SQLAlchemy Issue - __init__ function defined in table class causes issue with associations

Question

I'm new to SQLAlchemy, and ran into an issue when I tried to create __init__ function inside the table class. The script can run without exception but the association function won't take any values after I add the object to database (as an indicator the output results have "0" length of related attributes). However, as soon as I remove the __init__ function everything works as desired.

I wonder what the cause of this issue is, and if there is a work around that allows me to have some hints/arguments validator when insert new data.

working script:

Base = declarative_base()

a_b = Table('a_b', Base.metadata,
            Column('a_id', ForeignKey('a.a_id')),
            Column('b_id', ForeignKey('b.b_id'))
            )


class A(Base):
    __tablename__ = 'a'
    a_id = Column(Integer, primary_key=True)
    a_age = Column(Integer)
    b_lst = relationship('B', secondary=a_b, backref='a_lst')

    def __repr__(self):
        return f'A({self.a_age} >> length of b_lst: {len(self.b_lst)})'


class B(Base):
    __tablename__ = 'b'
    b_id = Column(Integer, primary_key=True)
    b_num = Column(Integer)

    def __repr__(self):
        return f'B({self.b_num} >> length of a_lst: {len(self.a_lst)})'


database_path = 'sqlite:///../data/database.db'
engine = create_engine(url=database_path, echo=False, future=True)
Base.metadata.create_all(engine)

with Session(engine) as session:
    a_lst = [
        A(a_age=1, b_lst=[B(b_num=11), B(b_num=22)]),
        A(a_age=2, b_lst=[B(b_num=33)])
    ]

    session.add_all(a_lst)
    session.commit()

    stmt = select(A)
    results = session.scalars(stmt)
    pprint([item for item in results])

• results

[A(1 >> length of b_lst: 2), A(2 >> length of b_lst: 1)]

issue script:

Base = declarative_base()

a_b = Table('a_b', Base.metadata,
            Column('a_id', ForeignKey('a.a_id')),
            Column('b_id', ForeignKey('b.b_id'))
            )


class A(Base):
    __tablename__ = 'a'
    a_id = Column(Integer, primary_key=True)
    a_age = Column(Integer)
    b_lst = relationship('B', secondary=a_b, backref='a_lst')

    def __init__(self, a_age, *args, **kwargs):
        self.a_age = a_age

    def __repr__(self):
        return f'A({self.a_age} >> length of b_lst: {len(self.b_lst)})'


class B(Base):
    __tablename__ = 'b'
    b_id = Column(Integer, primary_key=True)
    b_num = Column(Integer)

    def __init__(self, b_num, *args, **kwargs):
        self.b_num = b_num

    def __repr__(self):
        return f'B({self.b_num} >> length of a_lst: {len(self.a_lst)})'


database_path = 'sqlite:///../data/database.db'
engine = create_engine(url=database_path, echo=False, future=True)
Base.metadata.create_all(engine)

with Session(engine) as session:
    a_lst = [
        A(a_age=1, b_lst=[B(b_num=11), B(b_num=22)]),
        A(a_age=2, b_lst=[B(b_num=33)])
    ]

    session.add_all(a_lst)
    session.commit()

    stmt = select(A)
    results = session.scalars(stmt)
    pprint([item for item in results])

• results

[A(1 >> length of b_lst: 0), A(2 >> length of b_lst: 0)]