Creating Classes & Objects (Review) in Python0%

Creating Classes & Objects (Review) in Python

Intermediate8 min read•Updated: 2026-10-01

Creating Classes & Objects (Review & Deep Dive)

Object-Oriented Programming (OOP) is a programming paradigm based on the concept of objects, which bundle state (attributes/data) and behavior (methods/functions) together. In Python, everything is an object—from simple integers and strings to complex modules and custom classes.

1. Classes vs. Instances

A class serves as a blueprint or schema defining the attributes and operations that instances will possess. An instance (or object) is a concrete manifestation created in memory based on that blueprint.

1class Developer:
2    """Blueprint for a software developer profile."""
3    pass
4 
5# Creating two distinct instances in memory
6dev_a = Developer()
7dev_b = Developer()
8 
9print(dev_a)  # <__main__.Developer object at 0x7f8...>
10print(dev_a == dev_b)  # False (different memory addresses)

2. The `init` Constructor and `self`

The __init__ method is Python's initialization hook called immediately after an instance is created in memory:

self: A reference to the specific instance currently being initialized or manipulated. Python automatically passes the instance as the first argument when calling instance methods.

1class Developer:
2    def __init__(self, name: str, primary_language: str, years_exp: int):
3        # Instance attributes bound to this specific object
4        self.name = name
5        self.primary_language = primary_language
6        self.years_exp = years_exp
7        
8    def describe(self) -> str:
9        return f"{self.name} specializes in {self.primary_language} with {self.years_exp} years of experience."
10 
11dev = Developer("Vikram", "Python", 4)
12print(dev.describe())
13# dev.describe() is syntactic sugar for: Developer.describe(dev)

3. Instance Attributes vs. Class Attributes

Understanding the distinction between class-level and instance-level attributes is critical to avoiding state bugs:

Instance Attributes: Bound to self. Each instance maintains its own distinct copy in its __dict__.
Class Attributes: Defined directly inside the class body, shared across all instances of that class.

1class Student:
2    # Class attribute (shared by all instances)
3    institution = "MSK Institute of Technology"
4    student_count = 0
5    
6    def __init__(self, name, roll_no):
7        # Instance attributes (unique to each student)
8        self.name = name
9        self.roll_no = roll_no
10        
11        # Incrementing the shared class counter
12        Student.student_count += 1
13 
14s1 = Student("Aarav", 101)
15s2 = Student("Ananya", 102)
16 
17print(s1.institution)  # MSK Institute of Technology
18print(s2.institution)  # MSK Institute of Technology
19print(Student.student_count)  # 2

The Mutable Class Attribute Trap: Never define mutable defaults (like lists or dictionaries) as class attributes if they are meant to be per-instance! ```python class Team: members = [] # BAD: Shared across all Team instances! class CorrectTeam: def __init__(self): self.members = [] # GOOD: Each team has its own list ```

4. Instance, Class, and Static Methods

Python provides three distinct method types using decorators:

Method Type	Decorator	First Argument	Typical Use Case
Instance Method	(None)	`self`	Manipulates individual instance state
Class Method	`@classmethod`	`cls`	Factory methods, modifying class-level state
Static Method	`@staticmethod`	(None)	Self-contained utility functions logically grouped in the class

1class Temperature:
2    def __init__(self, celsius: float):
3        self.celsius = celsius
4 
5    # 1. Instance Method
6    def to_fahrenheit(self) -> float:
7        return (self.celsius * 9/5) + 32
8 
9    # 2. Class Method (Alternative Constructor / Factory)
10    @classmethod
11    def from_fahrenheit(cls, fahrenheit: float):
12        celsius = (fahrenheit - 32) * 5/9
13        return cls(celsius)
14 
15    # 3. Static Method (Pure utility)
16    @staticmethod
17    def is_freezing(celsius: float) -> bool:
18        return celsius <= 0.0
19 
20# Using Instance Method
21t1 = Temperature(25)
22print(f"25°C in F: {t1.to_fahrenheit()}°F")
23 
24# Using Class Method Factory
25t2 = Temperature.from_fahrenheit(98.6)
26print(f"98.6°F in C: {t2.celsius:.1f}°C")
27 
28# Using Static Method
29print(f"Is -5°C freezing? {Temperature.is_freezing(-5)}")

5. String Representations: `str` vs `repr`

Every Python class should define informative string representations:

__str__: User-friendly, readable string representation (returned by print(obj) or str(obj)).
__repr__: Unambiguous, developer-oriented string used for debugging (returned by typing obj in interactive shell or repr(obj)). Ideally should look like valid Python code to recreate the object.

