Problem in SQLAlchemy adding an object and returning value

Question

I am starting with sqlalchemy and I am implementing a project with the repository pattern.

In my project I have a class called BaseRepository:

class BaseRepository:

    def __init__(self, model):
        self.session = Session()
        self.model = model

    @db_complete
    def create(self, item):
        self.session.add(item)
        self.session.commit()
        return item

.... others methods.......

I have created a decorator whose function is to initialize the session and close it.

def db_complete(func):
    def wrapper(*args, **kwargs):
        session = args[0].session
        session.begin()
        try:
            response = func(*args, **kwargs)
            return response
        except:
            session.rollback()
            raise
        finally:
            session.close()

    return wrapper

With that code, sometimes it returns the value but sometimes it does not.

And if I update that code to this, calling refresh(), then it work well:

    @db_complete
    def create(self, item):
        self.session.add(item)
        self.session.commit()
        
        # new code
        self.session.refresh(item)

        return item

The question is: Why do I have to refresh?

I am working with mysql+pymysql

Thanks in advance.