Приложение на flask

Всем, привет. Я новичок в программировании. Написал приложение на flask (онлайн калькулятор). Столкнулся с проблемой, даже не знаю как правильно она называется. Приложение работает только для одного пользователя. Другой пользователь, если зайдёт одновременно, видит результаты запросов первого. Что это вообще такое и как можно это устранить, т. е. чтобы разные пользователи одновременно могли корректно пользоваться сервисом.

from collections import namedtuple
from flask import Flask, render_template, redirect, url_for, request
import xlrd


app = Flask(__name__)


rb_comfort = xlrd.open_workbook("/home/serjio/flask_app/Price_Comfort.xlsx")
sheet_comfort = rb_comfort.sheet_by_index(0)
vals_comfort = [sheet_comfort.row_values(rownum) for rownum in range(sheet_comfort.nrows)]


rb_prest = xlrd.open_workbook("/home/serjio/flask_app/Price_Prestige.xlsx")
sheet_prest = rb_prest.sheet_by_index(0)
vals_prest = [sheet_prest.row_values(rownum) for rownum in range(sheet_prest.nrows)]


Message = namedtuple('Message', 'modul cat cost value')
messages = []
a = []


divans_comfort = []
for names in range(6,283):
    x = vals_comfort[names][1]
    if x not in divans_comfort:
        divans_comfort.append(x)

divans_prest = []
for names1 in range(6,500):
    x1 = vals_prest[names1][1]
    if x1 not in divans_prest:
        divans_prest.append(x1)


@app.route('/', methods=['GET'])
def hello_world():
    return render_template('index.html')


@app.route('/overmain', methods=['GET'])
def overmain():
    coll = request.args.get('coll')
    if coll == 'index.html':
        return render_template('first.html')
    else:
        return render_template('first1.html')


@app.route('/main', methods=['GET', 'POST'])
def main():
    global name, possibile_moduls
    possibile_moduls = []
    sofa_name = request.args.get('sofa_name')
    a.append(sofa_name)
    if sofa_name != None:
        name = sofa_name
    if name in divans_comfort:
        for i in range(len(vals_comfort)):
            for j in range(50):
                if vals_comfort[i][1] == name:
                    possibile_moduls.append(vals_comfort[i][2])
                break
    elif name in divans_prest:
        for i in range(len(vals_prest)):
            for j in range(50):
                if vals_prest[i][1] == name:
                    possibile_moduls.append(vals_prest[i][2])
                break
    return render_template('main.html', messages=messages, name=name, possibile_moduls=possibile_moduls)


@app.route('/add_message', methods=['POST'])
def add_message():
    modul = request.form['modul']
    cat = request.form['cat']
    divan = a[0]
    cost = 0
    value = 0
    modul = modul.split()
    modul_dict = {}
    for ds in range(len(modul)):
        if modul[ds] not in modul_dict:
            modul_dict[modul[ds]] = 1
        else:
            modul_dict[modul[ds]] += 1
    if divan in divans_comfort:
        for i in range(len(vals_comfort)):
            for j in range(21):
                if vals_comfort[i][1] == divan:
                    for obj in modul_dict:
                        if obj == vals_comfort[i][2]:
                            cost += int(vals_comfort[i][2 + int(cat)]) * modul_dict[obj]
                            value += vals_comfort[i][3] * modul_dict[obj]

                break
    elif divan in divans_prest:
        for i in range(len(vals_prest)):
            for j in range(21):
                if vals_prest[i][1] == divan:
                    for obj in modul_dict:
                        if obj == vals_prest[i][2]:
                            cost += int(vals_prest[i][3 + int(cat)]) * modul_dict[obj]
                            value += vals_prest[i][3] * modul_dict[obj]
                break
    cost = str(cost)
    messages.append(Message(modul, cat, cost, value))
    return redirect(url_for('main'))


@app.route('/del_message', methods=['POST'])
def del_message():
    messages.clear()
    return redirect(url_for('main'))


@app.route('/return', methods=['POST'])
def return_():
    messages.clear()
    a.clear()
    return render_template('index.html')


if __name__ == '__main__':
    app.run(debug=True)