def _check_currency(currency: str):
    'Check that currency is in supported set.'
    if (currency not in currency_set):
        raise ValueError(f'currency {currency} not in supported currency set, {currency_set}')
Check that currency is in supported set.
def _check_currency(currency: str):
    
    if (currency not in currency_set):
        raise ValueError(f'currency {currency} not in supported currency set, {currency_set}')