Classes and OOP II

super() and Method Overriding

Inheritance gives you the parent's behaviour for free. Sometimes you want to change it for the subclass, or build on top of it. That is what overriding and super() are for.

Inheritance gives you the parent's behaviour for free. Sometimes the subclass needs to do things differently, or do a bit extra. That is what overriding solves.

Define a method with the same name in the subclass and Python uses the subclass version:

class Animal:
	def speak(self):
		return "some sound"
 
class Dog(Animal):
	def speak(self):
		return "woof"
 
d = Dog()
print(d.speak())

Python

Output

The parent class is not changed. Other subclasses or instances of Animal still see the original method.

What will be the output?

class Animal:
	def speak(self):
		return "sound"
 
class Cat(Animal):
	def speak(self):
		return "meow"
 
a = Animal()
print(a.speak())

Python

When a subclass defines its own __init__, the parent's setup does not run automatically. Use super() to call it.

super().__init__(name) runs the parent's __init__, so self.name gets set:

class Animal:
	def __init__(self, name):
		self.name = name
 
class Dog(Animal):
	def __init__(self, name, breed):
		super().__init__(name)
		self.breed = breed
 
d = Dog("Rex", "lab")
print(d.name, d.breed)

Python

Output

What will be the output?

class A:
	def __init__(self, x):
		self.x = x
 
class B(A):
	def __init__(self, x, y):
		super().__init__(x)
		self.y = y
 
b = B(1, 2)
print(b.x + b.y)

Python

Without super(), the parent setup is skipped and you lose those attributes.

Forgetting super().__init__() means self.name never gets set:

class Animal:
	def __init__(self, name):
		self.name = name
 
class Dog(Animal):
	def __init__(self, breed):
		self.breed = breed
 
d = Dog("lab")
print(hasattr(d, "name"))

Python

Output

super() works for any method, not just __init__. Use it when you want to extend a method instead of replacing it.

The subclass calls the parent's describe and adds extra detail:

class Animal:
	def describe(self):
		return "an animal"
 
class Dog(Animal):
	def describe(self):
		base = super().describe()
		return base + " that barks"
 
d = Dog()
print(d.describe())

Python

Output

What will be the output?

class Greeter:
	def hi(self):
		return "hi"
 
class Loud(Greeter):
	def hi(self):
		return super().hi() + "!"
 
l = Loud()
print(l.hi())

Python

What will be the output?

class Base:
	def __init__(self, n):
		self.n = n
 
class Sub(Base):
	def __init__(self, n, m):
		super().__init__(n)
		self.m = m
 
s = Sub(10, 5)
print(s.n - s.m)

Python

What will be the output?

class P:
	def tag(self):
		return "P"
 
class C(P):
	def tag(self):
		return "C"
 
p = P()
c = C()
print(p.tag(), c.tag())

Python

What will be the output?

class Shape:
	def __init__(self, sides):
		self.sides = sides
 
class Square(Shape):
	def __init__(self):
		super().__init__(4)
 
s = Square()
print(s.sides)

Python

What will be the output?

class Item:
	def label(self):
		return "item"
 
class Book(Item):
	def label(self):
		return super().label() + ":book"
 
b = Book()
print(b.label())

Python