Class definition syntax in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax to define a class in Python?

Use the <code>class</code> keyword followed by the class name and a colon. Inside, indent the class body.<br><br>Example:<br><code>class MyClass:<br>    pass</code>

beginner

What is the purpose of the __init__ method in a class?

It is a special method called a constructor. It runs automatically when you create an object from the class. It is used to set up initial values.

beginner

How do you create an object from a class named <code>Car</code>?

Call the class name like a function:<br><code>my_car = Car()</code><br>This creates a new object called <code>my_car</code>.

intermediate

What is the role of the <code>self</code> parameter in class methods?

self represents the instance of the class. It lets methods access or change the object's own data.

beginner

Can a Python class have multiple methods? How are they defined?

Yes. Define each method inside the class with <code>def</code> and indent them. Each method should have <code>self</code> as the first parameter.<br><br>Example:<br><code>class Dog:<br>    def bark(self):<br>        print('Woof!')</code>

Which keyword is used to define a class in Python?

A func

B def

C object

D class

The class keyword is used to define a class.

What does the __init__ method do?

A Deletes an object

B Prints the class name

C Initializes a new object when the class is called

D Runs only when the program ends

The __init__ method runs automatically to set up a new object.

What is the first parameter of a method inside a class usually called?

A self

B this

C obj

D instance

By convention, the first parameter is named self to refer to the object.

How do you create an instance of a class named Person?

A person = class Person

B person = Person()

C person = new Person

D person = Person[]

You call the class like a function to create an object: Person().

Which of these is a correct method definition inside a class?

A def greet(self):

B function greet():

C def greet():

D method greet(self):

Methods inside classes use def and include self as the first parameter.

Explain how to define a simple class in Python and create an object from it.

Think about the basic structure and how you make an object.

Describe the role of the __init__ method and the self parameter in a class.

Focus on how objects start and how methods know which object they belong to.

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__