自定制format

format_dic = {
    'y-m-d': '{0.year}-{0.mon}-{0.day}',
    'y:m:d': '{0.year}:{0.mon}:{0.day}',
    'dmy': '{0.day}{0.mon}{0.year}',
}


class Date:
    def __init__(self, year, mon, day):
        self.year = year
        self.mon = mon
        self.day = day

    def __format__(self, format_spec):
        if format_spec not in format_dic.keys() or not format_dic[format_spec]:
            format_spec = 'y-m-d'
        fm = format_dic[format_spec]
        return fm.format(self)


d1 = Date(2020, 3, 8)
print(format(d1))
输出：
2020-3-8

自己定制format方法