Let's import first the context for this chapter.

In [1]:
from context import *

Object-Oriented Design

In this session, we will finally discuss the thing most people think of when they refer to "Software Engineering": the deliberate design of software. We will discuss processes and methodologies for planned development of large-scale software projects: Software Architecture.

The software engineering community has, in large part, focused on an object-oriented approach to the design and development of large scale software systems. The basic concepts of object orientation are necessary to follow much of the software engineering conversation.

Design processes

In addition to object-oriented architecture, software engineers have focused on the development of processes for robust, reliable software development. These codified ways of working hope to enable organisations to repeatably and reliably complete complex software projects in a way that minimises both development and maintainance costs, and meets user requirements.

Design and research

Software engineering theory has largely been developed in the context of commercial software companies.

The extent to which the practices and processes developed for commercial software are applicable in a research context is itself an active area of research.

Recap of Object-Orientation

Classes: User defined types

In [2]:
class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def grow_up(self):
        self.age += 1

terry = Person("Terry", 76)
terry.home = "Colwyn Bay"

Notice, that in Python, you can add properties to an object once it's been defined. Just because you can doesn't mean you should!

Declaring a class

Class: A user-defined type

In [3]:
class MyClass:

Object instances

Instance: A particular object instantiated from a class.

In [4]:
my_object = MyClass()


Method: A function which is "built in" to a class

In [5]:
class MyClass:
    def someMethod(self, argument):

my_object = MyClass()


Constructor: A special method called when instantiating a new object

In [6]:
class MyClass:
    def __init__(self, argument):

my_object = MyClass(value)

Member Variable

Member variable: a value stored inside an instance of a class.

In [7]:
class MyClass:
    def __init__(self):
        self.member = "Value"

my_object = MyClass()
assert(my_object.member == "Value")

Object refactorings

Replace add-hoc structure with user defined classes

Smell: A data structure made of nested arrays and dictionaries becomes unwieldy.


In [8]:
from random import random
birds = [{"position": random(),
          "velocity": random(),
          "type": kind} for kind in bird_types]

average_position = average([bird["position"] for bird in birds])


In [9]:
class Bird:
    def __init__(self, kind):
        from random import random
        self.type = type
        self.position = random()
        self.velocity = random()

birds = [Bird(kind) for kind in bird_types]
average_position = average([bird.position for bird in birds])

Replace function with a method

Smell: A function is always called with the same kind of thing


In [10]:
def can_see(source, target):
    return (source.facing - target.facing) < source.viewport

if can_see(hawk, starling):


In [11]:
class Bird:
    def can_see(self, target):
        return (self.facing - target.facing) < self.viewport

if hawk.can_see(starling):

Replace method arguments with class members

Smell: A variable is nearly always used in arguments to a class.

In [12]:
class Person:
    def __init__(self, genes):
        self.genes = genes
    def reproduce_probability(self, age): pass
    def death_probability(self, age): pass
    def emigrate_probability(self, age): pass


In [13]:
class Person:
    def __init__(self, genes, age):
        self.age = age
        self.genes = genes
    def reproduce_probability(self): pass
    def death_probability(self): pass
    def emigrate_probability(self): pass

Replace global variable with class and member

Smell: A global variable is referenced by a few functions

In [14]:
name = "Terry Jones"
birthday = [1, 2, 1942]
today = [22, 11]

if today == birthday[0:2]:
    print(f"Happy Birthday, {name}")
    print("No birthday for you today.")
No birthday for you today.
In [15]:
class Person(object):
    def __init__(self, birthday, name):
        self.birth_day = birthday[0]
        self.birth_month = birthday[1]
        self.birth_year = birthday[2]
        self.name = name
    def check_birthday(self, today_day, today_month):
        if not self.birth_day == today_day:
            return False
        if not self.birth_month == today_month:
            return False
        return True
    def greet_appropriately(self, today):
        if self.check_birthday(*today):
            print(f"Happy Birthday, {self.name}")
            print("No birthday for you.")

john = Person([5, 5, 1943], "Michael Palin")
No birthday for you.

Object Oriented Refactoring Summary

  • Replace ad-hoc structure with a class
  • Replace function with a method
  • Replace method argument with class member
  • Replace global variable with class data