money.py

class Currency:
    _currencies = {}

    def __init__(self, code, symbol, name):
        self.code = code.upper()
        self.symbol = symbol
        self.name = name
        
        Currency.register(self)
        
    @classmethod
    def register(cls, currency):
         cls._currencies[currency.code] = currency

    @classmethod
    def get(cls, currency):
        if isinstance(currency, cls):
            return currency
        return cls._currencies[currency.upper()]
        
USD = Currency('USD', '$', 'Dollar')
ZERO = Decimal('0.00')

class Money:
    def __init__(self, amount=ZERO, currency=USD):
        if not isinstance(amount, Decimal):
            amount = Decimal(str(amount))
            
        self.amount = amount
        self.currency = Currency.get(currency)