Replaces all question mark ('?') occurrences with a random letter
        from postal_code_formats then passes result to
        numerify to insert numbers

def postcode(self):
        """
        Replaces all question mark ('?') occurrences with a random letter
        from postal_code_formats then passes result to
        numerify to insert numbers
        """
        temp = re.sub(r'\?',
                      lambda x: self.postal_code_letter(),
                      self.random_element(self.postal_code_formats))
        return self.numerify(temp)