Python Design Patterns - Facade Pattern

Facade design pattern provides a unified interface to a set of interfaces in a subsystem. It defines a higher-level interface that any subsystem can use.

A facade class knows which subsystem is responsible for a request.

Example - How to design a facade pattern?

Let us now see how to design a facade pattern.

main.py

class _IgnitionSystem(object):
   
   @staticmethod
   def produce_spark():
      return True

class _Engine(object):

   def __init__(self):
      self.revs_per_minute = 0

   def turnon(self):
      self.revs_per_minute = 2000
   
   def turnoff(self):
      self.revs_per_minute = 0

class _FuelTank(object):
   
   def __init__(self, level=30):
      self._level = level
   
   @property
   def level(self):
      return self._level
   
   @level.setter
   def level(self, level):
      self._level = level

class _DashBoardLight(object):

   def __init__(self, is_on=False):
      self._is_on = is_on

   def __str__(self):
      return self.__class__.__name__

   @property
   def is_on(self):
      return self._is_on
   
   @is_on.setter
   def is_on(self, status):
      self._is_on = status
   
   def status_check(self):
      if self._is_on:
         print("{}: ON".format(str(self)))
      else:
         print("{}: OFF".format(str(self)))

class _HandBrakeLight(_DashBoardLight):
   pass

class _FogLampLight(_DashBoardLight):
   pass

class _Dashboard(object):
   
   def __init__(self):
      self.lights = {"handbreak": _HandBrakeLight(), "fog": _FogLampLight()}
   
   def show(self):
      for light in self.lights.values():
         light.status_check()

# Facade
class Car(object):
   
   def __init__(self):
      self.ignition_system = _IgnitionSystem()
      self.engine = _Engine()
      self.fuel_tank = _FuelTank()
      self.dashboard = _Dashboard()
   
   @property
   def km_per_litre(self):
      return 17.0
   
   def consume_fuel(self, km):
      litres = min(self.fuel_tank.level, km / self.km_per_litre)
      self.fuel_tank.level -= litres
   
   def start(self):
      print("\nStarting...")
      self.dashboard.show()
      if self.ignition_system.produce_spark():
         self.engine.turnon()
      else:
         print("Can't start. Faulty ignition system")
   
   def has_enough_fuel(self, km, km_per_litre):
      litres_needed = km / km_per_litre
      if self.fuel_tank.level > litres_needed:
         return True
      else:
         return False
	   
   def drive(self, km = 100):
      print("\n")
      if self.engine.revs_per_minute > 0:
         while self.has_enough_fuel(km, self.km_per_litre):
            self.consume_fuel(km)
            print("Drove {}km".format(km))
            print("{:.2f}l of fuel still left".format(self.fuel_tank.level))
      else:
         print("Can't drive. The Engine is turned off!")
       
   def park(self):
      print("\nParking...")
      self.dashboard.lights["handbreak"].is_on = True
      self.dashboard.show()
      self.engine.turnoff()
         
   def switch_fog_lights(self, status):
      print("\nSwitching {} fog lights...".format(status))
      boolean = True if status == "ON" else False
      self.dashboard.lights["fog"].is_on = boolean
      self.dashboard.show()
         
   def fill_up_tank(self):
      print("\nFuel tank filled up!")
      self.fuel_tank.level = 100
				
# the main function is the Client
def main():
   car = Car()
   car.start()
   car.drive()
   car.switch_fog_lights("ON")
   car.switch_fog_lights("OFF")
   car.park()
   car.fill_up_tank()
   car.drive()
   car.start()
   car.drive()

if __name__ == "__main__":
   main()

Output

The above program generates the following output −

Starting...
_HandBrakeLight: OFF
_FogLampLight: OFF


Drove 100km
24.12l of fuel still left
Drove 100km
18.24l of fuel still left
Drove 100km
12.35l of fuel still left
Drove 100km
6.47l of fuel still left
Drove 100km
0.59l of fuel still left

Switching ON fog lights...
_HandBrakeLight: OFF
_FogLampLight: ON

Switching OFF fog lights...
_HandBrakeLight: OFF
_FogLampLight: OFF

Parking...
_HandBrakeLight: ON
_FogLampLight: OFF

Fuel tank filled up!


Can't drive. The Engine is turned off!

Starting...
_HandBrakeLight: ON
_FogLampLight: OFF


Drove 100km
94.12l of fuel still left
Drove 100km
88.24l of fuel still left
Drove 100km
82.35l of fuel still left
Drove 100km
76.47l of fuel still left
Drove 100km
70.59l of fuel still left
Drove 100km
64.71l of fuel still left
Drove 100km
58.82l of fuel still left
Drove 100km
52.94l of fuel still left
Drove 100km
47.06l of fuel still left
Drove 100km
41.18l of fuel still left
Drove 100km
35.29l of fuel still left
Drove 100km
29.41l of fuel still left
Drove 100km
23.53l of fuel still left
Drove 100km
17.65l of fuel still left
Drove 100km
11.76l of fuel still left
Drove 100km
5.88l of fuel still left
Drove 100km
0.00l of fuel still left

Explanation

This program is designed with a scenario. It is that of starting the engine of a car or any driving vehicle. If you observe the code, it includes the associated functions to drive, to park and to consume fuel as well.