这一节太好理解了，就粘贴上老师的代码，记录一下就行
实例化方法：tomorrow
静态：staticmethod
类方法：classmethod

class Date:
    #构造函数
    def __init__(self, year, month, day):
        self.year = year
        self.month = month
        self.day = day

    def tomorrow(self):
        self.day += 1

    @staticmethod
    def parse_from_string(date_str):
        year, month, day = tuple(date_str.split("-"))
        return Date(int(year), int(month), int(day))

    @staticmethod
    def valid_str(date_str):
        year, month, day = tuple(date_str.split("-"))
        if int(year)>0 and (int(month) >0 and int(month)<=12) and (int(day) >0 and int(day)<=31):
            return True
        else:
            return False
    // 传递第一个参数cls就是类本身，名字可以换
    @classmethod
    def from_string(cls, date_str):
        year, month, day = tuple(date_str.split("-"))
        return cls(int(year), int(month), int(day))

    def __str__(self):
        return "{year}/{month}/{day}".format(year=self.year, month=self.month, day=self.day)

if __name__ == "__main__":
    new_day = Date(2018, 12, 31)
    new_day.tomorrow()
    print(new_day)

    #2018-12-31
    date_str = "2018-12-31"
    year, month, day = tuple(date_str.split("-"))
    new_day = Date(int(year), int(month), int(day))
    print (new_day)

    #用staticmethod完成初始化
    new_day = Date.parse_from_string(date_str)
    print (new_day)

    #用classmethod完成初始化
    new_day = Date.from_string(date_str)
    print(new_day)

    print(Date.valid_str("2018-12-32"))