Applying Design Patterns in Python: The Art of Selecting the Right Solution

Lesson Introduction

Hello! Today, we'll venture into the realm of design patterns . Specifically, we'll tackle exercises that apply a single design pattern to problem-solving. Mastering these patterns is a surefire way to extend your coding skills. Our goal today is to fortify your understanding of when and how to apply specific Object-Oriented Programming (OOP) design patterns. These patterns include Encapsulation, Abstraction, Polymorphism, and Composition . We'll dissect four real-life scenarios and distinguish which pattern is applicable and why. Let's get underway!

Real-life Example 1: Database Management System (Encapsulation)

The Encapsulation pattern proves beneficial for the development of a Database Management System (DBMS). Each DBMS table represents a class, the fields represent private data members, and the functions operating on this data serve as methods. Encapsulation ensures that data members are accessed through methods that promote data integrity and prevent inadvertent anomalies. Here's a mini-code snippet to support this concept: Pythonclass Employees: def __init__(self): self.__employees = {} # private data member def add_employee(self, eid, name): # method to operate on private data self.__employees[eid] = name def update_employee(self, eid, new_name): # method to operate on private data if eid in self.__employees: self.__employees[eid] = new_name employees = Employees() employees.add_employee(1, "John") employees.add_employee(2, "Mark") employees.update_employee(2, "Jake")class Employees: def __init__(self): self.__employees = {} # private data member def add_employee(self, eid, name): # method to operate on private data self.__employees[eid] = name def update_employee(self, eid, new_name): # method to operate on private data if eid in self.__employees: self.__employees[eid] = new_name employees = Employees() employees.add_employee(1, "John") employees.add_employee(2, "Mark") employees.update_employee(2, "Jake") In this context, Encapsulation restricts direct access to employee data, presenting a protective layer via designated methods.

Real-life Example 2: Graphic User Interface (GUI) Development (Polymorphism)

When transitioning to GUI development, consider the creation of controls like buttons or checkboxes. Despite belonging to the same class, each responds differently when clicked. This situation illustrates Polymorphism, which allows us to handle different objects uniformly via a common interface. Check out this illustrative example: Python class Control: # common base class def click(self): # method that can be overridden pass class Button(Control): # derived class def click(self): print("Button Clicked!") # overridden method class CheckBox(Control): # derived class def click(self): print("CheckBox Clicked!") # overridden method # Create objects b = Button() c = CheckBox() # Click Controls b.click() # Outputs: Button Clicked! c.click() # Outputs: CheckBox Clicked! class Control: # common base class def click(self): # method that can be overridden pass class Button(Control): # derived class def click(self): print("Button Clicked!") # overridden method class CheckBox(Control): # derived class def click(self): print("CheckBox Clicked!") # overridden method # Create objects b = Button() c = CheckBox() # Click Controls b.click() # Outputs: Button Clicked! c.click() # Outputs: CheckBox Clicked! Despite sharing the common click interface, different controls display unique responses. This characteristic demonstrates Polymorphism .

Real-life Example 3: Creating a Web Page Structure (Composition)

