Money With a Currency

Floats cannot store most amounts exactly (0.1 + 0.2 is 0.30000000000000004), so programs that handle money count whole cents instead. Build a Money class that does this and also keeps track of the currency.

• Money(cents, currency) stores a whole number of cents and a currency code such as 'USD', as the attributes cents and currency.
• Money.from_string(text) is a class method that reads text like '12.50 USD' or '-0.05 EUR': an optional minus sign, digits, a dot, exactly two digits, a space and the currency code. It returns the matching Money.
• a + b and a - b return a new Money. If the two currencies differ, they raise ValueError.
• 0 + m returns m, so the built-in sum() works on a list of Money.
• a == b is True when both the cents and the currency are equal. Comparing with something that is not Money gives False.
• repr(m) returns the amount with exactly two decimals, a space and the currency, for example 12.50 USD, -0.05 EUR or 0.00 GBP.

Examples

lunch = Money.from_string('12.50 USD')
coffee = Money(375, 'USD')
lunch + coffee                 # 16.25 USD
coffee - lunch                 # -8.75 USD
sum([lunch, coffee, coffee])   # 20.00 USD
Money(5, 'EUR') == Money.from_string('0.05 EUR')    # True
lunch + Money(100, 'EUR')      # raises ValueError

Constraints

• Amounts can be negative and as large as you like

Note

Negative amounts are the tricky part, both when reading them and when writing them.