Task management program in Python


I made a simple program that stores tasks in the file and allows user to manage them. I’m interested if my implementation is efficient and could I do something better. I’m not sure whether loading all data from file to memory and again saving it in file is a good way to implement list saving tasks.

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import os


CHOICE_LIST_TASKS = 1
CHOICE_ADD_TASK = 2
CHOICE_REMOVE_TASK = 3
CHOICE_QUIT = 4


def read_int(prompt='> ', errmsg='Invalid number!'):
    num = None
    while num is None:
        try:
            num = int(input(prompt))
        except ValueError:
            print(errmsg)
    return num


def display_menu():
    print('What do you want to do?')
    print('(1) List all tasks')
    print('(2) Add a new task')
    print('(3) Delete task')
    print('(4) Quit')


choice = None
file = None
if not os.path.exists('tasks'):
    file = open('tasks', 'w+')
else:
    file = open('tasks', 'r+')
tasks = ()

for line in file.readlines():
    tasks.append(line.strip())

while choice != CHOICE_QUIT:
    display_menu()
    choice = read_int()
    if choice == CHOICE_LIST_TASKS:
        if len(tasks) == 0:
            print('Task list is empty!')
        else:
            print('Tasks:')
            for x in tasks:
                print(x)
    elif choice == CHOICE_ADD_TASK:
        desc = input('Which task do you want to do? ')
        index = tasks.append(desc)
        print('Successfully added a new task!')
    elif choice == CHOICE_REMOVE_TASK:
        if len(tasks) == 0:
            print('Task list is empty!')
        else:
            for x in tasks:
                print(tasks.index(x), x)
            index = read_int('Which task do you want to delete? ')
            try:
                tasks.pop(index)
                print('Successfully deleted task!')
            except IndexError:
                print('Please enter proper task number')
    elif choice == CHOICE_QUIT:
        print('Good bye!')
    else:
        print('Invalid choice!')

file = open('tasks', 'w+')
for x in tasks:
    file.write(x + 'n')