Classes and OOP II

Inheritance

In OOP I you defined a class from scratch every time. When two classes share a lot of behaviour, you can have one inherit from the other and reuse what is already there.

In OOP I you wrote each class on its own. When two classes share most of their behaviour, you can have one inherit from the other and reuse what is already there.

Pass the parent class in parentheses in the class header:

class Animal:
	pass
 
class Dog(Animal):
	pass

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Here Dog is the subclass and Animal is the parent. The subclass picks up everything the parent has.

Attributes defined on the parent are available on the subclass instance:

class Animal:
	kingdom = "Animalia"
 
class Dog(Animal):
	pass
 
d = Dog()
print(d.kingdom)

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Output

What will be the output?

class Vehicle:
	wheels = 4
 
class Truck(Vehicle):
	pass
 
t = Truck()
print(t.wheels)

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Methods are inherited the same way. The subclass can call any method the parent defines.

The describe method lives on the parent. The subclass uses it without redefining it:

class Animal:
	def __init__(self, name):
		self.name = name
 
	def describe(self):
		return f"I am {self.name}"
 
class Cat(Animal):
	pass
 
c = Cat("Whiskers")
print(c.describe())

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Output

What will be the output?

class Greeter:
	def hi(self):
		return "hello"
 
class Friend(Greeter):
	pass
 
f = Friend()
print(f.hi())

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A subclass can also add its own methods on top of what it inherits. The parent stays untouched.

Dog inherits describe and adds a bark method of its own:

class Animal:
	def describe(self):
		return "an animal"
 
class Dog(Animal):
	def bark(self):
		return "woof"
 
d = Dog()
print(d.describe())
print(d.bark())

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Output

What will be the output?

class Bird:
	def sound(self):
		return "tweet"
 
class Owl(Bird):
	def hunt(self):
		return "mouse"
 
o = Owl()
print(o.sound(), o.hunt())

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Remember isinstance() from OOP I? It also returns True when you check against the parent class.

A Dog is also an Animal:

class Animal:
	pass
 
class Dog(Animal):
	pass
 
d = Dog()
print(isinstance(d, Dog))
print(isinstance(d, Animal))

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Output

What will be the output?

class Shape:
	pass
 
class Circle(Shape):
	pass
 
c = Circle()
print(isinstance(c, Shape))

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What will be the output?

class A:
	x = 1
 
class B(A):
	x = 2
 
b = B()
print(b.x)

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What will be the output?

class Person:
	def __init__(self, name):
		self.name = name
 
class Student(Person):
	pass
 
s = Student("Ada")
print(s.name)

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What will be the output?

class Tool:
	def use(self):
		return "use"
 
class Hammer(Tool):
	def hit(self):
		return "bang"
 
h = Hammer()
print(h.use(), h.hit())

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