todo_manager.py

import json
import os


class Task:
    """Represents a single task in the to-do list."""

    def __init__(self, title, completed=False):
        self.title = title
        self.completed = completed

    def to_dict(self):
        """Convert the task into a dictionary for JSON storage."""
        return {
            "title": self.title,
            "completed": self.completed
        }

    @classmethod
    def from_dict(cls, data):
        """Create a Task object from stored JSON data."""
        return cls(
            data["title"],
            data.get("completed", False)
        )


class TodoManager:
    """Manages tasks and handles saving/loading task data."""

    def __init__(self, filename="tasks.json"):
        self.filename = filename
        self.tasks = []
        self.load_tasks()

    def add_task(self, title):
        """Add a new task to the list."""
        title = title.strip()

        if not title:
            raise ValueError("Task description cannot be empty.")

        self.tasks.append(Task(title))
        self.save_tasks()

    def view_tasks(self):
        """Return all tasks."""
        return self.tasks

    def complete_task(self, task_number):
        """Mark a selected task as completed."""
        index = task_number - 1

        if index < 0 or index >= len(self.tasks):
            raise IndexError("Invalid task number.")

        self.tasks[index].completed = True
        self.save_tasks()

    def remove_task(self, task_number):
        """Remove a selected task."""
        index = task_number - 1

        if index < 0 or index >= len(self.tasks):
            raise IndexError("Invalid task number.")

        removed_task = self.tasks.pop(index)
        self.save_tasks()

        return removed_task

    def save_tasks(self):
        """Save all tasks to a JSON file."""
        with open(self.filename, "w", encoding="utf-8") as file:
            json.dump(
                [task.to_dict() for task in self.tasks],
                file,
                indent=4
            )

    def load_tasks(self):
        """Load tasks from the JSON file when the application starts."""
        if not os.path.exists(self.filename):
            return

        try:
            with open(self.filename, "r", encoding="utf-8") as file:
                data = json.load(file)

            self.tasks = [Task.from_dict(item) for item in data]

        except (json.JSONDecodeError, KeyError, TypeError):
            # If the storage file is damaged, start with an empty list
            # instead of crashing the entire application.
            self.tasks = []