Project: A Small Bank

Why this matters

Time to design with objects. A small bank is a classic: it has several kinds of account, rules that must never be broken (no overdrafts, no negative deposits) and one object (the bank) that holds many others. It uses everything in this module: classes, properties, inheritance, composition, a dataclass and a custom exception.

The idea

You will complete these pieces. The starter has numbered TODOs:

Class	Role
Transaction	a frozen dataclass recording "deposit" or "withdraw" and an amount (given)
InsufficientFunds	your own error type (given)
Account	an owner, a private balance, a history; deposit, withdraw, __str__
SavingsAccount(Account)	adds a rate and add_interest(); its own __str__
Bank	has many accounts; opens them, finds them and moves money between them

Two design rules to notice: the balance can only change through deposit and withdraw (so every change is checked and recorded), and a failed operation must leave everything exactly as it was.

Try it

Before coding, read main() at the bottom of the starter and work out by hand what it should print. Which line triggers the InsufficientFunds error?

Your turn

Complete all thirteen TODOs. The error messages are exact, because the checks compare them. When you are done, running the program prints:

ada: 60.00
bo (savings): 40.00
Transfer failed: balance is 60.00, cannot withdraw 500.00

Recap

• Put the rules next to the data they protect (private balance, checked methods).
• Inheritance for “is a” (SavingsAccount), composition for “has a” (Bank holds accounts).
• Validate before you change anything, so failures leave no half-done work.

Common mistakes

• Changing the balance before checking it, so a rejected withdrawal still removes money.
• Returning the real history list, which lets callers rewrite it.
• Withdrawing before looking up the target account in transfer, so money disappears when the target is unknown.