Parent and child classes in Python

Introduction

Parent and child classes help organize code by sharing common features. The child class can use or change what the parent class has.

When you want to create a new type that is similar to an existing one but with some changes.

When you want to reuse code from a general class to avoid repeating yourself.

When you want to group related objects that share common behavior.

When you want to add new features to an existing class without changing it.

When you want to create a clear relationship between general and specific things.

Syntax

Python

class ParentClass:
    def __init__(self, value):
        self.value = value

class ChildClass(ParentClass):
    def __init__(self, value, extra):
        super().__init__(value)
        self.extra = extra

The child class name is followed by parentheses containing the parent class name.

Use super() to call the parent class's methods inside the child class.

Examples

The Dog class inherits from Animal and changes the speak method.

Python

class Animal:
    def speak(self):
        print("Animal speaks")

class Dog(Animal):
    def speak(self):
        print("Dog barks")

The Car class adds a new property model but keeps brand from Vehicle.

Python

class Vehicle:
    def __init__(self, brand):
        self.brand = brand

class Car(Vehicle):
    def __init__(self, brand, model):
        super().__init__(brand)
        self.model = model

Sample Program

This program shows a parent class Person and a child class Student. The child class adds a student ID and changes the greeting.

Python

class Person:
    def __init__(self, name):
        self.name = name
    def greet(self):
        print(f"Hello, my name is {self.name}.")

class Student(Person):
    def __init__(self, name, student_id):
        super().__init__(name)
        self.student_id = student_id
    def greet(self):
        print(f"Hi, I'm {self.name} and my student ID is {self.student_id}.")

p = Person("Alice")
p.greet()
s = Student("Bob", "S12345")
s.greet()

Important Notes

If the child class does not have a method, it uses the parent's method automatically.

You can add new methods or properties in the child class that the parent does not have.

Summary

Parent classes hold common features for child classes.

Child classes inherit and can change or add features.

Use super() to access parent class methods inside child classes.

Practice

1. What is the main purpose of a parent class in Python?

easy

A. To hold common features that child classes can inherit

B. To override methods in child classes

C. To create instances directly without child classes

D. To prevent child classes from adding new features

Solution

1. Step 1: Understand the role of parent class

   A parent class is designed to hold common features like methods and attributes that multiple child classes can share.

2. Step 2: Compare options with this role

   To hold common features that child classes can inherit correctly states this purpose. Other options describe incorrect or unrelated roles.

3. Final Answer:

   To hold common features that child classes can inherit -> Option A

4. Quick Check:

   Parent class = common features [OK]

Hint: Parent class shares features for children to reuse [OK]

Common Mistakes:

• Thinking parent classes prevent changes in children
• Believing parent classes are only for direct instances
• Confusing overriding with inheritance

2. Which of the following is the correct syntax to define a child class Dog that inherits from a parent class Animal?

easy

A. class Dog extends Animal:

B. class Dog inherits Animal:

C. class Dog < Animal:

D. class Dog(Animal):

Solution

1. Step 1: Recall Python inheritance syntax

   In Python, a child class inherits from a parent class by placing the parent class name in parentheses after the child class name.

2. Step 2: Match options with correct syntax

   class Dog(Animal): uses class Dog(Animal): which is correct. Other options use incorrect keywords or symbols not valid in Python.

3. Final Answer:

   class Dog(Animal): -> Option D

4. Quick Check:

   Child class syntax = class Child(Parent): [OK]

Hint: Use parentheses with parent class name after child class [OK]

Common Mistakes:

• Using 'inherits' or 'extends' keywords (not Python)
• Using symbols like '<' instead of parentheses
• Omitting the colon at the end

3. What will be the output of this code?

class Parent:
    def greet(self):
        return "Hello from Parent"

class Child(Parent):
    def greet(self):
        return "Hello from Child"

obj = Child()
print(obj.greet())

medium

A. Hello from ParentChild

B. Hello from Parent

C. Hello from Child

D. Error: greet method not found

Solution

1. Step 1: Understand method overriding in child class

   The child class Child defines its own greet method, which replaces the parent's method when called on a child instance.

2. Step 2: Check which method is called

   Since obj is an instance of Child, calling obj.greet() uses the child's method, returning "Hello from Child".

3. Final Answer:

   Hello from Child -> Option C

4. Quick Check:

   Child method overrides parent method = Hello from Child [OK]

Hint: Child method overrides parent method when called on child [OK]

Common Mistakes:

• Thinking parent method runs instead of child's
• Expecting combined output from both methods
• Assuming method not found error

4. Find the error in this code:

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

class Child(Parent):
    def __init__(self, name, age):
        self.age = age

c = Child('Anna', 10)
print(c.name, c.age)

medium

A. Child class __init__ does not call Parent __init__, so name is missing

B. Syntax error in class definition

C. Cannot create Child instance with two arguments

D. print statement syntax is wrong

Solution

1. Step 1: Check constructor chaining in child class

   The child class __init__ method sets age but does not call super().__init__(name) to set name from the parent.

2. Step 2: Understand consequence of missing super call

   Because name is not set in Child, accessing c.name will cause an error or missing attribute.

3. Final Answer:

   Child class __init__ does not call Parent __init__, so name is missing -> Option A

4. Quick Check:

   Missing super() call = missing parent attributes [OK]

Hint: Always call super().__init__ in child __init__ to set parent attributes [OK]

Common Mistakes:

• Forgetting to call super().__init__ in child constructor
• Assuming parent attributes set automatically
• Confusing syntax errors with logic errors

5. Given these classes, what will print(c.describe()) output?

class Parent:
    def describe(self):
        return "I am a parent"

class Child(Parent):
    def describe(self):
        parent_desc = super().describe()
        return parent_desc + " and I am a child"

c = Child()

hard

A. I am a parent

B. I am a parent and I am a child

C. I am a child

D. Error: super() used incorrectly

Solution

1. Step 1: Understand super() usage in child describe method

   The child method calls super().describe() which runs the parent method returning "I am a parent".

2. Step 2: Combine parent and child strings

   The child method adds " and I am a child" to the parent's string, so the full return is "I am a parent and I am a child".

3. Final Answer:

   I am a parent and I am a child -> Option B

4. Quick Check:

   super() calls parent method, combined output [OK]

Hint: Use super() to add parent behavior inside child method [OK]

Common Mistakes:

• Expecting only child or only parent output
• Thinking super() causes error without arguments
• Ignoring string concatenation