Purpose of constructors in Python - Time & Space Complexity

Time Complexity: Purpose of constructors

O(n)

Understanding Time Complexity

We want to understand how the time it takes to create objects grows as we create more of them.

How does the constructor's work change when we make many objects?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

cars = []
n = 10
for i in range(n):
    cars.append(Car('Toyota', 'Corolla'))

This code creates a list of n car objects, each initialized with the same make and model.

Identify Repeating Operations

• Primary operation: Calling the constructor Car() inside the loop.
• How many times: Exactly n times, once for each object created.

How Execution Grows With Input

Each new object requires the constructor to run once, so the total work grows directly with the number of objects.

Input Size (n)	Approx. Operations
10	10 constructor calls
100	100 constructor calls
1000	1000 constructor calls

Pattern observation: The work grows in a straight line as we add more objects.

Final Time Complexity

Time Complexity: O(n)

This means the time to create all objects grows directly in proportion to how many objects we make.

Common Mistake

[X] Wrong: "Creating many objects is always instant and does not depend on how many we make."

[OK] Correct: Each object needs its constructor to run, so more objects mean more work and more time.

Interview Connect

Understanding how constructors affect time helps you explain how object creation scales in programs, a useful skill when designing efficient code.

Self-Check

"What if the constructor did a complex calculation inside? How would that change the time complexity?"

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