OOP

Concepts

Classes and Instances

• Classes are also called Types in Python.
  • For example, these are classes: int, float, str, bool
• Instances are values of a given class or type.
  • For example: 'abc' is a str instance (also called a string)

Object and Object-Oriented Programming

• Every value in Python is an Object(Type).
  • Every instance is an object, and its type is some class.
  • Every class is an object, too (its type is type!).

Instance Level

Directly add property

• Quite like JavaScript, right?

class Dog(object): # inherit object by defaultpass

d1 = Dog()
d1.name = 'Jack'
d1.age = 3

Preload property with constructor

• Nothing need to be filled in Dog()

class Dog(): # no argument = objectdef __init__(self, name, age):
        self.name = name
        self.age = age

d1 = Dog("hello", 1)

Add class method, don't forget self!

• every method in class must have self as the 1st argument

class Dog():
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def sayHi(self): # don't forget self
        print(f'my name is {self.name}')

d1 = Dog('James', 2)
d1.sayHi()

Set property

class Dog():
    def __init__(self, name, age):
        self.name = name
        self.age = age
        self.barkCount = 0

    def sayHi(self): 
        print(f'my name is {self.name}')

    def bark(self, n):
        while self.barkCount < n:
            print('bark')
            self.barkCount += 1  # modify barkCount property

d1 = Dog('James', 2)
d1.bark(3)

Class Level

class variables

class A():
    x = 1 # x is a class variables
print(A.x) # access ✔️
a = A()
print(a.x) # access ✔️

class methods

class A():
    @staticmethoddef f(x):
        return 2*x

print(A.f(1))

Special Methods

Compare Object

• compare value without default behaviour

class A(object): pass
a1 == a2 # always true

• customize value comparison

class A(object):
    def __init__(self, x):
        self.x = x
    def __eq__(self,other):
        return isinstance(other, A) and self.x == other.x

• compare memory address

print(a1 is a2)

Print Object

• print(object)

class A():
    def __str__(self):
        return "A"
a = A()
print(a)

• print([object])

class A():
    def __str__(self):
        return "A"
    def __repr__(self):
        return "A"
a = A()
print([a])

Polymorphism

• require a method to run, you can pass any object with that method

reuse code by object's method

class Dog():
    def speak(self):
        print("woof!")

class Cat():
    def speak(self):
        print('meow')

for animal in [Dog(). Cat()]:
    animal.speak()

Inheritance

• subclass copy everything from parent class

class Animal():
    def run(self):
        print("animal is run")

class Dog(Animal):
    pass

d1 = Dog()
d1.run()

• override things from parent class

class Animal():
    def run(self):
        print("animal is run")

class Dog(Animal):
    def run(self):
        print('dog is running')
d1 = Dog()
d1.run()