model and instance methods: session aware (sqlalchemy)

So, using and learning with sqlalchemy.

I have an instance, I need to get a value. If that value exists, return it. If not calculate and return it.

Invariably someone will say 'you're doing it wrong' and input on improvement is appreciated in general. However I am looking into how I can do this without explicitly having to manage the session, because what I'm working on is starting to grow, and constantly managing the session for when I want to update an instance is problematic. It makes me think that I am in fact doing it wrong.

So how do I fix the method below to not have manage the session explicitly?

def method(self, session):
        if self.i_needed_this is None:
            self.i_needed_this = calculate(calcutron)
            session.add(self)
            session.commit()
            return self.i_needed_this                        
        else:
            return self.i_needed_this

Maybe this question should be titled 'making instances session aware so I'm not always explicitly managing it', and if it's a dumb question, at least show me why with examples and point me to where others have asked better.

Edit: apparently, importing the session I'm using works, and it is available, so maybe its a non issue or a future one for when I am more skilled with sqlalchemy.