Python 3- Deep Dive -part 4 - Oop- May 2026

This is an excellent topic. is the cornerstone of maintainable, scalable Object-Oriented Programming. In the context of Python 3: Deep Dive (Part 4) , we move beyond basic syntax into how these principles interact with Python’s dynamic nature, descriptors, metaclasses, and Abstract Base Classes (ABCs).

class Fax(Protocol): def fax(self, doc: str) -> None: ... class SimplePrinter: def print(self, doc: str) -> None: print(f"Printing doc") Multi-function device can compose multiple protocols class MultiFunctionDevice(Printer, Scanner, Fax): def print(self, doc): ... def scan(self, doc): ... def fax(self, doc): ... 5. D: Dependency Inversion Principle (DIP) Depend on abstractions, not concretions. High-level modules should not depend on low-level modules. Deep Dive Issue: Python's dynamic imports and global singletons (e.g., requests.get , open ) often hard-code dependencies, making unit testing impossible. Python 3- Deep Dive -Part 4 - OOP-

class EmployeeDiscount(DiscountStrategy): # Extension: No existing code modified def apply(self, amount: float) -> float: return amount * 0.5 This is an excellent topic

class Sparrow(FlyingBird): def move(self): return self.fly(100) def fly(self, altitude: int): return f"Flying at altitude" class Fax(Protocol): def fax(self, doc: str) -> None:

class Bird: def fly(self, altitude: int) -> None: return f"Flying at altitude" class Penguin(Bird): def fly(self, altitude: int) -> None: # Violation: Changes pre-condition (cannot fly) raise NotImplementedError("Penguins can't fly")

from dataclasses import dataclass @dataclass class Employee: name: str salary: float Responsibility 2: Business logic class PayCalculator: def calculate(self, emp: Employee) -> float: return emp.salary * 0.8 Responsibility 3: Persistence class EmployeeRepository: def save(self, emp: Employee) -> None: # Uses SQLAlchemy, filesystem, etc. pass 2. O: Open/Closed Principle (OCP) Classes should be open for extension, but closed for modification. Deep Dive Issue: Python is not statically typed. Without ABC or Protocol , developers often write long if/elif chains checking type() .

def generate_pdf_report(self): print(f"PDF: self.name") # Presentation