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L2 · Working engineerObject-oriented Python~6 min · 6 tests#71

BankAccount Class With Deposit and Withdraw

Design a BankAccount class in Python with deposit, withdraw and balance, rejecting overdrafts and bad amounts with ValueError. OOP basics, tested live.

The problem

Implement BankAccount:

  • deposit(amount) — adds money; raises ValueError if amount <= 0
  • withdraw(amount) — removes money; raises ValueError if amount <= 0 or greater than the balance
  • balance() — returns the current balance

A new account starts at 0, or at an optional opening balance.

Examples

  1. Example 1

    Input

    run_ops(BankAccount, [], [["deposit", 1000], ["withdraw", 300], ["balance"]])

    Expected output

    [None, None, 700]

  2. Example 2

    Input

    raises(BankAccount(100).withdraw, 500)

    Expected output

    'ValueError'

+ 4 hidden tests on Submit — opening balance, failed withdrawal changes nothing.

Edge cases to ask about

  • Overdraft
  • Zero/negative amounts
  • Opening balance

Hints

0/3

    How an interviewer scores this

    0/9
    Python 3.13 · BankAccount
    ⌘/Ctrl + Enter runs the examples

    Your code runs in real CPython inside your browser — nothing is sent anywhere. The first run downloads the interpreter (about 6 MB, once). Your code is saved on this device as you type.

    Complexity Lab

    What does this cost as n grows?

    Interviewers score the analysis as much as the code. Commit to an answer first — then check it, and read why.

    Time complexity of the optimal solution
    Space complexity (extra memory)

    Pick both to reveal the answer.

    From brute force to optimal

    The progression an interviewer wants to hear, one step at a time.

    ApproachTimeSpaceIdea
    State + validated methodsO(1)O(1)Every method guards its invariant: balance never negative.
    Walkthrough of the optimal approach (try it yourself first)

    Keep the balance on the instance and make every method protect the invariant — the balance can never go negative. Validate before mutating, so a rejected withdrawal leaves the account unchanged.

    For real money, use decimal.Decimal (or integer cents) rather than floats.

    Complexity: O(1) time, O(1) space. Each method does constant work on one number.

    Reveal the reference solution
    class BankAccount:
    def __init__(self, opening=0):
        self._balance = opening

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

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

    def balance(self):
        return self._balance

    Follow-ups interviewers ask

    • Why not store money as float?
    • Add a transaction history.

    Frequently asked interview questions

    Core interview concepts, complexities, and follow-ups scored by hiring teams.

    What is the time complexity of BankAccount Class With Deposit and Withdraw in Python?

    The optimal solution runs in O(1) time and O(1) auxiliary space. Each method does constant work on one number.

    What follow-up questions do interviewers ask about BankAccount Class With Deposit and Withdraw?

    Why not store money as float? Add a transaction history.

    Practise the same pattern