Inheritance

• A key feature of OOP is the ability to define a class which inherits from another class (a “base” or “parent” class)
• In Python, inheritance works by passing the parent class as an argument to the definition of a child class

class Animal:
	def make_sound(self,soud):
		print(sound)
	cool = True
 
class Cat(Animal):
	pass
 
gandalf = Cat()
gandalf.make_sound("meow") # "meow"
gandalf.cool # True

Using super()

class Animal:
	def __init__(self, name, species):
		self.name = name
		self.species = species
    
	def __repr__(self):
		return f"{self.name} is a {self.species}"
	
class Cat(Animal):
	def __init__(self, name, breed, toy):
		super().__init__(name, species = "Cat")
		# Animal.__init__(self, name, species = "Cat")
		# no need to set these: (duplicates with Animal class)
			# self.name = name
			# self.species = species
		self.breed = breed
		self.toy = toy 
 
 
cat1 = Cat("kitty", "Scottish Fold", "idk the breed","string")
print(cat1.breed)

Multiple inheritance

class A:
    def __init__(self,name):
        print("A init")
        self.name = name
    
    def greet(self):
        return f"Hello!"
 
class B:
    def __init__(self,name):
        print("B init")
        self.name = name
    
    def greet(self):
        return f"Hellooooo!"
	
	def cat(self):
		return "meow"
 
class C(A,B):
    def __init__(self,name):
        print("C init")
        super().__init__(name=name)
        # if we want to explicitly call B's init
        # B.__init__(self,name=name) 
 
test = C("test!")
print(test.cat()) # "meow" -> meow from B
print(test.greet()) # "Hello!" -> greet from A
 
# With debug statements
# C init
# A init 
# Hello!

• Just generally don’t do mutiple inheritances
• So the order of inheritance matters → if there’s overlapping methods, it takes the 1st class
  • Method Resolution Order(MRO)

Method Resolution Order(MRO)

• The order in which Python will look for methods on instances of that class
  • Python sets an MRO whenever you create a class
• How to programmatically reference the MRO
  • __mro__ attribute on the class
  • use the mro() method on the class
  • use the builtin help(cls) method (more readable human format)

class Mother:
    def __init__(self):
        #self.eye_color = "brown"
        self.hair_color = "dark brown"
        self.hair_type = "curly"
    
    
class Father:
    def __init__(self):
        self.eye_color = "blue"
        self.hair_color = "blond"
        self.hair_type = "straight"
    
    
class Child(Mother, Father):
    pass
    
    
j = Child()
    
print(j.eye_color) #ERROR

• j.eye_color gives ERROR
  • When the Child class inherits from both Mother and Father, it calls the __init__ method of only one of those classes (in this case, Mother),
  • Solution is to manually call the __init__ methods of both Mother and Father

class Child(Mother, Father):
    def __init__(self):
        Mother.__init__(self)  # Call Mother's __init__
        Father.__init__(self)  # Call Father's __init__