Open/Closed Principle

C#: give the discounts a common method

The C# snippets target .NET 8 and use System; add using System; at the top of the source file. Put class and interface declarations at file level, and the calling statements in Main or another method.

An interface declares operations that a class promises to provide. Here, IDiscount requires one method: Apply. The : IDiscount after a class name says that the class implements that interface. 2

public interface IDiscount
{
    decimal Apply(decimal subtotal);
}

public sealed class NoDiscount : IDiscount
{
    public decimal Apply(decimal subtotal)
    {
        return subtotal;
    }
}

public sealed class TenPercentDiscount : IDiscount
{
    public decimal Apply(decimal subtotal)
    {
        return subtotal * 0.90m;
    }
}

public sealed class Checkout
{
    public decimal CalculateTotal(decimal subtotal, IDiscount discount)
    {
        return discount.Apply(subtotal);
    }
}

NoDiscount.Apply returns the subtotal unchanged. TenPercentDiscount.Apply keeps ninety percent of it. The m suffix makes 0.90m a C# decimal literal.

Look at Checkout.CalculateTotal: it has no percentage and no check for a discount name. It simply calls Apply on the object it receives. That object supplies the calculation.

Use the existing discounts

These statements belong inside a calling method, such as Main:

Checkout checkout = new Checkout();

decimal fullPrice = checkout.CalculateTotal(100m, new NoDiscount());
decimal salePrice = checkout.CalculateTotal(100m, new TenPercentDiscount());

Console.WriteLine(fullPrice); // 100
Console.WriteLine(salePrice); // 90.00

The two calls use the same checkout method. Only the discount object changes. Display formatting can vary by culture; the amounts are 100 and 90.

Python: give the discounts a common method

Python does not require a declared interface for this example. Both discount classes provide apply, and checkout calls that method on the object it receives. Supplying an object without a suitable apply method would fail when the call is made.

from decimal import Decimal


class NoDiscount:
    def apply(self, subtotal):
        return subtotal


class TenPercentDiscount:
    def apply(self, subtotal):
        return subtotal * Decimal("0.90")


class Checkout:
    def calculate_total(self, subtotal, discount):
        return discount.apply(subtotal)

NoDiscount.apply returns the subtotal unchanged. TenPercentDiscount.apply keeps ninety percent of it. Decimal("0.90") constructs a decimal value from text, rather than introducing a binary floating-point approximation. 3

Look at Checkout.calculate_total: it has no percentage and no check for a discount name. It simply calls apply on the object it receives. That object supplies the calculation.

Use the existing discounts

Run these statements after the class definitions:

checkout = Checkout()

full_price = checkout.calculate_total(Decimal("100"), NoDiscount())
sale_price = checkout.calculate_total(Decimal("100"), TenPercentDiscount())

print(full_price)  # 100
print(sale_price)  # 90.00

The two calls use the same checkout method. Only the discount object changes.

Add the employee discount

public sealed class EmployeeDiscount : IDiscount
{
    public decimal Apply(decimal subtotal)
    {
        return subtotal * 0.80m;
    }
}

This class satisfies the same IDiscount interface. In a calling method, we can pass an instance to the existing checkout:

Checkout employeeCheckout = new Checkout();
IDiscount employeeDiscount = new EmployeeDiscount();

decimal employeeTotal = employeeCheckout.CalculateTotal(100m, employeeDiscount);
Console.WriteLine(employeeTotal); // 80.00

Checkout.CalculateTotal has not changed. Neither have NoDiscount, TenPercentDiscount, or IDiscount.

Add the employee discount

class EmployeeDiscount:
    def apply(self, subtotal):
        return subtotal * Decimal("0.80")

This class provides the same apply method. Run it after the earlier definitions, then pass an instance to the existing checkout:

employee_checkout = Checkout()
employee_discount = EmployeeDiscount()

employee_total = employee_checkout.calculate_total(
    Decimal("100"), employee_discount
)
print(employee_total)  # 80.00

Checkout.calculate_total has not changed. Neither have NoDiscount or TenPercentDiscount.