class Entity(object):

    def __init__(self, id, db_connection, data=None):
        self.id = id
self.db_connection = db_connection
if data is None:
    self.data = self.from_id(id, db_connection)
else:
    self.data = data

def from_id(cls, id_number, db_connection):
    pass
Использование статических методов в python — лучшая практика