LEARN COMPLETE PYTHON IN 24 HOURS

8. Real-World OOP Projects & Best Practices

8.1 Mini Project 1 – Bank Account System (Inheritance + Encapsulation)

Goal: Build a secure bank account system with different account types using inheritance and encapsulation.

Python

from abc import ABC, abstractmethod class BankAccount(ABC): def init(self, account_holder, account_number, initial_balance=0.0): self.account_holder = account_holder self.account_number = account_number self._balance = initial_balance # protected self.__transactions = [] # private @property def balance(self): return self._balance def deposit(self, amount): if amount <= 0: raise ValueError("Deposit amount must be positive") self._balance += amount self.__transactions.append(f"Deposit: +₹{amount}") print(f"Deposited ₹{amount}. New balance: ₹{self._balance}") @abstractmethod def withdraw(self, amount): pass def get_transaction_history(self): return self.__transactions.copy() # safe copy class SavingsAccount(BankAccount): INTEREST_RATE = 0.04 # 4% annual interest def withdraw(self, amount): if amount > self._balance: raise ValueError("Insufficient balance") self._balance -= amount self.__transactions.append(f"Withdrawal: -₹{amount}") print(f"Withdrew ₹{amount}. New balance: ₹{self._balance}") def add_interest(self): interest = self._balance * SavingsAccount.INTEREST_RATE self._balance += interest self.__transactions.append(f"Interest added: +₹{interest:.2f}") print(f"Interest added: ₹{interest:.2f}") # Usage acc = SavingsAccount("Anshuman", "SB123456", 10000) acc.deposit(5000) acc.withdraw(2000) acc.add_interest() print("Balance:", acc.balance) print("History:", acc.get_transaction_history())

Output example:

text

Deposited ₹5000. New balance: ₹15000 Withdrew ₹2000. New balance: ₹13000 Interest added: ₹520.00 Balance: 13520.0 History: ['Deposit: +₹5000', 'Withdrawal: -₹2000', 'Interest added: +₹520.00']

Key concepts used: Abstract base class, encapsulation (_balance, __transactions), properties, inheritance.

8.2 Mini Project 2 – Library Management System (Multiple Classes)

Goal: Model a library with books, members, and borrowing logic.

Python

class Book: def init(self, title, author, isbn): self.title = title self.author = author self.isbn = isbn self.is_available = True def str(self): return f"{self.title} by {self.author} (ISBN: {self.isbn})" class LibraryMember: def init(self, name, member_id): self.name = name self.member_id = member_id self.borrowed_books = [] def borrow_book(self, book): if book.is_available: book.is_available = False self.borrowed_books.append(book) print(f"{self.name} borrowed {book.title}") else: print(f"{book.title} is not available") def return_book(self, book): if book in self.borrowed_books: book.is_available = True self.borrowed_books.remove(book) print(f"{self.name} returned {book.title}") else: print("You didn't borrow this book") class Library: def init(self, name): self.name = name self.books = [] self.members = [] def add_book(self, book): self.books.append(book) print(f"Added book: {book}") def register_member(self, member): self.members.append(member) print(f"Registered member: {member.name}") def find_book(self, title): for book in self.books: if title.lower() in book.title.lower(): return book return None # Usage lib = Library("City Central Library") book1 = Book("Python Mastery", "Anshuman", "ISBN123") book2 = Book("Clean Code", "Robert C. Martin", "ISBN456") lib.add_book(book1) lib.add_book(book2) member = LibraryMember("Rahul", "M001") lib.register_member(member) member.borrow_book(book1) member.borrow_book(book2) member.return_book(book1)

Key concepts used: Multiple classes, composition (Library has Books and Members), encapsulation, clean interfaces.

8.3 Mini Project 3 – Employee Payroll System (Polymorphism)

Goal: Calculate salary for different employee types using polymorphism.

Python

from abc import ABC, abstractmethod class Employee(ABC): def init(self, name, employee_id): self.name = name self.employee_id = employee_id @abstractmethod def calculate_salary(self): pass def str(self): return f"{self.name} (ID: {self.employee_id})" class FullTimeEmployee(Employee): def init(self, name, employee_id, base_salary): super().__init__(name, employee_id) self.base_salary = base_salary def calculate_salary(self): return self.base_salary + 5000 # bonus class PartTimeEmployee(Employee): def init(self, name, employee_id, hourly_rate, hours_worked): super().__init__(name, employee_id) self.hourly_rate = hourly_rate self.hours_worked = hours_worked def calculate_salary(self): return self.hourly_rate self.hours_worked class ContractEmployee(Employee): def init(self, name, employee_id, project_fee): super().__init__(name, employee_id) self.project_fee = project_fee def calculate_salary(self): return self.project_fee 0.9 # 10% tax deduction # Payroll processing (polymorphism) employees = [ FullTimeEmployee("Anshuman", "FT001", 80000), PartTimeEmployee("Priya", "PT001", 500, 120), ContractEmployee("Rahul", "CT001", 150000) ] print("Payroll Report:") for emp in employees: salary = emp.calculate_salary() print(f"{emp} → Salary: ₹{salary:.2f}")

Output example:

text

Payroll Report: Anshuman (ID: FT001) → Salary: ₹85000.00 Priya (ID: PT001) → Salary: ₹60000.00 Rahul (ID: CT001) → Salary: ₹135000.00

Key concepts used: Abstract base class, polymorphism (different calculate_salary), inheritance.

8.4 OOP Best Practices – Clean Code, Naming Conventions, SOLID Principles in Python

Clean Code & Naming (PEP 8 + Pythonic style)

• Use snake_case for variables/methods: calculate_salary, get_full_name

• Use CamelCase for classes: BankAccount, LibraryMember

• Method names: verbs (deposit, withdraw, calculate_salary)

• Variables: nouns/adjectives (account_balance, is_active)

• Keep methods short (< 20–30 lines)

• One responsibility per method/class

SOLID Principles (adapted for Python)

• Single Responsibility Principle A class should have only one reason to change. → One class = one job (e.g., BankAccount handles balance, not printing statements)

• Open/Closed Principle Open for extension, closed for modification. → Use inheritance + polymorphism, decorators, strategy pattern

• Liskov Substitution Principle Subclasses should be substitutable for their base classes. → Child objects must behave correctly when used as parent

• Interface Segregation Principle Clients shouldn’t depend on methods they don’t use. → Prefer small, focused interfaces (abc classes)

• Dependency Inversion Principle Depend on abstractions, not concretions. → Use dependency injection (pass objects via constructor)

Pythonic Tips

• Prefer composition over deep inheritance

• Use @dataclass for data-heavy classes

• Use properties for controlled attributes

• Follow PEP 8 → use Black, isort, flake8, mypy

• Write docstrings (PEP 257) and type hints

Final Advice: Write small, testable classes. Use inheritance sparingly. Favor readability and simplicity.

This completes the full Real-World OOP Projects & Best Practices section — now you can confidently build professional, maintainable OOP systems in Python!

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