CSCI 127 Joy and Beauty of Data · Unit 3: Object-oriented programming · on Exam 2

3.2 More object orientation: inheritance

Study notes for More object orientation: inheritance in CSCI 127 Joy and Beauty of Data at Montana State University: the big ideas, code you should know, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in StudyCat.

Big ideas

Inheritance: class Dog(Animal): makes Dog a kind of Animal. Dog gets every attribute and method of Animal and can add or replace some.
Overriding: define a method with the same name in the subclass to replace it. Call the parent version with super().method(), especially super().__init__(...) to set up inherited attributes.
isinstance(d, Animal) is True for a Dog. Python looks for a method on the object’s class first, then its parents.
Class attributes (defined in the class body) are shared by all objects; instance attributes (self.x) belong to one object. A class attribute is handy for counting objects.
Composition: an object can hold other objects (a Course holds a list of Students). Prefer “has a” (composition) unless something really “is a” (inheritance).

Code you should know

Subclass, override, super

class Animal:
    count = 0

    def __init__(self, name):
        self.name = name
        Animal.count += 1

    def speak(self):
        return "..."

    def describe(self):
        return f"{self.name} says {self.speak()}"

class Dog(Animal):
    def speak(self):
        return "Woof"

class Puppy(Dog):
    def __init__(self, name, age_weeks):
        super().__init__(name)
        self.age_weeks = age_weeks

    def speak(self):
        return super().speak() + "!"

pets = [Animal("Rock"), Dog("Rex"), Puppy("Bit", 8)]
for p in pets:
    print(p.describe())
print(Animal.count, isinstance(pets[2], Dog), isinstance(pets[1], Puppy))

Output

Rock says ...
Rex says Woof
Bit says Woof!
3 True False

Composition: objects inside objects

class Student:
    def __init__(self, name, score):
        self.name = name
        self.score = score

class Course:
    def __init__(self, title):
        self.title = title
        self.students = []

    def add(self, student):
        self.students.append(student)

    def average(self):
        return sum(s.score for s in self.students) / len(self.students)

    def top(self):
        return max(self.students, key=lambda s: s.score).name

c = Course("CSCI 127")
c.add(Student("Ana", 91))
c.add(Student("Ben", 84))
print(c.title, len(c.students), c.average(), c.top())

Output

CSCI 127 2 87.5 Ana

Worked example

What does this print?

class A:
    def hi(self):
        return "A"
class B(A):
    def hi(self):
        return "B" + super().hi()
class C(B):
    pass

print(C().hi(), isinstance(C(), A))

Show the solution

BA True. C has no hi, so B’s runs; it adds "B" to A’s result. Every C is a B, and every B is an A.

Common mistakes

Forgetting super().__init__(), so inherited attributes never get set (AttributeError later).
Modifying a class attribute through self.count += 1, which creates an instance attribute instead.
Using inheritance for “has a” relationships.

Exam tipBe ready to write a small class hierarchy on paper and to state, for a given call, which class’s method runs.

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