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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 stop(self):
 """ Releases the db mutex lock. Throws an error if the lock was released before the function finished. """

        if not DBMutex.objects.filter(id=self.lock.id).exists():
            raise DBMutexTimeoutError('Lock {0} expired before function completed'.format(self.lock_id))
        else:
            self.lock.delete()