Returns a random credit card type instance.

def _credit_card_type(self, card_type=None):
        """ Returns a random credit card type instance. """
        if card_type is None:
            card_type = self.random_element(self.credit_card_types.keys())
        elif isinstance(card_type, CreditCard):
            return card_type
        return self.credit_card_types[card_type]