1class Book:
2    def __init__(self, title, author, isbn):
3        self.title = title
4        self.author = author
5        self.isbn = isbn
6 
7    def __str__(self):
8        return f"'{self.title}' by {self.author}"
9 
10    def __repr__(self):
11        return f"Book(title='{self.title}', author='{self.author}', isbn='{self.isbn}')"
12 
13book = Book("Fluent Python", "Luciano Ramalho", "978-1491946008")
14print(str(book))   # 'Fluent Python' by Luciano Ramalho
15print(repr(book))  # Book(title='Fluent Python', author='Luciano Ramalho', isbn='978-1491946008')

⚠️ Common Intermediate Mistakes & Gotchas

1. Confusing Class Attributes with Instance Attributes

A mutable class attribute (like a list) is shared across all instances of the class!

1# ❌ SHARED BUG: All students share the same grades list!
2class Student:
3    grades = []
4 
5# ✅ CORRECT: Each student gets an independent list in __init__
6class Student:
7    def __init__(self):
8        self.grades = []

2. Forgetting `self` in Method Signatures

Every instance method must take self as its first parameter; omitting it causes a TypeError: method takes 0 positional arguments but 1 was given.

💻 Try It Yourself: Product Inventory Item Class

Scenario

Create an Item class for an e-commerce store with attributes name, price, and quantity. Implement methods total_value(), apply_discount(pct), and a formatted __repr__() method.

Complete Solution

1class Item:
2    def __init__(self, name: str, price: float, quantity: int = 1):
3        if price < 0 or quantity < 0:
4            raise ValueError("Price and quantity must be non-negative.")
5        self.name = name
6        self.price = float(price)
7        self.quantity = int(quantity)
8 
9    def total_value(self) -> float:
10        return self.price * self.quantity
11 
12    def apply_discount(self, discount_pct: float):
13        self.price -= self.price * (discount_pct / 100.0)
14 
15    def __repr__(self) -> str:
16        return f"Item(name='{self.name}', price=₹{self.price:,.2f}, qty={self.quantity})"
17 
18# Testing
19phone = Item("Smartphone", 25000.0, 4)
20print(phone)
21print(f"Total Inventory Value: ₹{phone.total_value():,.2f}")
22phone.apply_discount(10)
23print(f"After 10% Discount: {phone}")

Expected Output

1Item(name='Smartphone', price=₹25,000.00, qty=4)
2Total Inventory Value: ₹100,000.00
3After 10% Discount: Item(name='Smartphone', price=₹22,500.00, qty=4)

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

What does the self parameter in a Python instance method represent?

Question 2

What happens if you define a mutable list as a class attribute and append items to it from an instance?

Question 3

Which decorator allows defining an alternative factory constructor that receives the class itself as cls?

Question 4

How does @staticmethod differ from regular instance methods and class methods?

Question 5

What is the standard purpose of the __repr__ special method in Python?

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Inheritance & Method Overriding (5: Object-Oriented Programming (Intermediate)).

👉 Continue to Next Lesson: Inheritance & Method Overriding →

Next Lesson

Inheritance & Method Overriding

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Project: CSV Contact Manager	Inheritance & Method Overriding

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

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Inheritance & Method Overriding

Creating Classes & Objects (Review) in Python

Intermediate8 min read•Updated: 2026-10-01

Study Materials Cheatsheet

Creating Classes & Objects (Review & Deep Dive)

1. Classes vs. Instances

1class Developer:
2    """Blueprint for a software developer profile."""
3    pass
4 
5# Creating two distinct instances in memory
6dev_a = Developer()
7dev_b = Developer()
8 
9print(dev_a)  # <__main__.Developer object at 0x7f8...>
10print(dev_a == dev_b)  # False (different memory addresses)

2. The `init` Constructor and `self`

The __init__ method is Python's initialization hook called immediately after an instance is created in memory:

self: A reference to the specific instance currently being initialized or manipulated. Python automatically passes the instance as the first argument when calling instance methods.

