ooad1-5  Exercise 2: From Code to Diagram
==============================================

shop_code.py (the code you were given; Python 3.14):

  from dataclasses import dataclass, field
  from typing import Protocol


  class Pricing(Protocol):
      def price(self, hours: float, parts: list["JobPart"]) -> int: ...


  @dataclass
  class Part:
      part_no: str
      price_pence: int


  @dataclass
  class JobPart:
      part: Part
      quantity: int


  @dataclass
  class Bike:
      description: str


  class StandardPricing:
      def price(self, hours: float, parts: list[JobPart]) -> int:
          return int(hours * 3000) + sum(p.part.price_pence * p.quantity for p in parts)


  @dataclass
  class RepairJob:
      bike: Bike
      hours: float = 0.0
      parts: list[JobPart] = field(default_factory=list)

      def add_part(self, part: Part, quantity: int) -> JobPart:
          used = JobPart(part, quantity)
          self.parts.append(used)
          return used

      def invoice(self, pricing: Pricing) -> int:
          return pricing.price(self.hours, self.parts)


  if __name__ == "__main__":
      job = RepairJob(Bike("red road bike"), hours=1.5)
      job.add_part(Part("TUBE-700", 450), 2)
      print("invoice (pence):", job.invoice(StandardPricing()))

Run with:  py shop_code.py

Output:

  invoice (pence): 5400

shop_answer.class.mmd (Mermaid, rendered with Mermaid 12.1.0):

  classDiagram
      direction LR
      class Pricing {
          <<interface>>
          +price(hours, parts) int
      }
      class StandardPricing
      class RepairJob {
          +hours
          +add_part(part, quantity) JobPart
          +invoice(pricing) int
      }
      class JobPart {
          +quantity
      }
      class Part {
          +part_no
          +price_pence
      }
      class Bike {
          +description
      }
      StandardPricing ..|> Pricing
      RepairJob ..> Pricing
      RepairJob "1" *-- "0..*" JobPart
      JobPart "*" --> "1" Part
      RepairJob "*" --> "1" Bike

HOW EACH LINK WAS DECIDED
  StandardPricing ..|> Pricing   Realisation: StandardPricing has the method price(hours, parts) that the Pricing Protocol requires, and does not inherit from it.
  RepairJob ..> Pricing          Dependency: invoice(pricing) takes it as a parameter and stores nothing.
  RepairJob *-- JobPart          Composition: JobPart objects are created inside the job (add_part) and live in the job's own list; a JobPart is meaningless without its job.
  JobPart --> Part               Association: a JobPart holds a Part (a field), but a Part exists independently of any job.
  RepairJob --> Bike             Association: RepairJob holds a Bike (a field); a Bike does not know its jobs, so the arrow is one way.
The 5400 pence in the output is 1.5 hours at 3000p plus two tubes at 450p: 4500 + 900.

WHY THIS WORKS AS AN ANSWER
---------------------------
The test for each link is what the code does: a field means association, creation inside the owner means composition, a parameter means dependency, a matching method set means realisation. The diagram omits private detail (dataclass boilerplate, the default list factory) because the reader does not need it.