object-oriented.md

Object-Oriented Python

• methods and properties can be added to object dynamically

Class

class Car:
    pass

Initializers (constructors)

• special name __init__

def __init__(self, number):
    self.number = number

Properties

Class properties

class Car:
    instance_counter = 0

Instance properties

class Car:
    def __init__(self, name):
        self.name = name

Methods

• can't explicitly overloaded - the latest definition will override the previous one
• implicit overloading - using default values
• overriding - is supported, parameters are not part of the method definition
  • see https://stackoverflow.com/questions/6034662/python-method-overriding-does-signature-matter

Instance methods

• access to instance (and class) properties
• name convention for 1st param - self

class Car:

    def print_id(self):
        print(id(self))

Class methods

• access to class properties
• use annotation @classmethod
• name convention for 1st param - cls

class Car:
    instance_counter = 0

    @classmethod
    def print_counter(cls):
        print(cls.instance_counter)

Static methods

• no access to any properties (util methods)
• use annotation @staticmethod

class Car:

    @staticmethod
    def print_hello():
        print("Hello from Car class")

Example

class Car:
    counter = 0

    def __init__(self, name="unknown"):
        self.name = name
        self.speed = 0

        Car.counter += 1

    def go(self, speed):
        self.speed = speed

    def print_status(self):
        print("%s is going %d mph" % (self.name, self.speed))


car1 = Car("Car1")
car1.go(150)
car1.print_status()

car2 = Car("Car2")
car2.print_status()

print(Car.counter)

Inheritance

• multiple inheritance is allowed
• super() (with parenthesis) to refer to the parent class/object
  • unlike Java, call to super doesn't have to be on the first line in a constructor

class MyChild(MyParent1, ...):
    def __init__(self, ...):
        super().__init__(...)
        ...

Abstract base class

• not natively supported, import from abc module

from abc import ABC, abstractmethod


class Shape(ABC):  # Shape is a child class of ABC
    @abstractmethod
    def area(self):
        pass