Build a Command-Line To-Do List Application with Data Persistence in Python

import json
import os

TODO_FILE = "todos.json"

def load_todos():
    if not os.path.exists(TODO_FILE):
        return []
    with open(TODO_FILE, "r") as f:
        return json.load(f)

def save_todos(todos):
    with open(TODO_FILE, "w") as f:
        json.dump(todos, f, indent=2)

def show_todos(todos):
    if not todos:
        print("No todos yet.")
        return
    for i, todo in enumerate(todos, 1):
        status = "[x]" if todo["done"] else "[ ]"
        print(f"{i}. {status} {todo['task']}")

def add_todo(todos, task):
    todos.append({"task": task, "done": False})

def toggle_todo(todos, index):
    if 1 <= index <= len(todos):
        todos[index - 1]["done"] = not todos[index - 1]["done"]

if __name__ == "__main__":
    todos = load_todos()
    while True:
        print("\nCommands: show, add <task>, done <num>, quit")
        cmd = input("> ").strip()
        if cmd == "quit":
            save_todos(todos)
            break
        elif cmd == "show":
            show_todos(todos)
        elif cmd.startswith("add "):
            add_todo(todos, cmd[4:])
        elif cmd.startswith("done "):
            try:
                toggle_todo(todos, int(cmd.split()[1]))
            except (ValueError, IndexError):
                print("Invalid number.")
        else:
            print("Unknown command.")