To-do manager class with JSON saving

Goal

Write a TodoList class with a Task dataclass: add tasks with priorities, complete and remove them, save the list to todos.json, load it into a new object with ids intact, list what is pending by priority, and search by word.

Why it matters

Every tutorial has a to-do app; this one is graded on what they skip: refusing bad input, raising the right error for a missing id, and not reusing ids after a reload. That is the difference between a class that works once and one that survives a restart.

What it proves

You can design a small class, use a dataclass, persist state as JSON and restore it without losing anything.

Steps · 6 checkpoints

Each step is one function in todo.py. In the workspace, press Check next to a step: it runs your file and tells you exactly what is still wrong.

1. 1Add tasks with ids. Write add(title, priority): refuse an empty title or an unknown priority with ValueError, store a Task under the next id (1, 2, 3...) and return that id.
2. 2Complete and remove. Write complete(task_id) and remove(task_id). Both raise KeyError for an id that does not exist, so a typo never fails silently.
3. 3Save to JSON. Write save(): write {"next_id": ..., "tasks": [...]} to self.path as JSON, one dict per task (dataclasses.asdict does the conversion).
4. 4Load it back. Write the load(path) classmethod: an empty list when the file is missing, otherwise rebuild every Task and restore next_id so ids never repeat after a reload.
5. 5What is still pending. Write pending(): the tasks that are not done, high before medium before low, and by id inside the same priority.
6. 6Search and the full demo. Write search(word): tasks whose title contains the word, ignoring case. Then press Run: the check reads the whole demo your program printed.

Requirements · rubric

Build against this exact list — it is the spec your published project is judged on.

• 01add() refuses empty titles and unknown priorities without using up an id
• 02complete() and remove() raise KeyError for an id that does not exist
• 03save() writes next_id and every task as JSON
• 04load() is a classmethod that restores tasks and next_id, and works with no file
• 05pending() orders high, medium, low, then by id; search() ignores case

Starter code

Copy this into a new file — or open it in a workspace as todo.py. The TODO parts are yours to fill.

"""
To-do manager: a class that saves itself to JSON.

A TodoList you can add to, tick off, save to todos.json and load back.
The demo in main() starts from a clean file every run, saves, then loads
the list into a brand-new object to prove the data survived.
Fill in one method per step, press Run, then press "Check step".
"""

import json
from dataclasses import asdict, dataclass
from pathlib import Path

PATH = Path("todos.json")
PRIORITIES = {"high": 0, "medium": 1, "low": 2}


@dataclass
class Task:
    id: int
    title: str
    priority: str = "medium"
    done: bool = False


class TodoList:
    def __init__(self, path: Path = PATH):
        self.path = Path(path)
        self.tasks: dict[int, Task] = {}
        self.next_id = 1

    def __len__(self) -> int:
        return len(self.tasks)

    def add(self, title: str, priority: str = "medium") -> int:
        # TODO: strip the title; ValueError if it is empty or if priority is
        # not a key of PRIORITIES. Create Task(self.next_id, title, priority),
        # store it in self.tasks under its id, bump self.next_id, return the id.
        raise NotImplementedError("step 1, add(): create a task with the next id")

    def complete(self, task_id: int) -> None:
        # TODO: mark the task done. KeyError if there is no such id.
        raise NotImplementedError("step 2, complete(): tick a task off")

    def remove(self, task_id: int) -> None:
        # TODO: delete the task. KeyError if there is no such id.
        raise NotImplementedError("step 2, remove(): delete a task")

    def save(self) -> None:
        # TODO: write {"next_id": ..., "tasks": [asdict(t), ...]} to self.path
        # as JSON. Hint: self.path.write_text(json.dumps(data, indent=2))
        raise NotImplementedError("step 3, save(): write the list to JSON")

    @classmethod
    def load(cls, path: Path = PATH) -> "TodoList":
        # TODO: build cls(path). If the file is missing, return it empty.
        # Otherwise read the JSON, rebuild each Task(**row) and restore next_id
        # (so ids never repeat after a reload).
        raise NotImplementedError("step 4, load(): rebuild the list from JSON")

    def pending(self) -> list[Task]:
        # TODO: tasks that are not done, high before medium before low, and
        # by id inside the same priority. Hint: key=lambda t: (PRIORITIES[t.priority], t.id)
        raise NotImplementedError("step 5, pending(): open tasks, most urgent first")

    def search(self, word: str) -> list[Task]:
        # TODO: tasks whose title contains word, ignoring upper/lower case.
        raise NotImplementedError("step 6, search(): find tasks by a word")


