Mediator Example: A Booking Form Whose Fields Depend on Each Other

Choosing a return trip shows the return date; choosing a date checks the seats.

A form whose fields affect each other is a natural Mediator, and this example maps the template onto it. Choosing a return trip shows the return date field. Choosing a departure date shows how many seats are left, and the book button is enabled only when there is a seat. The fields tell the dialog they changed, and the dialog applies the rules.

The template's parts in the booking form example
Template	Booking form example
ConcreteMediator	`BookingDialog`
Colleagues that report	`TripType` and `DepartureDate`
Colleagues that are updated	`ReturnDate`, `SeatInfo` and `BookButton`
notify	`changed(field)`

from datetime import date


class Field:
    def __init__(self, dialog: "BookingDialog") -> None:
        self.dialog = dialog


class TripType(Field):
    value = "one-way"

    def choose(self, value: str) -> None:
        self.value = value
        self.dialog.changed(self)


class DepartureDate(Field):
    value: date | None = None

    def choose(self, value: date) -> None:
        self.value = value
        self.dialog.changed(self)


class ReturnDate(Field):
    visible = False


class SeatInfo(Field):
    text = ""


class BookButton(Field):
    enabled = False


class BookingDialog:
    SEATS = {date(2026, 11, 2): 14, date(2026, 11, 3): 0}

    def __init__(self) -> None:
        self.trip = TripType(self)
        self.departure = DepartureDate(self)
        self.return_date = ReturnDate(self)
        self.seats = SeatInfo(self)
        self.book = BookButton(self)

    def changed(self, field: Field) -> None:
        if field is self.trip:
            self.return_date.visible = self.trip.value == "return"
        if field is self.departure and self.departure.value is not None:
            free = self.SEATS.get(self.departure.value, 0)
            self.seats.text = f"{free} seats left"
            self.book.enabled = free > 0
        self.show()

    def show(self) -> None:
        print(f"  return date shown: {self.return_date.visible}, "
              f"seats: {self.seats.text or '-'}, book enabled: {self.book.enabled}")


dialog = BookingDialog()
print("choose return trip:")
dialog.trip.choose("return")
print("choose 3 November:")
dialog.departure.choose(date(2026, 11, 3))
print("choose 2 November:")
dialog.departure.choose(date(2026, 11, 2))

OutputRan with python:3.12-slim (Python 3.12.13)

choose return trip:
  return date shown: True, seats: -, book enabled: False
choose 3 November:
  return date shown: True, seats: 0 seats left, book enabled: False
choose 2 November:
  return date shown: True, seats: 14 seats left, book enabled: True

The fields call `changed`; only the dialog knows the rules.

Choosing a return trip shows the return date. Choosing 3 November finds no seats and leaves the book button disabled; choosing 2 November finds 14 and enables it. TripType does not know that a return date field exists, and DepartureDate does not know about the button.

The rules are four lines of changed. A new rule, such as "a return trip must return after it departs", goes in the same method. The seat counts are fixed in a dictionary here so the listing runs offline.

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