Now we've laid the groundwork for our relocate_passenger() method:

class Flight:

    # ...

    def relocate_passenger(self, from_seat, to_seat):
        """Relocate a passenger to a different seat.

        Args:
            from_seat: The existing seat designator for the
                       passenger to be moved.

            to_seat: The new seat designator.
        """
        from_row, from_letter = self._parse_seat(from_seat)
        if self._seating[from_row][from_letter] is None:
           raise ValueError(
              "No passenger to relocate in seat {}".format(from_seat))

        to_row, to_letter = self._parse_seat(to_seat)
        if self._seating[to_row][to_letter] is not None:
           raise ValueError(
                "Seat {} already occupied".format(to_seat))

        self._seating[to_row][to_letter] = self._seating[from_row] 
        [from_letter]
        self._seating[from_row][from_letter] = None

This parses and validates the from_seat and to_seat arguments and then moves the passenger to the new location.

It's also getting tiresome recreating the Flight object each time, so we'll add a module level convenience function for that too:

def make_flight():
    f = Flight("BA758", Aircraft("G-EUPT", "Airbus A319",
               num_rows=22, num_seats_per_row=6))
    f.allocate_seat('12A', 'Guido van Rossum')
    f.allocate_seat('15F', 'Bjarne Stroustrup')
    f.allocate_seat('15E', 'Anders Hejlsberg')
    f.allocate_seat('1C', 'John McCarthy')
    f.allocate_seat('1D', 'Richard Hickey')
    return f

In Python it's quite normal to mix related functions and classes in the same module. Now, from the REPL:

>>> from airtravel import make_flight
>>> f = make_flight()
>>> f
<airtravel.Flight object at 0x1007a6690>

You may find it remarkable that we have access to the Flight class when we have only imported a single function, make_flight. This is quite normal and it's a powerful aspect of Python’s dynamic type system that facilitates this very loose coupling between code.

Let's get on and move Guido back to row 15 with his fellow Europeans:

>>> f.relocate_passenger('12A', '15D')
>>> from pprint import pprint as pp
>>> pp(f._seating)
[None,
  {'A': None,'B': None, 'C': 'John McCarthy', 'D': 'Richard Hickey',
  'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None,'B': None,'C': None,'D': 'Guido van Rossum',
   'E': 'Anders Hejlsberg','F': 'Bjarne Stroustrup'},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None},
  {'A': None, 'B': None, 'C': None, 'D': None, 'E': None, 'F': None}]