Practice: Bank Account

Lesson

Practice: Bank Account

This exercise combines state, methods, and raise — a class that protects its own rules.

Exercise

Define a class named BankAccount.
__init__ takes owner and stores it; it also sets a balance attribute to 0.
Add a method named deposit with one parameter, amount. It adds amount to the balance. A zero or negative amount raises ValueError with the message amount must be positive.
Add a method named withdraw with one parameter, amount. It subtracts amount from the balance. A zero or negative amount raises ValueError with the message amount must be positive; an amount above the balance raises ValueError with the message insufficient funds.

account = BankAccount("Ada")
account.deposit(100)
account.withdraw(30)
print(account.balance)

Output:

Run your code to see the output, then press Submit.

Hints

This is the withdraw function from the exceptions section, moved onto an object that owns the balance.
Check the amount before touching self.balance, so a rejected call leaves the state unchanged.

Tests

import unittest


class TestBankAccount(unittest.TestCase):
    def test_new_account_has_zero_balance(self):
        self.assertEqual(BankAccount("Ada").balance, 0)

    def test_owner_is_stored(self):
        self.assertEqual(BankAccount("Ada").owner, "Ada")

    def test_deposit_and_withdraw(self):
        account = BankAccount("Ada")
        account.deposit(100)
        account.withdraw(30)
        self.assertEqual(account.balance, 70)

    def test_negative_deposit_raises(self):
        with self.assertRaises(ValueError):
            BankAccount("Ada").deposit(-10)

    def test_overdraw_raises_and_preserves_balance(self):
        account = BankAccount("Ada")
        account.deposit(50)
        with self.assertRaises(ValueError):
            account.withdraw(80)
        self.assertEqual(account.balance, 50)

Solution

class BankAccount:
    def __init__(self, owner):
        self.owner = owner
        self.balance = 0

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("amount must be positive")
        self.balance += amount

    def withdraw(self, amount):
        if amount <= 0:
            raise ValueError("amount must be positive")
        if amount > self.balance:
            raise ValueError("insufficient funds")
        self.balance -= amount


account = BankAccount("Ada")
account.deposit(100)
account.withdraw(30)
print(account.balance)

Solution hidden. Give it a real try first.

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main.py

Console

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