How would you write a custom class in python using different objects?

Question

I'm trying to learn how to code in python, but am having trouble on finding ways to create custom classes online. I wrote a program in java and I am trying to convert it in python. I think I have the custom class down (I'm not sure), and I'm definitely having trouble with the driver.

my custom class (python):

class CostCalculator:
    __item = ""
    __costOfItem = 0.0
    __tax = 0.0
    __tip = 0.0

    def set_item(self, item):
        self.__item = item
    def get_name(self):
        return self.__item

    def set_costOfItem(self, costOfItem):
        self.__costOfItem = costOfItem
    def get_costOfItem(self):
        return self.__costOfItem

    def get_tax(self):
        __tax = self.__costOfItem * .0875
        return self.__tax

    def get_tip(self):
        __tip = self.__costOfItem * .15
        return self.__tip

My python driver attempt

import sys
from CostCalculator import CostCalculator

item = ""
cost = 0.0
totalTip = 0.0
totalTax = 0.0
overallTotal = 0.0
subtotal = 0.0

print("Enter the name of 3 items and their respective costs to get the total value of your meal")
print ("\n Enter the name of your first item: ")
item = sys.stdin.readline()
print("How much is " + item + "?")
cost = sys.stdin.readLine()

My java custom class and driver:

public class TotalCost
{
   String item = " ";
   double costOfItem = 0;
   double tax = 0;
   double tip = 0;

   public void setItem ( String i )
   {
       item = i;
    }

   public String getItem()
   {
       return item;
    }

   public void setCostOfItem ( double c )
   {
       costOfItem = c;
    }

   public double getCostOfItem ()
   {
       return costOfItem;
    }

   public double getTax ()
   {
       double tax = costOfItem * .0875;
       return tax;
    }

   public double getTip()
   {
      double tip = costOfItem * .15;
      return tip;
    }

   public String toString()
   {
      String str;
        str = "\nMeal: " + getItem() +
        "\nCost of " + getItem() + ": " + getCostOfItem() +
        "\nTax of " + getItem() + ": " + getTax() +
        "\nTip of " + getItem() + ": " + getTip();
      return str;

    }

}

import java.util.Scanner;
public class Driver
{
    public static void main (String args[])
    {
        Scanner input = new Scanner (System.in);

        String item ;
        double cost ;
        double totalTip = 0;
        double totalTax = 0;
        double OverallTotal = 0;
        double subtotal;
        TotalCost a = new TotalCost ();
        TotalCost b = new TotalCost ();
        TotalCost c = new TotalCost ();

        System.out.println("Enter the name of 3 items and their respective costs to get the total value of your meal");
        System.out.println("Enter the name of your first item: ");
        item = input.nextLine();
        a.setItem ( item );
        System.out.println("How much is " + a.getItem() + "?" );
        cost = input.nextDouble();
        a.setCostOfItem (cost);

        input.nextLine();

        System.out.println("Enter the name of your second item: ");
        item = input.nextLine();
        b.setItem (item);
        System.out.println("How much is a " + b.getItem() + "?");
        cost = input.nextDouble();
        b.setCostOfItem (cost);

        input.nextLine();

        System.out.println("Enter the name of your third item: ");
        item = input.nextLine();
        c.setItem (item);
        System.out.println("How much is a " +c.getItem() + "?" );
        cost = input.nextDouble();
        c.setCostOfItem(cost);

        System.out.println(a + "\n" + b + "\n" + c);
        subtotal = a.getCostOfItem() + b.getCostOfItem() + c.getCostOfItem();
        totalTip = a.getTip() + b.getTip() + c.getTip();
        totalTax = a.getTax() + b.getTax() + c.getTax();
        OverallTotal = subtotal + totalTip + totalTax;

        System.out.println("\n\tSubtotal: $" + subtotal);
        System.out.println("\tTax: $" + totalTax);
        System.out.println("\tTip: $" + totalTip);
        System.out.println("\tMeal Total: $" + OverallTotal);
    }   
}

Answer 1

In Python, there is no notion of public vs private, everything is public so you do not need setters or getters.

What you do need is the __init__ function, which is similar to a constructor. You can initialize the member variables here so they are not static and shared among all instances of your class. You can also add default arguments so you many pass in any, all, or none of the arguments to the class upon instantiation.

class CostCalculator:
    def __init__(self, item = "", cost = 0.0):
        self.item = item
        self.cost = cost

    def __str__(self):
        return 'Meal: {item}\nCost of {item}: {cost}\nTax of {item}: {tax}\nTip of {item}: {tip}'.format(item = self.item, cost = self.cost, tax = self.calc_tax(), tip = self.calc_tip())

    def calc_tax(self):
        return self.cost * 0.0875

    def calc_tip(self):
        return self.cost * 0.15

    def calc_total(self):
        return self.cost + self.calc_tax() + self.calc_tip()

Then you can create an instance of this class. Again note that you can directly access the members without setters or getters, for better or worse ;)

>>> c = CostCalculator('cheese', 1.0)
>>> c.item
'cheese'
>>> c.calc_tip()
0.15

Now you can invoke print on your object

>>> c = CostCalculator('cheese', 1.0)
>>> print(c)
Meal: cheese
Cost of cheese: 1.0
Tax of cheese: 0.085
Tip of cheese: 0.15

Lastly, the way you accept input from a user is generally via input (although messing around with stdin isn't necessarily wrong)

>>> tax = input('how much does this thing cost? ')
how much does this thing cost? 15.0
>>> tax
'15.0'

Answer 2

Another nice feature of Python is the built-in @property decorator, which helps to replace setters and getters from Java. The @property decorator allows you to create early versions of a class using property attributes (i.e., self.tax). If it later becomes necessary to perform calculations on the attribute or move it to a calculated attribute, the @property attribute allows this to be done transparently to any code that depends on the existing implementation. See example, below.

TAX_RATE = 0.0875
TIP_RATE = 0.15

class CostCalculator(object):
    def __init__(self, item='', cost=0):
        self.item = item
        self.cost = cost


    @property
    def tax(self):
        """Tax amount for item."""

        return self.cost * TAX_RATE


    @property
    def tip(self):
        """Tip amount for item."""

        return self.cost * TIP_RATE



if __name__ == '__main__':
    item = CostCalculator('Steak Dinner', 21.50)
    assert item.tax == 1.8812499999999999
    assert item.tip == 3.225