Python Inheritance

Python Inheritance: Subclassing, super(), & MRO

Inheritance allows a child class (subclass) to inherit attributes and methods from a parent class (superclass), promoting code reuse and polymorphic method overriding.

Method Resolution Order (MRO)

flowchart TD
    Base["Base Class (Animal)"] --> Child1["Subclass 1 (Dog)"]
    Base --> Child2["Subclass 2 (Cat)"]
    Child1 --> Overridden["Method Override (.speak())"]
    Child2 --> Overridden2["Method Override (.speak())"]

Key Inheritance Concepts

• super(): Returns a proxy object delegating method calls to a parent class.
• Method Overriding: Redefining a parent class method inside a subclass.
• Class.mro(): Inspects the Method Resolution Order algorithm (C3 Linearization) used by CPython to resolve class attributes.

Practical Code Example

from typing import List

class User:
    def __init__(self, username: str, email: str) -> None:
        self.username = username
        self.email = email

    def get_permissions(self) -> List[str]:
        return ["read_content"]

    def __repr__(self) -> str:
        return f"User({self.username!r})"

class AdminUser(User):
    """Subclass extending Base User with elevated administrative roles."""
    def __init__(self, username: str, email: str, admin_level: int) -> None:
        super().__init__(username, email)  # Invoke parent constructor cleanly
        self.admin_level = admin_level

    # Overriding method
    def get_permissions(self) -> List[str]:
        base_permissions = super().get_permissions()
        return base_permissions + ["write_content", "delete_users", "manage_system"]

if __name__ == "__main__":
    standard_user = User("alice", "[email protected]")
    admin_user = AdminUser("bob_admin", "[email protected]", admin_level=1)

    print(f"Standard Permissions: {standard_user.get_permissions()}")
    print(f"Admin Permissions: {admin_user.get_permissions()}")
    print("Method Resolution Order (MRO):", AdminUser.mro())

Best Practices & Gotchas

• Always Call super().__init__(): In subclasses, call super().__init__(...) to ensure parent state is properly initialized.
• Prefer Composition Over Inheritance: If relationship is "has-a" rather than "is-a", use object composition instead of deep inheritance hierarchies.
• Check Subclassing with issubclass(): Use issubclass(Child, Parent) or isinstance(obj, Parent) for dynamic type verification.

Self-Check Challenge

Create a base class Vehicle(max_speed) and a subclass ElectricCar(max_speed, battery_capacity) that calls super().__init__() and adds a charge() method.