Object Oriented Programming

Object Oriented Programming

OO Principles

• Procedural: there’s data, and there’s code

• ⟶ relationship is not aways clear

• OO: data and code aggregated together, into classes*

• ⟶ Methods operate on objects that have members*

• Encapsulation: implementation is hidden from the public

End effect: you talk about your code in the same way that you program it

OO Everywhere

Strings

s = 'Jörg'
enc_s = s.encode(encoding='utf-8')

Lists

list = ['Hello', 'World']
list.extend(['!'])

Batteries

from http.client import HTTPConnection
connection = HTTPConnection('www.google.com')
connection.connect()

The class Statement

Defining a class: the class statement

class MakesNoSense:
    ...

• class creates a “class” object (⟶ Metaprogramming)

• MakesNoSense is the name of a variable (that refers to the class object)

• ⟶ like with functions, the class object can be assigned, passed as parameter, …

The Constructor

class MakesNoSense:
    def __init__(self, parameter1, parameter2):
        ...

mns = MakesNoSense('Hello', 666)

• __init__: special method name ⟶ constructor

• self: the object being initialized/constructed

• Python does not require the name self, but it is “good style”. IDE’s may rely upon it, but no requirement otherwise.

Attributes/Members

class MakesNoSense:
    def __init__(self, parameter1, parameter2):
        self.member1 = parameter1
        self.member2 = parameter2

...
print(mns.member1)
mns.member2 = 42

• There is no information hiding in Python

• Members are visible to outside users

• … by default at least

Attributes/Members: Hiding

class MakesNoSense:
    def __init__(self, parameter1, parameter2):
        self.__member1 = parameter1
        self.__member2 = parameter2

# error!
mns.__member2 = 42

• Python recognizes __ as something special

• Mangles the name ⟶ visible as-is only within class’s methods

Methods

A Method is a function that is called on an object …

class MakesNoSense:
    def __init__(self, parameter1, parameter2):
        self.__member1 = parameter1
        self.__member2 = parameter2

    def do_make_sense(self, value):
        print('I try to but fail: {} {} {}'.format(
            self.__member1, self.__member2, value))

mns = MakesNoSense(1, 'one')
mns.do_make_sense('bummer')