1class Developer:
2    def __init__(self, name: str, primary_language: str, years_exp: int):
3        # Instance attributes bound to this specific object
4        self.name = name
5        self.primary_language = primary_language
6        self.years_exp = years_exp
7        
8    def describe(self) -> str:
9        return f"{self.name} specializes in {self.primary_language} with {self.years_exp} years of experience."
10 
11dev = Developer("Vikram", "Python", 4)
12print(dev.describe())
13# dev.describe() is syntactic sugar for: Developer.describe(dev)

3. Instance Attributes vs. Class Attributes

Understanding the distinction between class-level and instance-level attributes is critical to avoiding state bugs:

Instance Attributes: Bound to self. Each instance maintains its own distinct copy in its __dict__.
Class Attributes: Defined directly inside the class body, shared across all instances of that class.

1class Student:
2    # Class attribute (shared by all instances)
3    institution = "MSK Institute of Technology"
4    student_count = 0
5    
6    def __init__(self, name, roll_no):
7        # Instance attributes (unique to each student)
8        self.name = name
9        self.roll_no = roll_no
10        
11        # Incrementing the shared class counter
12        Student.student_count += 1
13 
14s1 = Student("Aarav", 101)
15s2 = Student("Ananya", 102)
16 
17print(s1.institution)  # MSK Institute of Technology
18print(s2.institution)  # MSK Institute of Technology
19print(Student.student_count)  # 2

4. Instance, Class, and Static Methods

Python provides three distinct method types using decorators:

Method Type	Decorator	First Argument	Typical Use Case
Instance Method	(None)	`self`	Manipulates individual instance state
Class Method	`@classmethod`	`cls`	Factory methods, modifying class-level state
Static Method	`@staticmethod`	(None)	Self-contained utility functions logically grouped in the class

1class Temperature:
2    def __init__(self, celsius: float):
3        self.celsius = celsius
4 
5    # 1. Instance Method
6    def to_fahrenheit(self) -> float:
7        return (self.celsius * 9/5) + 32
8 
9    # 2. Class Method (Alternative Constructor / Factory)
10    @classmethod
11    def from_fahrenheit(cls, fahrenheit: float):
12        celsius = (fahrenheit - 32) * 5/9
13        return cls(celsius)
14 
15    # 3. Static Method (Pure utility)
16    @staticmethod
17    def is_freezing(celsius: float) -> bool:
18        return celsius <= 0.0
19 
20# Using Instance Method
21t1 = Temperature(25)
22print(f"25°C in F: {t1.to_fahrenheit()}°F")
23 
24# Using Class Method Factory
25t2 = Temperature.from_fahrenheit(98.6)
26print(f"98.6°F in C: {t2.celsius:.1f}°C")
27 
28# Using Static Method
29print(f"Is -5°C freezing? {Temperature.is_freezing(-5)}")

5. String Representations: `str` vs `repr`

Every Python class should define informative string representations:

__str__: User-friendly, readable string representation (returned by print(obj) or str(obj)).
__repr__: Unambiguous, developer-oriented string used for debugging (returned by typing obj in interactive shell or repr(obj)). Ideally should look like valid Python code to recreate the object.

1class Book:
2    def __init__(self, title, author, isbn):
3        self.title = title
4        self.author = author
5        self.isbn = isbn
6 
7    def __str__(self):
8        return f"'{self.title}' by {self.author}"
9 
10    def __repr__(self):
11        return f"Book(title='{self.title}', author='{self.author}', isbn='{self.isbn}')"
12 
13book = Book("Fluent Python", "Luciano Ramalho", "978-1491946008")
14print(str(book))   # 'Fluent Python' by Luciano Ramalho
15print(repr(book))  # Book(title='Fluent Python', author='Luciano Ramalho', isbn='978-1491946008')

⚠️ Common Intermediate Mistakes & Gotchas

1. Confusing Class Attributes with Instance Attributes

A mutable class attribute (like a list) is shared across all instances of the class!

