Specific Patterns Prep

Overview

There are four design patterns that you’ve likely used and will find useful, so we’re going to focus on those today.

Basic Learning Objectives

Before class, you should be able to:

Define what the Observer, Iterator, Decorator, and State design patterns are

Advanced Learning Objectives

After class, you should be able to:

Implement these four patterns in Python
Explain how code smells, class design principles, and design patterns reinforce each other or possibly conflict

Readings

To achieve the basic learning objectives, read the following:

Observer Design Pattern - skip the pseudocode, look at the Python example if you want
Iterator Design Pattern - ditto, Python example
Decorator Design Pattern - ditto, Python example
State Design Pattern - ditto, Python

Checks

Submit answers to the following on Moodle. Given the following code snippets that should be refactored, which pattern do you think they need and why? (Note, the TODOs are hints, not things that you should do.)

 class WeatherStation:
     def __init__(self):
         self.temp = 0
         self.phone_app = PhoneApp()
         self.window_display = WindowDisplay()
         #TODO: Add the rest of the displays

     def set_temperature(self, temp):
         self.temp = temp
         self.phone_app.update(temp)
         self.window_display.update(temp)
         #TODO: Add the rest of the displays

 class BookShelf:
     '''Must store books as a dictionary of lists grouped by genre'''
     def __init__(self):
         self.books_by_genre = {"Sci-Fi": ["Dune"], "Tech": ["Clean Code"]}
         #TODO: add the rest of the books from the library

 #Client code below
 shelf = BookShelf()
 for genre in shelf.books_by_genre:
     for book in shelf.books_by_genre[genre]:
         print(book)

 class SimpleCoffee:
 def get_cost(self): return 2.0

 class CoffeeWithMilk(SimpleCoffee):
     def get_cost(self): return 2.5

 class CoffeeWithMilkAndSugar(CoffeeWithMilk):
     def get_cost(self): return 2.7
    
 #TODO: Add the rest of the extra option subclasses along with all combos

 class VendingMachine:
     def __init__(self, count):
         self.count = count
         self.state = "NO_QUARTER" if count > 0 else "OUT_OF_STOCK"

     def insert_quarter(self):
         if self.state == "HAS_QUARTER":
             print("You can't insert another quarter.")
         elif self.state == "NO_QUARTER":
             self.state = "HAS_QUARTER"
             print("Quarter inserted.")
         elif self.state == "OUT_OF_STOCK":
             print("Machine is out of stock.")
         elif self.state == "SOLD":
             print("Please wait, we're already giving you a soda.")
         #TODO: Add those additional states the customer was asking about

     def turn_crank(self):
         if self.state == "HAS_QUARTER":
             print("You turned...")
             self.state = "SOLD"
             self.dispense()
         elif self.state == "NO_QUARTER":
             print("You turned, but there's no quarter.")
         #TODO: add the rest of the states, my fingers are tired...