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Coding problem · Medium · Language Idioms

Immutable Money Value Object (dataclasses)

Medium Language practicals Seen at Stripe, Wise, Revolut

Create a value object Money with a @dataclass:

  • fields amount_cents: int and currency: str;
  • immutable — assigning to a field raises dataclasses.FrozenInstanceError;
  • Money + Money returns a new Money; adding different currencies raises ValueError;
  • str(Money(1999, "USD")) → "19.99 USD" (also negative: "-5.00 EUR");
  • instances sort by amount_cents, then currency.

Hints

Hint 1

@dataclass(frozen=True, order=True) gives immutability, __eq__, __hash__ and ordering by field order.

Hint 2

Format with divmod(abs(cents), 100) and add the sign back.

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Solution and explanation

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Time O(1) · Space O(1)

frozen=True makes instances immutable and hashable (safe as dict keys and in sets); order=True generates comparison methods using field order — here amount, then currency. Value objects like Money are compared by value, never by identity. Returning NotImplemented from __add__ for unknown types lets Python try the other operand and produce a proper TypeError.

Reference solution

from dataclasses import dataclass


@dataclass(frozen=True, order=True)
class Money:
    amount_cents: int
    currency: str

    def __add__(self, other: "Money") -> "Money":
        if not isinstance(other, Money):
            return NotImplemented
        if other.currency != self.currency:
            raise ValueError(f"cannot add {other.currency} to {self.currency}")
        return Money(self.amount_cents + other.amount_cents, self.currency)

    def __str__(self) -> str:
        units, cents = divmod(abs(self.amount_cents), 100)
        sign = "-" if self.amount_cents < 0 else ""
        return f"{sign}{units}.{cents:02d} {self.currency}"

Common mistakes

  • Floats for money.
  • Mutable value objects used as dict keys.
  • Formatting negatives as -0.-5.

Follow-up questions

  • Add __mul__ for quantities and allocation without losing cents.
  • Use Decimal for currencies with 3 decimal places.