Let's explore the Composition design pattern through a Pythonic approach to create a simple web page structure. Here, we'll build a fundamental structure representing a webpage composed of various elements like headers, paragraphs, and lists. This abstraction allows us to understand how composite objects work together to form a larger system. Pythonclass WebPageElement: def render(self): pass class Header(WebPageElement): def __init__(self, text): self.text = text def render(self): return f"<h1>{self.text}</h1>" class Paragraph(WebPageElement): def __init__(self, text): self.text = text def render(self): return f"<p>{self.text}</p>" class List(WebPageElement): def __init__(self, items): self.items = items def render(self): items_str = "\n".join([f"<li>{item}</li>" for item in self.items]) return f"<ul>{items_str}</ul>" class WebPage: def __init__(self, title): self.title = title self.elements = [] def add_element(self, element: WebPageElement): self.elements.append(element) def display(self): elements_str = "\n".join([element.render() for element in self.elements]) return f"<html>\n<head>\n <title>{self.title}\n</title>\n</head>\n<body>\n {elements_str}\n</body>\n</html>" # Example usage page = WebPage("My Web Page") page.add_element(Header("Welcome to My Web Page")) page.add_element(Paragraph("This is a paragraph of text on the web page.")) page.add_element(List(["Item 1", "Item 2", "Item 3"])) print(page.display()) """ Outputs: <html> <head> <title>My Web Page </title> </head> <body> <h1>Welcome to My Web Page</h1> <p>This is a paragraph of text on the web page.</p> <ul><li>Item 1</li> <li>Item 2</li> <li>Item 3</li></ul> </body> </html> """class WebPageElement: def render(self): pass class Header(WebPageElement): def __init__(self, text): self.text = text def render(self): return f"<h1>{self.text}</h1>" class Paragraph(WebPageElement): def __init__(self, text): self.text = text def render(self): return f"<p>{self.text}</p>" class List(WebPageElement): def __init__(self, items): self.items = items def render(self): items_str = "\n".join([f"<li>{item}</li>" for item in self.items]) return f"<ul>{items_str}</ul>" class WebPage: def __init__(self, title): self.title = title self.elements = [] def add_element(self, element: WebPageElement): self.elements.append(element) def display(self): elements_str = "\n".join([element.render() for element in self.elements]) return f"<html>\n<head>\n <title>{self.title}\n</title>\n</head>\n<body>\n {elements_str}\n</body>\n</html>" # Example usage page = WebPage("My Web Page") page.add_element(Header("Welcome to My Web Page")) page.add_element(Paragraph("This is a paragraph of text on the web page.")) page.add_element(List(["Item 1", "Item 2", "Item 3"])) print(page.display()) """ Outputs: <html> <head> <title>My Web Page </title> </head> <body> <h1>Welcome to My Web Page</h1> <p>This is a paragraph of text on the web page.</p> <ul><li>Item 1</li> <li>Item 2</li> <li>Item 3</li></ul> </body> </html> """ In this code, we've designed a web page structure using the Composition design pattern. Each web page element (Header, Paragraph, and List) is a WebPageElement, allowing for unified handling while maintaining their specific behaviors (rendering as HTML elements). The WebPage class acts as a composite object that can contain an arbitrary number of WebPageElement instances, each representing different parts of a web page. By adding these elements to the WebPage and invoking the display method, we dynamically compose a complete web page structure in HTML format.

Real-life Example 4: Creating a Vehicle (Abstraction)

Design Pattern Identification

Let's recap the major OOP patterns: Encapsulation : This pattern confines data and related methods into one unit, veiling direct data access. Abstraction : This pattern offers a simplified interface, cloaking complexity. Polymorphism : This pattern facilitates treating different objects as related objects of a common superclass. Composition : This pattern builds elaborate systems by composing closely related objects. Reflect on these principles and practice applying them to a variety of scenarios to better recognize suitable patterns.

Lesson Summary and Practice

Great job! You've poked and prodded at the practical applications of OOP design patterns. We've explored the use of Encapsulation in Database Management Systems, the pivotal role of Polymorphism in GUI development, the important of composition when designing a web page builder, and how abstraction helps to build a vehicle structure. Next up are hands-on exercises to reinforce these concepts. Remember, practice is the master key to understanding these concepts. So keep coding!

Applying Design Patterns in Python: The Art of Selecting the Right Solution

Lesson Introduction

Real-life Example 1: Database Management System (Encapsulation)

Real-life Example 2: Graphic User Interface (GUI) Development (Polymorphism)

Real-life Example 3: Creating a Web Page Structure (Composition)

Real-life Example 4: Creating a Vehicle (Abstraction)

Design Pattern Identification

Lesson Summary and Practice

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