Class definition syntax in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple class attribute access

What is the output of this Python code?

Python

class Dog:
    sound = "bark"

print(Dog.sound)

A bark

B Sound

C dog

D None

💡 Hint

Look at how class attributes are accessed directly from the class.

The class Dog has a class attribute 'sound' set to 'bark'. Accessing Dog.sound prints 'bark'.

❓ Predict Output

intermediate

Output of instance method call

What is the output of this Python code?

Python

class Cat:
    def meow(self):
        return "meow"

c = Cat()
print(c.meow())

A meow

B Meow

C None

D TypeError

💡 Hint

Instance methods need to be called on an object instance.

The method meow returns the string 'meow'. Calling c.meow() prints 'meow'.

❓ Predict Output

advanced

Output when __init__ is missing

What happens when you run this code?

Python

class Bird:
    def fly(self):
        return "flying"

b = Bird()
print(b.fly())

A None

B TypeError: missing 1 required positional argument

C AttributeError: 'Bird' object has no attribute 'fly'

D flying

💡 Hint

If __init__ is missing, Python uses a default constructor.

Python provides a default __init__ if none is defined, so b = Bird() works and b.fly() returns 'flying'.

❓ Predict Output

advanced

Output of class with instance variable

What is the output of this code?

Python

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

f = Fish("Nemo")
print(f.name)

A AttributeError

B Fish

C Nemo

D name

💡 Hint

Instance variables are set inside __init__ and accessed via self.

The instance variable 'name' is set to 'Nemo' during object creation and printed.

❓ Predict Output

expert

Output of class with method overriding and super()

What is the output of this code?

Python

class Animal:
    def speak(self):
        return "sound"

class Dog(Animal):
    def speak(self):
        return super().speak() + " bark"

d = Dog()
print(d.speak())

A bark

B sound bark

C sound

D TypeError

💡 Hint

super() calls the parent class method inside the child method.

Dog's speak calls Animal's speak via super(), returning 'sound', then adds ' bark'.

Practice

1. What keyword is used to define a class in Python?

easy

A. def

B. class

C. function

D. object

Solution

1. Step 1: Identify the keyword for class definition

   In Python, the keyword class is used to start a class definition.

2. Step 2: Differentiate from function and other keywords

   def defines functions, function and object are not Python keywords for class definition.

3. Final Answer:

   class -> Option B

4. Quick Check:

   Class keyword = class [OK]

Hint: Remember: classes start with 'class' keyword [OK]

Common Mistakes:

• Using def instead of class
• Confusing function keyword with class
• Trying to use object keyword

2. Which of the following is the correct syntax to define a class named Car?

easy

A. class Car:

B. class Car()

C. def Car():

D. class Car[]:

Solution

1. Step 1: Check class header syntax

   Python allows defining a class with or without parentheses if no base class is specified. So class Car: is correct.

2. Step 2: Identify incorrect options

   def Car(): defines a function, not a class. class Car() is valid syntax but less common; however, it requires a colon at the end. class Car[]: is invalid syntax.

3. Final Answer:

   class Car: -> Option A

4. Quick Check:

   Class header ends with colon, no brackets [OK]

Hint: Class header ends with colon, no brackets needed [OK]

Common Mistakes:

• Using def instead of class
• Adding square brackets [] in class header
• Omitting colon at end

3. What will be the output of this code?

class Dog:
    def __init__(self, name):
        self.name = name
    def bark(self):
        return f"{self.name} says Woof!"

my_dog = Dog("Buddy")
print(my_dog.bark())

medium

A. Dog says Woof!

B. Woof!

C. Buddy says Woof!

D. Error: missing self parameter

Solution

1. Step 1: Understand the __init__ method

   The __init__ method sets self.name to "Buddy" when my_dog is created.

2. Step 2: Analyze the bark method output

   The bark method returns a string using self.name, so it returns "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option C

4. Quick Check:

   Method uses self.name = Buddy [OK]

Hint: Methods use self to access object data [OK]

Common Mistakes:

• Ignoring self and expecting just 'Woof!'
• Confusing class name with instance name
• Forgetting to pass name argument

4. Find the error in this class definition:

class Person:
    def __init__(name):
        self.name = name

p = Person("Alice")

medium

A. self.name should be name.name

B. Missing colon after class name

C. Incorrect object creation syntax

D. Missing self parameter in __init__ method

Solution

1. Step 1: Check __init__ method parameters

   The first parameter of instance methods must be self. Here, __init__ lacks self.

2. Step 2: Confirm other syntax correctness

   Class header has colon, object creation syntax is correct, and self.name assignment is proper.

3. Final Answer:

   Missing self parameter in __init__ method -> Option D

4. Quick Check:

   Instance methods need self as first parameter [OK]

Hint: Always include self as first method parameter [OK]

Common Mistakes:

• Omitting self in methods
• Forgetting colon after class name
• Misusing self in attribute assignment

5. You want to create a class Book that stores title and author. Which is the best way to define the __init__ method to set these attributes?

hard

A. def __init__(self, title, author):
self.title = title
self.author = author

B. def __init__(title, author):
self.title = title
self.author = author

C. def __init__(self):
title = None
author = None

D. def __init__(self, title, author):
title = self.title
author = self.author

Solution

1. Step 1: Define __init__ with self and parameters

   The method must have self as first parameter, then title and author to receive values.

2. Step 2: Assign parameters to object attributes

   Use self.title = title and self.author = author to store values in the object.

3. Final Answer:

   def __init__(self, title, author): self.title = title self.author = author -> Option A

4. Quick Check:

   Init method sets attributes using self [OK]

Hint: Use self.param = param to store values in __init__ [OK]

Common Mistakes:

• Omitting self parameter
• Assigning attributes backwards
• Not passing parameters to __init__