18

18 — Practice solutions

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These are independently written explanations, not an official marking scheme.

18A

Workout: date (string/date representation as specified), duration (number), summary() → str. Cardio inherits Workout and adds distance (number), overriding summary() → str. Strength inherits Workout and adds repetitions (integer), also overriding summary. A loop calling each workout’s summary can display distance for Cardio and repetitions for Strength while using the same method name.

classDiagram
    Workout <|-- Cardio
    Workout <|-- Strength
    class Workout {
        +str date
        +float duration
        +summary() str
    }
    class Cardio {
        +float distance
        +summary() str
    }
    class Strength {
        +int repetitions
        +summary() str
    }

Reasoning

Date and duration apply to all workouts; distance and repetitions distinguish specialised types. Both specialised objects are workouts, giving an inheritance relationship. A polymorphism example must show how one summary() call produces type-appropriate content, rather than merely using a common method name.

18B

class Device:
    def __init__(self, device_id):
        self.device_id = device_id
        self.is_on = False
 
    def activate(self):
        self.is_on = True
 
class Camera(Device):
    def __init__(self, device_id):
        super().__init__(device_id)
        self.recording = False
 
    def activate(self):
        super().activate()  # Preserve the inherited switch-on behaviour.
        self.recording = True  # Add camera-specific activation behaviour.
 
class SmartCamera(Camera):
    pass

After camera = SmartCamera("C1"), is_on and recording are False. After camera.activate(), both are True. This simplified question gives SmartCamera no additional state; it inherits Camera’s behaviour. The original paper has further details to model.

Reasoning

A SmartCamera is a Camera and a Camera is a Device. Camera adds recording state but still needs the inherited device ID/on-off state. Its activation must do both jobs, so invoke the base activation and then add recording behaviour. No extra SmartCamera attribute was specified in this adapted question.

18C

Insert this method inside Account (same indentation as deposit):

def withdraw(self, amount):
    if amount <= 0 or amount > self.__balance:  # Validate before mutating state.
        return False
    self.__balance -= amount
    return True

The method provides a controlled interface and keeps validation with the state change. A withdrawal exactly equal to the balance succeeds and leaves zero; an unsuccessful withdrawal leaves the previous balance intact.

Reasoning

Unsuccessful withdrawals must leave balance unchanged, so both rejection conditions precede subtraction. Equality with balance is valid: only an amount greater than balance fails. The method returns a Boolean, not the updated balance.