To the Flight class we add a new method make_boarding_cards() which accepts a card_printer:

class Flight:

    # ...

    def make_boarding_cards(self, card_printer):
        for passenger, seat in sorted(self._passenger_seats()):
            card_printer(passenger, seat, self.number(), 
            self.aircraft_model())

This tells the card_printer to print each passenger, having sorted a list of passenger-seat tuples obtained from a _passenger_seats() implementation detail method (note the leading underscore). This method is in fact a generator function which searches all seats for occupants, yielding the passenger and the seat number as they are found:

def _passenger_seats(self):
    """An iterable series of passenger seating allocations."""
    row_numbers, seat_letters = self._aircraft.seating_plan()
    for row in row_numbers:
        for letter in seat_letters:
            passenger = self._seating[row][letter]
                if passenger is not None:
                yield (passenger, "{}{}".format(row, letter))

Now if we run this on the REPL, we can see that the new boarding card printing system works:

>>> from airtravel import console_card_printer, make_flight
>>> f = make_flight()
>>> f.make_boarding_cards(console_card_printer)
+--------------------------------------------------------------------+
|                                                                         |
|Name: Anders Hejlsberg Flight: BA758  Seat: 15E  Aircraft:Airbus A319 |
|                                                                         |
+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
|                                                                          |
| Name:Bjarne Stroustrup  Flight:BA758  Seat:15F  Aircraft:Airbus A319 |
|                                                                          |
+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
|                                                                         |
| Name:Guido van Rossum  Flight:BA758  Seat:12A  Aircraft:Airbus A319 |
|                                                                         |
+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
|                                                                     |
| Name:John McCarthy  Flight:BA758  Seat:1C  Aircraft:Airbus A319     |
|                                                                     |
+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
|                                                                      |
| Name: Richard Hickey  Flight: BA758  Seat: 1D  Aircraft: Airbus A319 |
|                                                                      |
+--------------------------------------------------------------------+