Dave++

This is the first of actual posts in my series on design patterns in python.

This will be about the Facade pattern which is a structural pattern.

The Purpose

The facade pattern is used to make one object with a simple interface represent a complicated system. The problem often occurs in programming where you have a series of interconnected classes where the functions must be called in a certain order or have complicated interdependencies.

This pattern is to give a standard interface to such a system, so you don’t have to rely on reading how you call the system in one of the files or look at example usage.

The Pattern

Here is a UML diagram representing the pattern.

UML is a very useful tool however I have not used a lot yet. It provides a (mostly) standardised way of representing system structure for object oriented programming. I intend to use a UML diagram to represent each pattern in this series, I will explain them so hopefully you don’t need to know UML to follow what’s going on.

This is a nice and simple UML diagram with the following elements; the facade box represents a class, the boxes in the subsystem box could be any object such as a class or a function and the lines between them indicate some relationship.

This shows that the pattern is just one class which groups together a lot of other objects and uses them in some way.

An Example Usage

Here is an implementation of this in python. For this example I am using a car. As this is a small example I will not create a whole complex sub-system, just a few classes.

Here are the three classes which make up the sub-system; Engine, StarterMotor and Battery.

#==============================================================================
class Engine(object):
    
    def __init__(self):
        # how much the motor is spinning in revs per minute
        self.spin = 0

    def start(self, spin):
        if (spin > 2000):
            self.spin = spin // 15

#==============================================================================
class StarterMotor(object):
    
    def __init__(self):
        # how much the starter motor is spinning in revs per minute
        self.spin = 0

    def start(self, charge):
        # if there is enough power then spin fast
        if (charge > 50):
            self.spin = 2500

#==============================================================================
class Battery(object):

    def __init__(self):
        # % charged, starts flat
        self.charge = 0

So now we need our facade object which will work as a common interface for the car.

#==============================================================================
class Car(object):
    # the facade object that deals with the battery, engine and starter motor.
    
    def __init__(self):
        self.battery = Battery()
        self.starter = StarterMotor()
        self.engine = Engine()
        
    def turn_key(self):
        # use the battery to turn the starter motor
        self.starter.start(self.battery.charge)

        # use the starter motor to spin the engine
        self.engine.start(self.starter.spin)
        
        # if the engine is spinning the car is started
        if (self.engine.spin > 0):
            print("Engine Started.")
        else:
            print("Engine Not Started.")

    def jump(self):
        self.battery.charge = 100
        print("Jumped")

This enables the user of Car to use the system as it was intended to be used. I include this at the bottom of the python module so that on running it makes a car, tries to start it, then jumps it to start it.

#==============================================================================
if (__name__ == "__main__"):
    corsa = Car()
    corsa.turn_key()
    corsa.jump()
    corsa.turn_key()

The output of this program is the following;

Engine Not Started.
Jumped
Engine Started.

That is a simple example of the facade design pattern. The code for this is on my GitHub here.

All of the code for this series can be found in this repository.