Purpose of constructors in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constructor in Python?

A constructor is a special method in a class that runs automatically when an object is created. It usually sets up the initial state of the object.

beginner

Why do we use constructors in classes?

We use constructors to initialize an object's properties with values right when the object is created, so it is ready to use.

beginner

What is the name of the constructor method in Python?

The constructor method in Python is called __init__ (double underscore init double underscore).

intermediate

How does a constructor help when creating multiple objects?

A constructor lets you create many objects with different starting values easily, without writing extra code to set each property after creating the object.

intermediate

Can a class have more than one constructor in Python?

Python does not support multiple constructors directly, but you can use default values or class methods to simulate different ways to create objects.

What does the constructor method __init__ do in a Python class?

A Initializes the object when it is created

B Deletes the object

C Prints the object

D Runs only when the program ends

The __init__ method runs automatically to set up the object when it is created.

When is the constructor called in Python?

A When the class is defined

B When a method is called

C When the program finishes

D When an object of the class is created

The constructor runs automatically right after an object is created.

Which of these is the correct way to define a constructor in Python?

A def __init__(self):

B def constructor(self):

C def start(self):

D def create(self):

The constructor method must be named __init__ with self as the first parameter.

What happens if you do not define a constructor in your Python class?

A Python will give an error

B The program will crash

C Python uses a default constructor that does nothing

D The class cannot create objects

Python provides a default constructor that does nothing if you don't define one.

How can you give default values to constructor parameters in Python?

A By using a separate method

B By assigning values in the __init__ method parameters

C By calling the constructor twice

D By creating multiple __init__ methods

You can assign default values to parameters in the __init__ method to make them optional.

Explain in your own words what a constructor does in a Python class.

Think about what happens right after you make a new object.

Why is it useful to have a constructor when creating multiple objects from the same class?

Consider how you would set up each object without a constructor.

Practice

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

easy

A. To print information about the class

B. To delete objects when they are no longer needed

C. To initialize new objects with starting values

D. To create new functions inside the class

Solution

1. Step 1: Understand what a constructor does

   A constructor is a special method that runs when a new object is created.

2. Step 2: Identify the purpose of initialization

   It sets up the object with initial values so it is ready to use.

3. Final Answer:

   To initialize new objects with starting values -> Option C

4. Quick Check:

   Constructor = initialize objects [OK]

Hint: Constructors set starting values when creating objects [OK]

Common Mistakes:

• Confusing constructors with methods that delete objects
• Thinking constructors print information automatically
• Believing constructors create new functions

2. Which of the following is the correct way to define a constructor in a Python class?

easy

A. def __init__(self):

B. def constructor(self):

C. def init(self):

D. def __start__(self):

Solution

1. Step 1: Recall Python constructor syntax

   Python uses a special method named __init__ to define constructors.

2. Step 2: Match the exact method name

   The method must be named exactly __init__ with double underscores before and after.

3. Final Answer:

   def __init__(self): -> Option A

4. Quick Check:

   Constructor method = __init__ [OK]

Hint: Constructor method is always named __init__ in Python [OK]

Common Mistakes:

• Using 'constructor' or 'init' without underscores
• Using wrong method names like __start__
• Forgetting double underscores before and after init

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. Buddy says Woof!

B. Woof!

C. my_dog says Woof!

D. Error: missing argument

Solution

1. Step 1: Understand the constructor usage

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

2. Step 2: Check the bark method output

   bark returns a string with self.name followed by "says Woof!" so it returns "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option A

4. Quick Check:

   Constructor sets name, bark uses it [OK]

Hint: Constructor sets name; bark prints name with Woof [OK]

Common Mistakes:

• Ignoring the name argument in constructor
• Expecting bark to print only 'Woof!'
• Thinking my_dog is printed instead of its name

4. Identify the error in this class definition:

class Car:
    def __init__(self, model):
        model = model

my_car = Car("Tesla")
print(my_car.model)

medium

A. The print statement should be print(model)

B. The constructor name is incorrect

C. The class is missing a return statement

D. The constructor does not assign model to self.model

Solution

1. Step 1: Check constructor assignment

   The constructor assigns model to local variable model, not to self.model.

2. Step 2: Understand attribute access

   Without self.model, the object has no model attribute, causing an error on print.

3. Final Answer:

   The constructor does not assign model to self.model -> Option D

4. Quick Check:

   Use self.model = model to store attribute [OK]

Hint: Always assign to self.attribute inside __init__ [OK]

Common Mistakes:

• Assigning to local variable instead of self.attribute
• Thinking constructor name is wrong
• Expecting print(model) to work outside class

5. You want to create a class Book that stores title and author. Which constructor correctly initializes these attributes and allows creating a Book object with both values?

hard

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

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

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

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

Solution

1. Step 1: Check parameters needed

   Both title and author must be passed to the constructor to initialize attributes.

2. Step 2: Verify attribute assignment

   Constructor must assign both self.title and self.author from parameters.

3. Final Answer:

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

4. Quick Check:

   Constructor with all attributes assigned = def __init__(self, title, author): self.title = title self.author = author [OK]

Hint: Constructor needs all attributes as parameters and assigns them [OK]

Common Mistakes:

• Missing parameters for all attributes
• Assigning attributes without parameters
• Using local variables instead of self attributes