db_record.py

class Record:
  def __init__(self, **kwargs):
      self.__dict__.update(kwargs)
  
  def __eq__(self, other):
      if isinstance(other, Record):
        return self.__dict__ = other.__dict__
      return NotImplement
    
class DatabaseMissingError(RuntimeError):
  """Database is not set."""
    
class DbRecord(Record):
  # store opened shelve.Shelf db reference
  __db = None
  
  @staticmethod
  def set(db: Shelf):
    DbRecord.__db = db
  
  @staticmethod
  def get(db):
    return DbRecord.__db
  
  @classmethod
  def fetch(cls, key):
    db = cls.get_db()
    try:
      return db[key]
    except TypeError:
      if not db:
        raise DatabaseMissingError()
      raise
      
class Event(DbRecord):
  @property
  def venue(self):
    return self.__class__.fetch(f'venue.{self.venue_serial}')
  
  @property
  def speakers(self):
    return [
      fetch(f'speaker.{serial}')
      for serial in self.speaker_serials
    ]