Object-oriented programming

1 · Learn and explore

Study this example, identify inputs and outputs, then trace the key variables. Run and adapt it in Python 3.

class BankAccount:
    def __init__(self, owner, balance=0):
        if balance < 0: raise ValueError("negative opening balance")
        self.owner = owner
        self._balance = balance
    @property
    def balance(self): return self._balance
    def deposit(self, amount):
        if amount <= 0: raise ValueError("deposit must be positive")
        self._balance += amount
    def withdraw(self, amount):
        if amount <= 0 or amount > self._balance: raise ValueError("invalid withdrawal")
        self._balance -= amount

account = BankAccount("Sam", 50)
account.deposit(25)
account.withdraw(10)

2 · Independent programming challenge

Create a BankAccount class with encapsulated balance and deposit/withdraw methods. Download the starter, design your solution, and add tests before revealing the answer.

Download starter + solution ZIP ↓

Stretch task: investigate time/space complexity or data validation, then write an evaluation of your approach.

class BankAccount:
    def __init__(self, owner, balance=0):
        if balance < 0: raise ValueError("negative opening balance")
        self.owner = owner
        self._balance = balance
    @property
    def balance(self): return self._balance
    def deposit(self, amount):
        if amount <= 0: raise ValueError("deposit must be positive")
        self._balance += amount
    def withdraw(self, amount):
        if amount <= 0 or amount > self._balance: raise ValueError("invalid withdrawal")
        self._balance -= amount

account = BankAccount("Sam", 50)
account.deposit(25)
account.withdraw(10)
assert account.balance == 65
print(account.balance)

3 · Debug and explain

Change one boundary condition in the starter or reference program. Predict which test fails, then repair it and document why.

Concept check: Why hide a class balance behind methods?

4 · Assess your learning

Describe the algorithm or design in your own words; include a normal test, a boundary test and an invalid input. Compare your answers with the reference solution and record one improvement.

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