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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 credit_card_number(self, card_type=None):
 """ Returns a valid credit card number. """

        card = self._credit_card_type(card_type)
        prefix = self.random_element(card.prefixes)
        number = self._generate_number(self.numerify(prefix), card.length)
        return number