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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 start(self):
 """ Acquires the db mutex lock. Takes the necessary steps to delete any stale locks. Throws a DBMutexError if it can't acquire the lock. """

        # Delete any expired locks first
        self.delete_expired_locks()
        try:
            with transaction.atomic():
                self.lock = DBMutex.objects.create(lock_id=self.lock_id)
        except IntegrityError:
            raise DBMutexError('Could not acquire lock: {0}'.format(self.lock_id))