Method overriding in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is method overriding in Python?

Method overriding occurs when a subclass provides a specific implementation of a method that is already defined in its superclass. The subclass's method replaces the superclass's method when called on an instance of the subclass.

beginner

Why do we use method overriding?

We use method overriding to change or extend the behavior of a method inherited from a parent class, allowing subclasses to customize or improve functionality without changing the parent class.

beginner

Example of method overriding in Python:<br><pre>class Animal:
    def sound(self):
        return "Some sound"

class Dog(Animal):
    def sound(self):
        return "Bark"</pre><br>What will <code>Dog().sound()</code> return?

It will return "Bark" because the Dog class overrides the sound method of the Animal class with its own version.

intermediate

Can method overriding be used to call the parent class method as well?

Yes. Inside the overriding method, you can call the parent class method using <code>super().method_name()</code> to reuse or extend the original behavior.

beginner

What happens if a subclass does not override a method from its superclass?

If a subclass does not override a method, it inherits the method from the superclass and uses that implementation when the method is called.

What does method overriding allow a subclass to do?

A Create a new method with a different name

B Prevent the superclass method from being called

C Provide a new version of a method already defined in the superclass

D Change the superclass's source code

Method overriding lets a subclass provide its own version of a method that exists in the superclass.

How do you call the parent class method inside an overriding method?

A Using <code>override.method_name()</code>

B Using <code>self.method_name()</code>

C Using <code>parent.method_name()</code>

D Using <code>super().method_name()</code>

The super() function is used to call the parent class method inside an overriding method.

If a subclass does not override a method, what happens when the method is called on the subclass instance?

A The method from the superclass is used

B An error occurs

C The method is ignored

D The program crashes

The subclass inherits the method from the superclass and uses it if it does not override it.

Which of the following is NOT true about method overriding?

A It requires changing the superclass code

B It supports polymorphism

C It allows customizing inherited methods

D It happens in subclass methods

Method overriding does NOT require changing the superclass code; it happens by defining a method with the same name in the subclass.

What will this code print?

class A:
    def greet(self):
        return 'Hello from A'

class B(A):
    def greet(self):
        return 'Hello from B'

print(B().greet())

A Hello from A

B Hello from B

C Error

D Hello from A and B

Class B overrides the greet method, so calling greet on B's instance returns 'Hello from B'.

Explain method overriding and why it is useful in object-oriented programming.

Think about how a child class can change a parent's method.

Describe how to call a parent class method from an overriding method in Python.

Use a built-in function to access the parent method.

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