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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def atomic(self, func):
 """A decorator that wraps a function in an atomic block. Example:: db = CustomSQLAlchemy() @db.atomic def f():
 write_to_db('a message') return 'OK' assert f() == 'OK' This code defines the function ``f``, which is wrapped in an atomic block. Wrapping a function in an atomic block gives several guarantees: 1. The database transaction will be automatically committed if the function returns normally, and automatically rolled back if the function raises unhandled exception. 2. When the transaction is committed, all objects in ``db.session`` will be expunged. This means that no lazy loading will be performed on them. 3. If a transaction serialization error occurs during the execution of the function, the function will be re-executed. (It might be re-executed several times.) Atomic blocks can be nested, but in this case the outermost block takes full control of transaction's life-cycle, and inner blocks do nothing. """


        @wraps(func)
        def wrapper(*args, **kwargs):
            session = self.session
            session_info = session.info
            if session_info.get(_ATOMIC_FLAG_SESSION_INFO_KEY):
                return func(*args, **kwargs)
            f = retry_on_deadlock(session)(func)
            session_info[_ATOMIC_FLAG_SESSION_INFO_KEY] = True
            try:
                result = f(*args, **kwargs)
                session.flush()
                session.expunge_all()
                session.commit()
                return result
            except Exception:
                session.rollback()
                raise
            finally:
                session_info[_ATOMIC_FLAG_SESSION_INFO_KEY] = False

        return wrapper