Is there any reason why session_factory method includes the create_all() statement? Why not use the create_all() globally just below Base is created?

def session_factory():
    Base.metadata.create_all(engine) # why is this a part of session_factory
    return _SessionFactory()

I'm using sqlalchemy with my Flask application and getting intermittent "MySQL server has gone away" error on the create_all() line. I believe due to inactivity Base doesn't exist anymore. Your thoughts?