The Proxy pattern in the real world

We will take up a payment use case to demonstrate a real-world scenario for the Proxy pattern. Let's say that you go to shop at a mall and like a nice denim shirt there. You would like to purchase the shirt but you don't have enough cash to do so.

In yesteryears, you'd go to an ATM, take out the money, then come to the mall, and pay for it. Even earlier, you had a bank check for which you had to go to the bank, withdraw money, and then come back to pay for your expense.

Thanks to the banks, we now have something called a debit card. So now, when you want to purchase something, you present your debit card to the merchant. When you punch in your card details, the money is debited in the merchant's account for your expense.

Let's develop an application in Python v3.5 and implement the above use case. We start with the client first. You went to the shopping mall and now would like to purchase a nice denim shirt. Lets see how Client code is written:

• Your behavior...