class MaskedCreditCardField(models.CharField):
    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = 20
        super().__init__(*args, **kwargs)
        
    def to_python(self, value):
        if not isinstance(value, str):
            raise ValidationError("The card number must be a string")
            
        if not value.isdigit():
            raise ValidationError("The card number must contain only digits")
            
        if len(value) != 16:
            raise ValidationError("The card number must be exactly 16 characters long")
            
        return f"****-****-****-{value[-4:]}"
            
class CreditCard(models.Model):
    card_owner = models.CharField(max_length=100)
    card_number - MaskedCreditCardField()