Imagine that you have written an e-commerce website. You have displayed the product prices all over the pages. Imagine that the prices in your database are stored with no VAT (sales tax), but you want to display them on the website with VAT at 20%. Here's a few ways of calculating the VAT-inclusive price from the VAT-exclusive price:

# vat.py
price = 100  # GBP, no VAT
final_price1 = price * 1.2
final_price2 = price + price / 5.0
final_price3 = price * (100 + 20) / 100.0
final_price4 = price + price * 0.2

All these four different ways of calculating a VAT-inclusive price are perfectly acceptable, and I promise you I have found them all in my colleagues' code, over the years. Now, imagine that you have started selling your products in different countries and some of them have different VAT rates, so you need to refactor your code (throughout the website) in order to make that VAT calculation dynamic.

How do you trace all the places in which you are performing a VAT calculation? Coding today is a collaborative task and you cannot be sure that the VAT has been calculated using only one of those forms. It's going to be hell, believe me.

So, let's write a function that takes the input values, vat and price (VAT-exclusive), and returns a VAT-inclusive price:

# vat.function.py
def calculate_price_with_vat(price, vat):
    return price * (100 + vat) / 100

Now you can import that function and use it in any place in your website where you need to calculate a VAT-inclusive price, and when you need to trace those calls, you can search for calculate_price_with_vat.