ooad1-10  Exercise 1: Generate the Bike Shop, Then Finish It by Subclassing
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shop.mmd (the design, Mermaid):

  classDiagram
      class JobStatus {
          <<enumeration>>
          OPEN
          DONE
      }
      class Pricing {
          <<interface>>
          +price(hours: float, part_pence: int) int
      }
      class StandardPricing {
          +hourly_pence: int
          +price(hours: float, part_pence: int) int
      }
      class Part {
          +part_no: str
          +price_pence: int
      }
      class JobPart {
          +quantity: int
      }
      class RepairJob {
          +hours: float
          +status: JobStatus
          +total(pricing: Pricing) int
      }
      class Bike {
          +description: str
      }
      StandardPricing ..|> Pricing
      RepairJob "1" *-- "0..*" JobPart
      JobPart "0..*" --> "1" Part
      RepairJob "0..*" --> "1" Bike

Generate with:  py uml2py.py shop.mmd shop_generated.py     (uml2py.py is the chapter's generator)

shop_generated.py (generated, never edited):

  """Generated from the class diagram by uml2py.py. Structure only: every operation is a stub."""
  from __future__ import annotations

  from abc import ABC, abstractmethod
  from dataclasses import dataclass, field
  from enum import Enum
  from typing import Protocol, runtime_checkable



  class JobStatus(Enum):
      OPEN = "OPEN"
      DONE = "DONE"


  @runtime_checkable
  class Pricing(Protocol):
      def price(self, hours: float, part_pence: int) -> int: ...


  @dataclass
  class StandardPricing:
      hourly_pence: int

      def price(self, hours: float, part_pence: int) -> int:
          raise NotImplementedError("StandardPricing.price")


  @dataclass
  class Part:
      part_no: str
      price_pence: int


  @dataclass
  class JobPart:
      quantity: int
      part: Part


  @dataclass
  class RepairJob:
      hours: float
      status: JobStatus
      bike: Bike
      job_parts: list[JobPart] = field(default_factory=list)

      def total(self, pricing: Pricing) -> int:
          raise NotImplementedError("RepairJob.total")


  @dataclass
  class Bike:
      description: str

shop_impl.py (hand-written; subclasses the generated classes):

  """Hand-written behaviour, added by subclassing the generated classes (the 'generation gap' pattern), so regenerating never overwrites it."""
  import unittest
  from dataclasses import dataclass

  import shop_generated as g


  @dataclass
  class StandardPricing(g.StandardPricing):
      def price(self, hours: float, part_pence: int) -> int:
          return int(hours * self.hourly_pence) + part_pence


  @dataclass
  class RepairJob(g.RepairJob):
      def total(self, pricing: g.Pricing) -> int:
          parts = sum(jp.part.price_pence * jp.quantity for jp in self.job_parts)
          return pricing.price(self.hours, parts)


  class Tests(unittest.TestCase):
      def job(self):
          job = RepairJob(1.5, g.JobStatus.OPEN, g.Bike("red road bike"))
          job.job_parts.append(g.JobPart(2, g.Part("TUBE", 450)))
          return job

      def test_generated_stub_alone_does_nothing_useful(self):
          with self.assertRaises(NotImplementedError):
              g.StandardPricing(3000).price(1.5, 900)

      def test_the_finished_classes_compute_a_total(self):
          self.assertEqual(self.job().total(StandardPricing(3000)), 5400)

      def test_the_implementation_still_fits_the_interface(self):
          self.assertIsInstance(StandardPricing(3000), g.Pricing)

      def test_a_required_link_cannot_be_omitted(self):
          with self.assertRaises(TypeError):
              RepairJob(1.0, g.JobStatus.OPEN)                       # the diagram says exactly one Bike


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

Run with:  py shop_impl.py

Output:

  ----------------------------------------------------------------------
  Ran 4 tests in 0.000s

  OK

WHY THIS WORKS AS AN ANSWER
---------------------------
Regenerating shop_generated.py after a design change cannot erase the behaviour, because the behaviour lives in subclasses in a different file. The tests cover what each file contributes: the generated stub does nothing useful on its own (first test); the finished classes compute 5400p for 1.5 hours plus two 450p tubes (second); the subclass still fits the Pricing interface (third); and a link the diagram marks as exactly one (the Bike) is a required field, so omitting it fails (fourth). One thing the design did not say, found while writing the subclasses: the generated RepairJob names its list of parts job_parts, so the hand-written code has to use the generator's naming.