def main() -> None:
    if PATH.exists():
        PATH.unlink()  # the demo starts clean every run
    todo = TodoList(PATH)
    todo.add("Revise DSA: two pointers", "high")
    todo.add("Submit resume on placement portal", "high")
    todo.add("Buy notebook", "low")
    todo.add("Practise 2 SQL joins")
    call = todo.add("Call home", "low")
    todo.complete(2)
    todo.remove(call)
    todo.save()
    print(f"Saved {len(todo)} tasks to {PATH}")

    again = TodoList.load(PATH)
    done = sum(t.done for t in again.tasks.values())
    print(f"Reloaded {len(again)} tasks, {done} done")
    print("Pending:")
    for t in again.pending():
        print(f"  #{t.id} {'[' + t.priority + ']':<8} {t.title}")
    print(f"Next id: {again.next_id}")
    hits = again.search("dsa")
    print(f"Search 'dsa': {', '.join(f'#{t.id} {t.title}' for t in hits)}")


if __name__ == "__main__":
    try:
        main()
    except NotImplementedError as todo:
        # A fresh starter is SUPPOSED to stop here. Say which step is next
        # instead of printing a traceback that looks like a bug.
        print(f"Not built yet: {todo}")
        print("Write that method, then press Run again. Each step you finish moves this message forward.")

Reference solution

"""
To-do manager: a class that saves itself to JSON.

A TodoList you can add to, tick off, save to todos.json and load back.
The demo in main() starts from a clean file every run, saves, then loads
the list into a brand-new object to prove the data survived.
"""

import json
from dataclasses import asdict, dataclass
from pathlib import Path

PATH = Path("todos.json")
PRIORITIES = {"high": 0, "medium": 1, "low": 2}


@dataclass
class Task:
    id: int
    title: str
    priority: str = "medium"
    done: bool = False


class TodoList:
    def __init__(self, path: Path = PATH):
        self.path = Path(path)
        self.tasks: dict[int, Task] = {}
        self.next_id = 1

    def __len__(self) -> int:
        return len(self.tasks)

    def add(self, title: str, priority: str = "medium") -> int:
        title = title.strip()
        if not title:
            raise ValueError("title cannot be empty")
        if priority not in PRIORITIES:
            raise ValueError(f"priority must be one of {list(PRIORITIES)}")
        task = Task(self.next_id, title, priority)
        self.tasks[task.id] = task
        self.next_id += 1
        return task.id

    def complete(self, task_id: int) -> None:
        if task_id not in self.tasks:
            raise KeyError(f"no task #{task_id}")
        self.tasks[task_id].done = True

    def remove(self, task_id: int) -> None:
        if task_id not in self.tasks:
            raise KeyError(f"no task #{task_id}")
        del self.tasks[task_id]

    def save(self) -> None:
        data = {"next_id": self.next_id, "tasks": [asdict(t) for t in self.tasks.values()]}
        self.path.write_text(json.dumps(data, indent=2), encoding="utf-8")

    @classmethod
    def load(cls, path: Path = PATH) -> "TodoList":
        todo = cls(path)
        if not todo.path.exists():
            return todo
        data = json.loads(todo.path.read_text(encoding="utf-8"))
        for row in data["tasks"]:
            task = Task(**row)
            todo.tasks[task.id] = task
        todo.next_id = data["next_id"]
        return todo

    def pending(self) -> list[Task]:
        open_tasks = [t for t in self.tasks.values() if not t.done]
        return sorted(open_tasks, key=lambda t: (PRIORITIES[t.priority], t.id))

    def search(self, word: str) -> list[Task]:
        word = word.lower()
        return [t for t in self.tasks.values() if word in t.title.lower()]


def main() -> None:
    if PATH.exists():
        PATH.unlink()  # the demo starts clean every run
    todo = TodoList(PATH)
    todo.add("Revise DSA: two pointers", "high")
    todo.add("Submit resume on placement portal", "high")
    todo.add("Buy notebook", "low")
    todo.add("Practise 2 SQL joins")
    call = todo.add("Call home", "low")
    todo.complete(2)
    todo.remove(call)
    todo.save()
    print(f"Saved {len(todo)} tasks to {PATH}")

    again = TodoList.load(PATH)
    done = sum(t.done for t in again.tasks.values())
    print(f"Reloaded {len(again)} tasks, {done} done")
    print("Pending:")
    for t in again.pending():
        print(f"  #{t.id} {'[' + t.priority + ']':<8} {t.title}")
    print(f"Next id: {again.next_id}")
    hits = again.search("dsa")
    print(f"Search 'dsa': {', '.join(f'#{t.id} {t.title}' for t in hits)}")


if __name__ == "__main__":
    main()