Question

I am trying to integrate a Singleton Pattern into this code I had previously made.

Here is my code to a previous project:

radius = float(input("Please enter the radius: "))
area = 3.14 * radius**2;
print("The area is ", area, "square units.")

For this project it must be coded in python.

Here are the instructions for the project for reference.

Implementing Design Patterns with Python

Software Design Patterns may be thought of as blue prints or recipes for implementing common models in software. Much like re-using proven classes in Object Oriented design, re-using proven patterns tends to produce more secure and reliable results often in reduced time compared to re-inventing the wheel. In fact, some high-level languages have integrated facilities to support many common patterns with only minimal, if any, user written software. That said, in software design and construction, often the challenge is to know that a pattern exists and be able to recognize when it is a good fit for a project. To give you a familiar frame of reference, below is a Singleton Pattern implemented in Java:

public class MySingleton {

    // reference to class instance

    private static MySingleton instance = null;

    // Private Constructor

    private MySingleton() {

        instance = this;

    }

    // Returns single instance to class

    public static MySingleton getInstance() {

        if (instance == null) {

            instance = new MySingleton();

        }

        return instance;

    }

    public static void main(String[] args)

    {

        MySingleton s1 = MySingleton.getInstance();

        MySingleton s2 = MySingleton.getInstance();

        System.out.println("s1: " + s1 + " s2: " + s2);

    }

}

If you run this example, you will see that the address for s1 and s2 are exactly the same. That is because the pattern restricts the existence of more than one instance in a process. Of course this example only implements the pattern but other functionality can be added to this class just as any other class. The primary features of a singleton are:

1. Private or restricted constructor
2. Internal variable for holding reference to single instance
3. Static method for retrieving the instance

The primary goal of this assignment is not to teach you how to write singleton patterns in Python, (though that’s part of it), but to familiarize you with the concept of design patterns as well as give you experience in adapting one into one of your own designs.

Description

1. Using an Internet Search Engine, search for common software design patterns. Describe, in your own words, how one of these patterns could be used in an application or software task. You are expected to write professionally, with proper grammar and spelling, but you are not required to adhere to a specific format or cite any sources.
2. Pick one of your previous assignments, either from MindTap or one of your Distributed Systems assignments and integrate a Singleton Pattern into it. Hint: Any class can be made into a Singleton.

Answer 1

Hi. I have answered similar questions before. Here is the completed code for this problem. Comments are included, go through it, learn how things work and let me know if you have any doubts or if you need anything to change. If you are satisfied with the solution, please rate the answer. Thanks

#code

class AreaCalculator:
    __instance = None # single instance

    # Initialize the constant
    PI = 3.14
    # variable for storing radius
    __radius = 0

    # method to create the instance of AreaCalculator, constructor should not be used
    @staticmethod
    def getInstance():
        # if __instance is None, initializing it
        if AreaCalculator.__instance == None:
            AreaCalculator.__instance = AreaCalculator()
        # returning __instance
        return AreaCalculator.__instance

    # constructor
    def __init__(self):
        # if __instance is not None, throwing exception, because all objects (copies) should
        # be created using getInstance method only
        if AreaCalculator.__instance != None:
            raise Exception('AreaCalculator should be initialized using getInstance() method only')

    # method to set radius of the circle
    @staticmethod
    def setRadius(radius):
        AreaCalculator.__radius=radius

    # method to compute area and return it
    @staticmethod
    def calculateArea():
        area=AreaCalculator.PI* AreaCalculator.__radius**2
        return area


# main method for testing
def main():
    # creating two instances (will only be a single instance internally)
    a1 = AreaCalculator.getInstance()
    a2 = AreaCalculator.getInstance()

    # printing both instances to let the user know that they are same
    print('First object:', a1)
    print('Second object:', a2)

    # Request the inputs
    radius = float(input("Please enter the radius: "))
    # setting inputs using setRadius method, only using one object
    a1.setRadius(radius)
    # Display the area using both objects
    print("(Using First object) The area is", a1.calculateArea(),"square units.")
    print("(Using Second object) The area is", a2.calculateArea(), "square units.")


main()

#output

First object: <__main__.AreaCalculator object at 0x03A66070>

Second object: <__main__.AreaCalculator object at 0x03A66070>

Please enter the radius: 52

(Using First object) The area is 8490.56 square units.

(Using Second object) The area is 8490.56 square units.