Update main.py

from flask import Flask

from flask import request

import threading

import time

app = Flask(__name__)

queue = []

def processQueue():

    while True:

        if (len(queue) == 0):

            time.sleep(1)

            print("no messages, checking in 1 seconds")

        else:

            with open('/dev/usb/lp0', 'w') as printer:

                printer.write(queue[0] + '\n')

                print(queue[0])

            queue.pop(0)

@app.route('/msg', methods=['POST'])

def postmsg():

    queue.append(request.form.get('message'))

    return "posted!\n"

def server():

    app.run(host='0.0.0.0')

threading.Thread(target=processQueue).start()

threading.Thread(target=server).start()