@classmethod
def _largest_nonfractional_unit(cls, amount):
    """
        Return largest unit such that `amount` is larger than 1 when
        expressed in that unit.
        """
    units = sorted(cls._UNITS.values())
    prev = units[0].base
    for unit in units:
        if prev.amount(unit.base) <= amount < unit.amount(unit.base):
            return prev
        prev = unit
    return unit
