ooad1-9  Exercise 3: An Object Diagram of a Refused Loan
=============================================================

ex3.py (needs objectdiagram.py and the library package from the chapter folder; run from there):

  """Exercise 3: an object diagram of a situation: a junior member at the loan limit asks for a fourth book.
  Run from the chapter folder:  py ex3/ex3.py"""
  import pathlib
  import sys
  from datetime import date

  HERE = pathlib.Path(__file__).resolve().parent.parent
  sys.path.insert(0, str(HERE))
  from library.application import LoanDesk  # noqa: E402
  from library.domain import Copy, Member, Refusal  # noqa: E402
  from library.infrastructure import InMemoryCatalogue  # noqa: E402
  from objectdiagram import walk  # noqa: E402

  catalogue = InMemoryCatalogue([Member("J1", date(2027, 1, 1), "junior")], [Copy(f"B{i}") for i in range(1, 5)])
  desk = LoanDesk(catalogue)
  for code in ("B1", "B2", "B3"):
      desk.borrow("J1", code, date(2026, 10, 10))
  try:
      desk.borrow("J1", "B4", date(2026, 10, 11))
  except Refusal as e:
      print("fourth loan refused:", e)

  member = catalogue.member("J1")
  print("open loans:", len(member.open_loans()), "| copies on loan:", sorted(c.barcode for c in catalogue.copies.values() if c.on_loan))
  text = walk([member, catalogue.copy("B4")])
  (pathlib.Path(__file__).parent / "limit.object.puml").write_text(text, encoding="utf8")
  print()
  print(text)

Run with:  py ex3/ex3.py

Output:

  fourth loan refused: loan limit reached
  open loans: 3 | copies on loan: ['B1', 'B2', 'B3']

  @startuml
  scale 1.2
  hide empty members
  object "j1 : Member" as member_0 {
    card = J1
    expires_on = 2027-01-01
    kind_name = junior
    fines_pence = 0
  }
  object "b4 : Copy" as copy_1 {
    barcode = B4
    on_loan = False
  }
  object "b1 : Loan" as loan_2 {
    barcode = B1
    borrowed_on = 2026-10-10
    due_on = 2026-10-31
    returned_on = None
  }
  object "b2 : Loan" as loan_3 {
    barcode = B2
    borrowed_on = 2026-10-10
    due_on = 2026-10-31
    returned_on = None
  }
  object "b3 : Loan" as loan_4 {
    barcode = B3
    borrowed_on = 2026-10-10
    due_on = 2026-10-31
    returned_on = None
  }
  member_0 --> loan_2 : loans
  member_0 --> loan_3 : loans
  member_0 --> loan_4 : loans
  @enduml

What the diagram shows: member J1 (junior) with three loans, none returned; and the fourth copy, B4, still on the shelf (on_loan = False). The refusal left no trace in the objects: nothing was created or changed.
What it cannot show: the refusal itself (an object diagram is one moment, so it shows the state that makes the refusal inevitable, three open loans against a limit of 3, but not the event); that the limit is 3 (the junior's loan limit lives in the kind object, which the diagram does not include because the member holds only its name, kind_name); and why B1 to B3 are not drawn (the loans refer to them by barcode, not by link).

WHY THIS WORKS AS AN ANSWER
---------------------------
It is generated from the real objects after the real refusal, so it cannot show a situation the code does not allow. The omissions listed are the useful part: each is a question a reader will ask, and each points at a different diagram (a sequence diagram for the event, the class diagram for the limit).