class Product:
    pay_rate = 0.8    # Class variable for a standard discount of 20%
    def __init__(self, name, price, quantity):
        assert price >= 0, f"Product price {price} is not greater than or equal to zero!"
        assert quantity >= 0, f"Product quantity {quantity} is not greater than or equal to zero!"
        self.name = name
        self.price = price
        self.quantity = quantity
        
    def calculate_total_price(self):
        return self.price * self.quantity   
    
    def apply_discount(self):
        return self.calculate_total_price() * (self.pay_rate) 

# Example usage:
if __name__ == "__main__":
    
    item1 = Product("Laptop", 1000, 2)     # Item with standard discount    
    item2 = Product("Smartphone", 100, 3)  # Item with special discount

    print(f'Price of {item1.name}: ${item1.price:.2f}')
    print(f'Price of {item1.name} after 20% discount: ${item1.price * Product.pay_rate:.2f}')    
    print(f"Total price for {item1.name}(s) after 20% discount: ${item1.apply_discount():.2f}")  # Applying standard discount
    print()
    item2.pay_rate = 0.7  # Instance variable for a special discount of 30%
    print(f'Price of {item2.name}: ${item2.price:.2f}')
    print(f'Price of {item2.name}(s) after 30% discount: ${item2.price * item2.pay_rate:.2f}')    
    print(f"Total price for {item2.name}(s) after 30% discount: ${item2.apply_discount():.2f}")  # Applying item-specific discount