ooad1-1  Exercise 3: From Sketch to Code, and What the Sketch Missed
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ex3.py (run with Python 3.14):

  from dataclasses import dataclass, field
  from datetime import date


  @dataclass
  class Book:
      title: str


  @dataclass
  class Reservation:
      book: Book
      reserved_on: date


  @dataclass
  class Member:
      name: str
      reservations: list[Reservation] = field(default_factory=list)

      def reserve(self, book: Book, today: date) -> Reservation:
          r = Reservation(book, today)
          self.reservations.append(r)
          return r


  ada = Member("Ada")
  ada.reserve(Book("Dune"), date(2026, 10, 10))
  ada.reserve(Book("Dune"), date(2026, 10, 11))      # the diagram does not forbid this
  print(len(ada.reservations), "reservations, both for", ada.reservations[0].book.title)

Output:

  2 reservations, both for Dune

What the diagram does not say, and the code therefore allows:
  1. The same member reserving the same book twice (the output shows it happening). Is that allowed?
  2. How long a reservation lasts, and what happens when it expires.
  3. What happens when the book is available now: is a reservation still needed?
  4. Whether there is a limit on reservations per member.
  5. The order of the queue when several members reserve one book: first come first served, or something else?
  6. Whether a member can cancel, and who may cancel for them.
Questions for the librarian: the six above, in plain words. Questions 1, 4 and 5 are business rules; 2, 3 and 6 also decide the states a reservation can be in, which is chapter 8.

WHY THIS WORKS AS AN ANSWER
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The code was run, and the second reserve() call succeeding is a concrete demonstration that the diagram's multiplicity "0..*" allows duplicates. A sketch states structure, not rules; turning it into code makes the missing rules show up as places where you had to choose, or where nothing stops a bad case.