Design Patterns

Notes and exercises for learning design patterns

View the Project on GitHub Claptar/design-patterns

Bridge Exercise 1: Building the Basic Shape

Goal

Practice the core Bridge mechanic: two separate hierarchies connected by a reference.

You will build a simple smart-home control system. Don’t worry about realism — the goal is to get comfortable with the structural shape before applying it to harder problems.

The domain

A smart-home app controls devices (a light, a fan). Devices can be operated at different intensity levels (low, high). The app sends commands, but the device decides how to carry them out.

The two dimensions are:

Devices:    Light, Fan
Intensity:  LowIntensity, HighIntensity

Without Bridge you’d end up with LowIntensityLight, HighIntensityLight, LowIntensityFan, HighIntensityFan — four classes for two dimensions, and it only gets worse from there.

What you need to build

Implementor interface: Device

class Device(ABC):
    @abstractmethod
    def turn_on(self) -> None: ...

    @abstractmethod
    def turn_off(self) -> None: ...

    @abstractmethod
    def set_level(self, level: int) -> None:
        """Set the device's output level (0–100)."""
        ...

    @abstractmethod
    def status(self) -> str: ...

Concrete implementors: Light, Fan

Light.set_level(level) should print something like "Light brightness set to 75%".

Fan.set_level(level) should print something like "Fan speed set to 40%".

Both should track whether they are on or off and their current level.

Abstraction base: RemoteControl

class RemoteControl(ABC):
    def __init__(self, device: Device):
        self._device = device   # <-- the bridge

    @abstractmethod
    def toggle_power(self) -> None: ...

Refined abstractions: LowIntensityRemote, HighIntensityRemote

LowIntensityRemote.toggle_power() turns the device on at level 30, or off.

HighIntensityRemote.toggle_power() turns the device on at level 90, or off.

Both should track whether the device is currently on so they can toggle correctly.

Expected output

light = Light()
fan = Fan()

low_remote = LowIntensityRemote(light)
high_remote = HighIntensityRemote(fan)

low_remote.toggle_power()
# Light turned on
# Light brightness set to 30%

low_remote.toggle_power()
# Light turned off

high_remote.toggle_power()
# Fan turned on
# Fan speed set to 90%

print(light.status())  # Light: OFF, level=0
print(fan.status())    # Fan: ON, level=90

Skeleton

See exercise1.py.

Things to notice while you work

Exercise 2