1# ❌ SHARED BUG: All students share the same grades list!
2class Student:
3    grades = []
4 
5# ✅ CORRECT: Each student gets an independent list in __init__
6class Student:
7    def __init__(self):
8        self.grades = []

2. Forgetting `self` in Method Signatures

Every instance method must take self as its first parameter; omitting it causes a TypeError: method takes 0 positional arguments but 1 was given.

💻 Try It Yourself: Product Inventory Item Class

Scenario

Create an Item class for an e-commerce store with attributes name, price, and quantity. Implement methods total_value(), apply_discount(pct), and a formatted __repr__() method.

Complete Solution

1class Item:
2    def __init__(self, name: str, price: float, quantity: int = 1):
3        if price < 0 or quantity < 0:
4            raise ValueError("Price and quantity must be non-negative.")
5        self.name = name
6        self.price = float(price)
7        self.quantity = int(quantity)
8 
9    def total_value(self) -> float:
10        return self.price * self.quantity
11 
12    def apply_discount(self, discount_pct: float):
13        self.price -= self.price * (discount_pct / 100.0)
14 
15    def __repr__(self) -> str:
16        return f"Item(name='{self.name}', price=₹{self.price:,.2f}, qty={self.quantity})"
17 
18# Testing
19phone = Item("Smartphone", 25000.0, 4)
20print(phone)
21print(f"Total Inventory Value: ₹{phone.total_value():,.2f}")
22phone.apply_discount(10)
23print(f"After 10% Discount: {phone}")

Expected Output

1Item(name='Smartphone', price=₹25,000.00, qty=4)
2Total Inventory Value: ₹100,000.00
3After 10% Discount: Item(name='Smartphone', price=₹22,500.00, qty=4)

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

What does the self parameter in a Python instance method represent?

Question 2

What happens if you define a mutable list as a class attribute and append items to it from an instance?

Question 3

Which decorator allows defining an alternative factory constructor that receives the class itself as cls?

Question 4

How does @staticmethod differ from regular instance methods and class methods?

Question 5

What is the standard purpose of the __repr__ special method in Python?

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Inheritance & Method Overriding (5: Object-Oriented Programming (Intermediate)).

👉 Continue to Next Lesson: Inheritance & Method Overriding →

Next Lesson

Inheritance & Method Overriding

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Project: CSV Contact Manager	Inheritance & Method Overriding

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

Creating Classes & Objects (Review & Deep Dive)

1. Classes vs. Instances

2. The __init__ Constructor and self

3. Instance Attributes vs. Class Attributes

4. Instance, Class, and Static Methods

5. String Representations: __str__ vs __repr__

⚠️ Common Intermediate Mistakes & Gotchas

1. Confusing Class Attributes with Instance Attributes

2. Forgetting self in Method Signatures

💻 Try It Yourself: Product Inventory Item Class

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Inheritance & Method Overriding

Related Lessons

Practice Quiz

Loading Page Content...

Creating Classes & Objects (Review & Deep Dive)

1. Classes vs. Instances

2. The __init__ Constructor and self

3. Instance Attributes vs. Class Attributes

4. Instance, Class, and Static Methods

5. String Representations: __str__ vs __repr__

⚠️ Common Intermediate Mistakes & Gotchas

1. Confusing Class Attributes with Instance Attributes

2. Forgetting self in Method Signatures

💻 Try It Yourself: Product Inventory Item Class

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Inheritance & Method Overriding

Related Lessons

Practice Quiz

2. The `init` Constructor and `self`

5. String Representations: `str` vs `repr`

2. Forgetting `self` in Method Signatures

2. The `init` Constructor and `self`

5. String Representations: `str` vs `repr`

2. Forgetting `self` in Method Signatures