Inheritance

Why this matters

Often one kind of thing is a more specific version of another: a manager is an employee, a square is a shape. Inheritance lets a new class start from an existing one and add or change only what is different, without copying code.

The idea

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        return f"{self.name} makes a sound"

class Dog(Animal):                       # Dog inherits from Animal
    def __init__(self, name, trick):
        super().__init__(name)           # run Animal's __init__ first
        self.trick = trick

    def speak(self):                     # override: replaces Animal's version
        return f"{self.name} barks"

• super() reaches the parent’s version of a method.
• A Dog can use every Animal method it does not override.
• isinstance(rex, Animal) is True for a Dog; issubclass(Dog, Animal) is True.
• Polymorphism: code written for Animal works for any subclass, and each one answers speak() in its own way.

Use inheritance only for a true “is-a” relationship, and keep hierarchies shallow.

Try it

Add a Cat(Animal) whose speak returns "... meows". Loop over [Dog("Rex", "sit"), Cat("Tom")] and call speak() on each.

Your turn

Write:

• Employee(name, monthly_salary) with annual_pay() (twelve months of salary) and describe() returning text like Ann earns 36000.00 a year (two decimals).
• Manager(Employee) with an extra reports list (given as an optional third argument; each manager gets their own list). A manager’s annual_pay() is the normal pay plus 1000 for each report. Do not rewrite describe: it should work for both classes.
• total_payroll(people): the sum of annual_pay() for a list of employees and managers.

Recap

• class Child(Parent): inherits everything; override methods to change behaviour.
• super() calls the parent’s version.
• Code that works with the parent type works with every child (polymorphism).

Common mistakes

• Forgetting super().__init__(...), so the parent’s attributes are never set (AttributeError).
• Copying the parent’s code into the child instead of calling super().
• Inheriting for convenience when the relationship is not really “is a” (use composition, next lesson).