Delete parent if the child is deleted

I want to delete the parent row if the associated rows in child tables have been removed.

class Child(Base):
    __tablename__ = "children"

id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey("parents.id", ondelete='CASCADE'))

class Parent(Base):
    __tablename__ = "parents"
id = Column(Integer, primary_key=True)
child = relationship(Child, backref="parent", passive_deletes=True)

If I remove the child

child_obj = session.query(Child).first()
session.delete(child_obj)
session.commit()

It does delete the child obj but parent remains as it is. I want to remove the parent as well using cascading.

from Delete parent if the child is deleted