def is_valid(self):
        if self.subject == '':
            return False
        # todo: check if email is valid
        return True
Raises `RuntimeError` if the model is not ready for prediction.
dataset/ETHPy150Open allink/pennyblack/pennyblack/models/newsletter.py/Newsletter.is_valid--crossover with--dataset/ETHPy150Open UDST/urbansim/urbansim/models/dcm.py/MNLDiscreteChoiceModel.assert_fitted