Method overriding in Python - Step-by-Step Execution

Concept Flow - Method overriding

Define Parent class with method

↓

Define Child class overriding method

↓

Create Child object

↓

Call method on Child object

↓

Child's method runs, not Parent's

↓

End

This flow shows how a child class replaces a parent's method with its own when called on a child object.

Execution Sample

Python

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

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

c = Child()
c.greet()

This code shows a child class overriding a method from its parent and calling the child's version.

Execution Table

Step	Action	Evaluation	Output
1	Define class Parent with method greet	Parent.greet defined
2	Define class Child overriding greet	Child.greet defined
3	Create object c of class Child	c is Child instance
4	Call c.greet()	Calls Child.greet	Hello from Child
5	End of execution	No more code

💡 Program ends after calling overridden method on Child instance

Variable Tracker

Variable	Start	After Step 3	Final
c	undefined	Child instance created	Child instance

Key Moments - 2 Insights

Why does calling c.greet() run Child's method, not Parent's?

Because c is an instance of Child, and Child has its own greet method that overrides Parent's. See execution_table step 4 where Child.greet is called.

What if Child did not have greet method?

Then calling c.greet() would run Parent's greet method, since Child inherits it. This is shown by the absence of Child.greet in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is printed at step 4?

A "Hello from Parent"

B "Hello from Child"

C Nothing is printed

D Error occurs

💡 Hint

Check the Output column at step 4 in execution_table

At which step is the Child object created?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the Action column describing object creation in execution_table

If Child did not override greet, what would c.greet() call?

A No method found error

B Child.greet

C Parent.greet

D It would call a random method

💡 Hint

Refer to key_moments answer about inheritance when method is not overridden

Concept Snapshot

Method overriding lets a child class replace a parent's method.
Syntax: define method with same name in child class.
When called on child object, child's method runs.
If child lacks method, parent's runs.
Used to customize behavior in subclasses.

Full Transcript

Method overriding happens when a child class defines a method with the same name as one in its parent class. When you create an object of the child class and call that method, the child's version runs instead of the parent's. In the example, the Parent class has a greet method that prints 'Hello from Parent'. The Child class overrides greet to print 'Hello from Child'. When we create a Child object and call greet, it prints the child's message. This shows how overriding changes behavior in subclasses. If the child did not override greet, calling greet on the child object would run the parent's method instead.

Practice

1.

What is method overriding in Python?

Choose the best description.

easy

A. A child class provides a new version of a method from its parent class.

B. A method that is called automatically when an object is created.

C. A method that cannot be changed once defined in a class.

D. A method that is only accessible inside the class it is defined.

Solution

1. Step 1: Understand method overriding concept

   Method overriding means a child class changes a method from its parent by defining a method with the same name.

2. Step 2: Match description to concept

   A child class provides a new version of a method from its parent class. correctly describes this behavior, while others describe different concepts.

3. Final Answer:

   A child class provides a new version of a method from its parent class. -> Option A

4. Quick Check:

   Method overriding = child changes parent method [OK]

Hint: Child class method with same name replaces parent method [OK]

Common Mistakes:

• Confusing overriding with overloading
• Thinking overriding means creating a new method
• Believing methods cannot be changed in child classes

2.

Which of the following is the correct way to override a method greet in a child class?

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

class Child(Parent):
    # What goes here?

easy

A. def greet():
print("Hello from Child")

B. def greet(self):
print("Hello from Child")

C. def greet(self, name):
print(f"Hello {name} from Child")

D. def greet(self):
return "Hello from Child"

Solution

1. Step 1: Check method signature for overriding

   The child method must have the same name and parameters as the parent method to override it properly.

2. Step 2: Compare options

   def greet(self): print("Hello from Child") matches the parent's method signature exactly. def greet(): print("Hello from Child") misses 'self', C adds a parameter, and D returns a string instead of printing.

3. Final Answer:

   def greet(self):\n print("Hello from Child") -> Option B

4. Quick Check:

   Same method name and parameters = correct override [OK]

Hint: Match method name and parameters exactly to override [OK]

Common Mistakes:

• Omitting 'self' parameter in method
• Changing method parameters when overriding
• Returning value instead of matching parent's behavior

3.

What will be the output of the following code?

class Animal:
    def sound(self):
        print("Some sound")

class Dog(Animal):
    def sound(self):
        print("Bark")

pet = Dog()
pet.sound()

medium

A. Some sound

B. None

C. Error: Method not found

D. Bark

Solution

1. Step 1: Identify method overriding in Dog class

   Dog class overrides the sound() method from Animal to print "Bark" instead of "Some sound".

2. Step 2: Check which method is called

   When pet.sound() is called, it uses Dog's version, printing "Bark".

3. Final Answer:

   Bark -> Option D

4. Quick Check:

   Child method called = overridden output [OK]

Hint: Child method replaces parent method output [OK]

Common Mistakes:

• Expecting parent method output instead of child's
• Thinking method call causes error
• Confusing print output with return value

4.

Find the error in this code that tries to override a method:

class Vehicle:
    def start(self):
        print("Vehicle started")

class Car(Vehicle):
    def start():
        print("Car started")

c = Car()
c.start()

medium

A. Missing 'self' parameter in Car's start method

B. Car class should not inherit Vehicle

C. Parent method start() is private and cannot be overridden

D. Calling start() without parentheses

Solution

1. Step 1: Check method signature in child class

   Car's start method is missing the 'self' parameter, which is required for instance methods.

2. Step 2: Understand impact of missing 'self'

   Without 'self', Python treats start as a static method, causing a TypeError when called on an instance.

3. Final Answer:

   Missing 'self' parameter in Car's start method -> Option A

4. Quick Check:

   Instance methods need 'self' parameter [OK]

Hint: Always include 'self' as first parameter in instance methods [OK]

Common Mistakes:

• Forgetting 'self' in child method
• Thinking inheritance causes error
• Misunderstanding method call syntax

5.

Given the classes below, what will be the output when c.describe() is called?

class Parent:
    def describe(self):
        print("Parent description")

class Child(Parent):
    def describe(self):
        print("Child description")
        super().describe()

c = Child()
c.describe()

hard

A. Child description

B. Parent description\nChild description

C. Child description\nParent description

D. Error: super() used incorrectly

Solution

1. Step 1: Understand method overriding with super()

   Child's describe() overrides Parent's but calls super().describe() to run parent's method too.

2. Step 2: Trace method calls

   Calling c.describe() prints "Child description" first, then calls Parent's describe() printing "Parent description".

3. Final Answer:

   Child description\nParent description -> Option C

4. Quick Check:

   super() calls parent method after child override [OK]

Hint: super() calls parent method inside child override [OK]

Common Mistakes:

• Expecting only child's print output
• Thinking super() causes error
• Confusing order of prints