ooad1-12  Exercise 2: Requirement R1, 'See the Sessions and the Seats Left'
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ex2/browse.py:

  """Exercise 2: requirement R1 ('see the upcoming sessions and how many seats are left'), added as a read-only query beside the desk."""
  from dataclasses import dataclass
  from datetime import date

  from workshops.application import SessionStore


  @dataclass(frozen=True)
  class SessionRow:
      session_id: str
      workshop: str
      starts_on: date
      seats_left: int
      fee_pence: int


  class Browser:
      def __init__(self, store: SessionStore):
          self.store = store

      def upcoming(self, today: date) -> list[SessionRow]:
          rows = [SessionRow(s.id, s.workshop, s.starts_on, s.capacity - s.seats_in_use(), s.fee_pence)
                  for s in self.store.all_sessions() if not s.cancelled and s.starts_on > today]
          return sorted(rows, key=lambda r: (r.starts_on, r.session_id))

ex2/test_browse.py:

  import sys
  import unittest
  from datetime import date

  sys.path.insert(0, "ex2")
  from browse import Browser
  from workshops.application import BookingDesk
  from workshops.domain import Session
  from workshops.infrastructure import FakePayments, InMemoryStore

  TODAY = date(2026, 11, 1)


  class Tests(unittest.TestCase):
      def setUp(self):
          self.sessions = [Session("S2", "Coding", date(2026, 11, 20), 5, 4000), Session("S1", "Pottery", date(2026, 12, 1), 2, 2500),
                           Session("S0", "Old", date(2026, 10, 1), 5, 1000), Session("S3", "Cancelled one", date(2026, 12, 5), 5, 1000)]
          self.store = InMemoryStore(self.sessions)
          self.desk = BookingDesk(self.store, FakePayments())
          self.sessions[3].cancelled = True

      def test_only_upcoming_uncancelled_sessions_in_date_order(self):
          self.assertEqual([r.session_id for r in Browser(self.store).upcoming(TODAY)], ["S2", "S1"])

      def test_seats_left_goes_down_as_people_book(self):
          self.desk.book("ann", "S1", TODAY)
          rows = {r.session_id: r for r in Browser(self.store).upcoming(TODAY)}
          self.assertEqual((rows["S1"].seats_left, rows["S2"].seats_left), (1, 5))

      def test_a_full_session_shows_zero_seats_left_not_a_negative_number(self):
          self.desk.book("ann", "S1", TODAY)
          self.desk.book("bo", "S1", TODAY)
          self.desk.book("cy", "S1", TODAY)                                  # waitlisted: holds no seat
          self.assertEqual({r.session_id: r.seats_left for r in Browser(self.store).upcoming(TODAY)}["S1"], 0)


  if __name__ == "__main__":
      unittest.main()

Run from the chapter folder with:  py -m unittest ex2.test_browse

Output:

  Ran 3 tests in 0.000s

  OK

THE DESIGN CHOICES
  - A query is separate from the desk because it changes nothing: it has no payment, no events, and no state machine. Putting it on BookingDesk would grow the class that exists to coordinate changes.
  - It depends only on the SessionStore interface, so it works with any provider of it.
  - seats_left is capacity minus the seats in use (Requested, Offered and Confirmed bookings hold seats, Waitlisted ones do not), so a full session shows 0 and a waitlist never makes it negative.
  - Upcoming means 'starts after today', matching the rule that a session that has started cannot be booked, and cancelled sessions are not shown.

WHY THIS WORKS AS AN ANSWER
---------------------------
The requirement was missing from the design (a gap in my first reading, which went straight to booking), and adding it needed no change to the existing code: one new class that depends on an existing interface. The class diagram would gain Browser and SessionRow, and the design check could be extended to cover them. The tests pin the three decisions above.