Inheritance
Build new classes on top of existing ones and override their behaviour.
Inheritance lets one class reuse the code of another. The new class (the child) gets all the attributes and methods of the existing class (the parent) and can add or change things.
Python
class Animal:
def __init__(self, name):
self.name = name
def speak(self):
return "..."
def introduce(self):
return f"I am {self.name} and I say {self.speak()}"
class Cat(Animal):
def speak(self):
return "meow"
class Cow(Animal):
def speak(self):
return "moo"
for pet in [Cat("Kitty"), Cow("Gauri"), Animal("Mystery")]:
print(pet.introduce())Output
I am Kitty and I say meow I am Gauri and I say moo I am Mystery and I say ...
Cat and Cow override speak(), but they reuse __init__ and introduce() from Animal. Calling the same method on different types and getting type-specific behaviour is called polymorphism.
Extending the parent with super()
When the child needs extra data, call the parent's constructor with super() and then add the new parts.
Python
class Employee:
def __init__(self, name, salary):
self.name = name
self.salary = salary
def yearly_pay(self):
return self.salary * 12
class Manager(Employee):
def __init__(self, name, salary, bonus):
super().__init__(name, salary)
self.bonus = bonus
def yearly_pay(self):
return super().yearly_pay() + self.bonus
e = Employee("Ravi", 50000)
m = Manager("Anita", 80000, 100000)
print(e.name, e.yearly_pay())
print(m.name, m.yearly_pay())Output
Ravi 600000 Anita 1060000
Checking types
isinstance() checks whether an object belongs to a class, including any parent class.
Python
class Employee:
pass
class Manager(Employee):
pass
e = Employee()
m = Manager()
print(isinstance(m, Manager))
print(isinstance(m, Employee))
print(isinstance(e, Manager))Output
True True False
A Manager is an Employee, but not every Employee is a Manager.