 

Obviously the 2nd function is not called.

Is there a way to achieve this?

P.S: My real life problem is different where trigger function is time consuming and I don't want user to wait for it to finish execution.

Solution

One solution would be to have a background thread that will watch a queue. You put your csv data in the queue and the background thread will consume it. You can start such a thread before first request:

import threading
from multiprocessing import Queue

class CSVWriterThread(threading.Thread):
    def __init__(self, *args, **kwargs):
        threading.Thread.__init__(self, *args, **kwargs)
        self.input_queue = Queue()

    def send(self, item):
        self.input_queue.put(item)

    def close(self):
        self.input_queue.put(None)
        self.input_queue.join()

    def run(self):
        while True:
            csv_array = self.input_queue.get()
            if csv_array is None:
                break

            # Do something here ...
            df = pd.DataFrame({'x': csv_array})
            df.to_csv("docs/xyz.csv", index=False)


            self.input_queue.task_done()
            time.sleep(1)
        # Done
        self.input_queue.task_done()
        return

@app.before_first_request
def activate_job_monitor():
    thread = CSVWriterThread()
    app.csvwriter = thread
    thread.start()

And in your code put the message in the queue before returning:

@app.route("/test", methods=['GET','POST'])
def check():
    arr.append(request.form['a'])
    arr.append(request.form['b'])
    res = {'Status': True}
    app.csvwriter.send(arr)
    return json.dumps(res)

Answered By - mehdix