I'm trying to understand the DDD architecture pattern. I wrote a simple project in which I tried to use DDD arch. Here are my doubts after implementing it:

1. Does it make sense to use an ORM for a DDD project? Maybe I should drop the use of Base class from declarative_base?
2. Where should I define validators - in domain objects or in repository objects?
3. Should SimpleService(session) take a session in the initializer or specific repositories that I will create in the main function? Now I create it here:

class SimpleService:
    """Creates simple service."""

    def __init__(self, session):
        self.session = session
        self.post_repository = PostRepository(session)
        self.comment_repository = CommentRepository(session)

4. What should I change or improve in whole project? Please give me a quick review.