Fastest Way to Create a New Object Only if it Doesn't Already Exist (SQLAlchemy)

Question

I want to get an object from the database if it already exists (based on provided parameters) or create it if it does not.

Django's get_or_create (or source) does this. Is there an equivalent shortcut in SQLAlchemy?

I'm currently writing it out explicitly like this:

def get_or_create_instrument(session, serial_number):
    instrument = session.query(Instrument).filter_by(serial_number=serial_number).first()
    if instrument:
        return instrument
    else:
        instrument = Instrument(serial_number)
        session.add(instrument)
        return instrument