Object-Oriented Programming (OOP)

# Object-Oriented Programming (OOP)

Welcome to Chapter 18! OOP is one of the most important programming paradigms. It lets you model real-world entities as objects with properties and behaviors.

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1. Learning Objectives #

• Understand OOP concepts (classes, objects, encapsulation).

• Create classes with constructors and methods.

• Use instance and class variables.

• Implement special/magic methods.

• Build a bank account system.

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2. Classes and Objects #

```python id="py18_ex1" class Dog: # Class variable (shared by all instances) species = "Canis familiaris" # Constructor (initializer) def __init__(self, name, breed, age): # Instance variables (unique to each object) self.name = name self.breed = breed self.age = age # Instance method def bark(self): return f"{self.name} says Woof! 🐕" def info(self): return f"{self.name} ({self.breed}), {self.age} years old"

# Creating objects (instances) dog1 = Dog("Buddy", "Labrador", 3) dog2 = Dog("Luna", "German Shepherd", 5)

print(dog1.bark()) # Buddy says Woof! 🐕 print(dog2.info()) # Luna (German Shepherd), 5 years old print(Dog.species) # Canis familiaris

Class vs Object: ┌──────────────────┐ │ Class: Dog │ ← Blueprint │ - name │ │ - breed │ │ - bark() │ └──────┬───────────┘ │ Creates ┌───┴───┐ ▼ ▼ ┌──────┐ ┌──────┐ │dog1 │ │dog2 │ ← Instances │Buddy │ │Luna │ └──────┘ └──────┘

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## 3. The self Parameter

`self` refers to the **current instance** of the class:

python id="py18_ex2" class Student: def __init__(self, name, grade): self.name = name # self.name = instance variable self.grade = grade def promote(self): self.grade += 1 # Modify instance variable print(f"{self.name} promoted to grade {self.grade}!")

s = Student("Alice", 10) s.promote() # Alice promoted to grade 11!

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## 4. Class Variables vs Instance Variables

python id="py18_ex3" class Employee: # Class variable company = "TechCorp" employee_count = 0 def __init__(self, name, salary): self.name = name # Instance variable self.salary = salary # Instance variable Employee.employee_count += 1

e1 = Employee("Alice", 80000) e2 = Employee("Bob", 75000)

print(Employee.company) # TechCorp print(Employee.employee_count) # 2 print(e1.name) # Alice

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## 5. Methods Types

python id="py18_ex4" class MyClass: count = 0 def __init__(self, value): self.value = value MyClass.count += 1 # Instance method — accesses instance data def show(self): return f"Value: {self.value}" # Class method — accesses class data @classmethod def get_count(cls): return f"Total instances: {cls.count}" # Static method — no access to instance or class @staticmethod def validate(value): return isinstance(value, (int, float))

obj1 = MyClass(10) obj2 = MyClass(20)

print(obj1.show()) # Value: 10 print(MyClass.get_count()) # Total instances: 2 print(MyClass.validate(42)) # True

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## 6. Magic/Dunder Methods

python id="py18_ex5" class Vector: def __init__(self, x, y): self.x = x self.y = y def __str__(self): return f"Vector({self.x}, {self.y})" def __repr__(self): return f"Vector(x={self.x}, y={self.y})" def __add__(self, other): return Vector(self.x + other.x, self.y + other.y) def __len__(self): return int((self.x2 + self.y2)**0.5) def __eq__(self, other): return self.x == other.x and self.y == other.y

v1 = Vector(3, 4) v2 = Vector(1, 2)

print(v1) # Vector(3, 4) print(v1 + v2) # Vector(4, 6) print(len(v1)) # 5 print(v1 == v2) # False

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## 7. Properties (Getters and Setters)

python id="py18_ex6" class Temperature: def __init__(self, celsius): self._celsius = celsius @property def celsius(self): return self._celsius @celsius.setter def celsius(self, value): if value < -273.15: raise ValueError("Below absolute zero!") self._celsius = value @property def fahrenheit(self): return (self._celsius * 9/5) + 32

temp = Temperature(100) print(temp.celsius) # 100 print(temp.fahrenheit) # 212.0 temp.celsius = 0 print(temp.fahrenheit) # 32.0

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## 8. Mini Project: Bank Account System

python id="py18_project" class BankAccount: bank_name = "PyBank" def __init__(self, owner, balance=0): self.owner = owner self._balance = balance self._transactions = [] @property def balance(self): return self._balance def deposit(self, amount): if amount <= 0: raise ValueError("Deposit must be positive!") self._balance += amount self._transactions.append(f"+${amount:.2f}") print(f" ✅ Deposited ${amount:.2f}") def withdraw(self, amount): if amount <= 0: raise ValueError("Withdrawal must be positive!") if amount > self._balance: print(f" ❌ Insufficient funds!") return self._balance -= amount self._transactions.append(f"-${amount:.2f}") print(f" ✅ Withdrew ${amount:.2f}") def statement(self): print(f"\n {'='*35}") print(f" {self.bank_name} - Account Statement") print(f" {'='*35}") print(f" Owner: {self.owner}") print(f" Balance: ${self._balance:.2f}") print(f" Transactions: {len(self._transactions)}") for t in self._transactions[-5:]: print(f" {t}") print(f" {'='*35}") def __str__(self): return f"BankAccount({self.owner}, ${self._balance:.2f})"

# Demo account = BankAccount("Alice", 1000) account.deposit(500) account.withdraw(200) account.withdraw(100) account.statement() ``

Q1: __init__ is: A) Destructor B) Constructor C) Iterator D) Generator Answer: B

Q2: self refers to: A) The class B) Current instance C) Parent class D) Module Answer: B

Q4: @classmethod receives: A) self B) cls C) Nothing D) Both Answer: B

Q5: @staticmethod receives: A) self B) cls C) Nothing special D) Both Answer: C

Q6: __str__ is called by: A) repr() B) print() C) len() D) init() Answer: B

Q7: @property creates: A) Class variable B) Getter method C) Static method D) Private variable Answer: B

Q8: _variable convention means: A) Public B) Private (by convention) C) Protected D) Constant Answer: B

Q10: Can a class have multiple constructors? A) Yes, directly B) No, use @classmethod alternatives C) Only in Python 2 D) Only with metaclasses Answer: B — Python uses @classmethod for alternative constructors.

1. 1. What is self? Reference to the current instance, passed automatically as the first argument to instance methods.

1. 3. What are magic/dunder methods? Special methods with double underscores that Python calls implicitly (e.g., __init__, __str__, __add__).

1. 5. Can you have multiple __init__ methods? No. Use @classmethod for alternative constructors.

• __init__ initializes objects; self refers to the current instance.

• Magic methods (__str__, __add__`, etc.) customize object behavior.

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12. Next Chapter Recommendation #

In Chapter 19: Inheritance and Polymorphism, you'll learn to build class hierarchies and reuse code! 🚀