class InsufficientCreditError(Exception):
    """Raised when trying to subtract more than available"""
    pass


def upgrade(amount, rower):
    if rower.eurocredits > amount:  # pragma: no cover
        return rower.eurocredits
    else:
        rower.eurocredits = amount
        rower.save()
        return rower.eurocredits
    return rower.eurocredits  # pragma: no cover


def withdraw(amount, rower):
    if rower.eurocredits < amount:  # pragma: no cover
        rower.eurocredits = 0
        rower.save()
        raise InsufficientCreditError
    else:
        rower.eurocredits = rower.eurocredits - amount
        rower.save()
        return rower.eurocredits

    return rower.eurocredits  # pragma: no cover


def discount(amount, rower):  # pragma: no cover
    if amount < rower.eurocredits:
        return amount
    else:
        return rower.eurocredits

    return 0


def discounted(amount, rower):
    if amount > rower.eurocredits:  # pragma: no cover
        return amount-rower.eurocredits
    else:
        return 0

    return 0  # pragma: no cover