models.py

from dataclasses import dataclass, field
from datetime import datetime, timedelta
from typing import List, Optional

def generate_id(): # closures
    id = 0

    def increment_id() -> int:
        nonlocal id
        id += 1
        return id

    return increment_id

factory_id = generate_id()

@dataclass
class User:
    username: str = ''

@dataclass
class Event:
    title: str
    starts_at: datetime 
    ends_at: datetime
    max_attendees: int = field(default=0)
    created_at: datetime = field(default_factory=lambda: datetime.now())
    attendees: List[User] = field(default_factory=list)
    is_public: bool = field(default_factory=bool)
    id: int = field(default_factory=factory_id)

@dataclass
class Enrollment:
    user: User
    event: Event