alphaPlan · Data Structures, Algorithms & the Web · Cheat-sheet 05

Object-oriented programming

Describe the things in your program as classes, build objects from them, and let them share, guard and vary their behaviour.

Ideas to remember

01A class is a blueprint and an object is one thing built from it; each object carries its own data, so changing one leaves the others alone.
02Attributes are the data an object carries and methods are the actions it can take; self always means this particular object.
03An instance attribute is set in __init__ and belongs to one object; a class attribute is written in the class body and shared by all.
04A child class names its parent in parentheses and inherits its attributes and methods; a method with the same name in the child overrides the parent's.
05Polymorphism lets one loop call the same method name on a mixed pile of objects and let each answer in its own way.
06Encapsulation hides data behind guarded methods, and an abstract base class states what every child must provide without saying how.

def __init__(self, title, pages):: Runs automatically whenever a new object is created and sets up its starting data.
self.balance: The balance of this account, not some global number.
class Bus(Vehicle):: Bus inherits everything Vehicle has and adds or replaces only what is different.
super().__init__(make, wheels): Runs the parent's __init__ first, so the child reuses its setup instead of copying it.
self.__balance: A double leading underscore makes the attribute private, so code outside the class cannot reach it by the obvious path.
@abstractmethod: Marks a method with no body that every child must supply; a bare base class marked this way cannot be created at all.

Read self as the specific object this is running on.
Go through a method like deposit instead of setting balance by hand, so the guard inside it can refuse bad input.
Use inheritance only when you can honestly say a child is a kind of parent; a wheel is a part of a car, not a child.
Give every shape class the same method name, area, so one loop handles them all and a new kind fits without touching the loop.
Do not force classes on a five line script; their value shows when several things each carry state and behaviour.

A single underscore like _balance is only a polite sign; Python enforces privacy only for the double underscore __balance.
Changing a class attribute such as interest_rate changes it for every object at once, so use it only for values that are the same for the whole kind.
Reading card.__balance from outside raises an AttributeError; that failure is the encapsulation working.

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