Basic Inheritance & Hierarchy

Inheritance is a fundamental pillar of Object-Oriented Programming that allows you to create new classes based on existing ones. It facilitates code reuse by allowing a Subclass (Child) to inherit the attributes and behaviors of a Base Class (Parent).

In Python, inheritance is not just about copying code; it’s about establishing a logical hierarchy where specialized objects can leverage and extend the functionality of more general ones.

1. The “is-a” Relationship

Inheritance should only be used when there is a clear “is-a” relationship between the two entities.

A Manager is a Employee.
A Circle is a Shape.
A Smartphone is a Device.

class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def get_details(self):
        return f"{self.name} earns ${self.salary}"

# Manager inherits from Employee
class Manager(Employee):
    def plan_meeting(self):
        print(f"{self.name} is planning a meeting.")

m = Manager("Alice", 90000)
print(m.get_details()) # Inherited from Employee

2. Extending Logic with `super()`

Often, you don’t want to completely replace a parent’s method; you want to run the parent’s logic and then add your own. The super() function allows you to call methods from the parent class without explicitly naming it.

class Manager(Employee):
    def __init__(self, name, salary, department):
        # Let the parent class handle name and salary
        super().__init__(name, salary)
        # Add the manager-specific attribute
        self.department = department

    def get_details(self):
        # Call the parent's version and append more info
        basic_info = super().get_details()
        return f"{basic_info} and manages {self.department}"

3. Introspection: `isinstance` and `issubclass`

Because of inheritance, an object can be an instance of multiple classes.

m = Manager("Alice", 90000, "HR")

print(isinstance(m, Manager))  # True
print(isinstance(m, Employee)) # True (Because Manager is an Employee)
print(isinstance(m, object))   # True (Everything is an object in Python)

print(issubclass(Manager, Employee)) # True

4. Under the Hood: The `bases` Attribute

How does Python know which class is the parent? Every class has a hidden attribute called __bases__ that stores a tuple of its parent classes.

print(Manager.__bases__)
# Output: (<class '__main__.Employee'>,)

When you call a method like m.get_details(), Python first looks in the Manager class. If it’s not there, it checks the classes listed in __bases__.

5. Summary Table

Term	Meaning
Base Class	The parent class being inherited from.
Subclass	The child class that is inheriting.
Overriding	Replacing a parent’s method with a child’s version.
Extending	Using `super()` to build upon a parent’s method.
Object	The root of all classes in Python 3.