import datetime
import time

class PK(object):
    def __init__(self,name,attack=0.0,defence=0.0):
        self.name = name
        self.attack = attack
        self.defence = defence
        self._obtain_time = datetime.datetime.now()

    @property
    def defence(self):
        return self._defence

    @defence.setter
    def defence(self,value):
        if value < 0 :
            print("防禦值小於0設置為0")
            self._defence = 0
        elif value > 1000:
            print("防禦值大於1000被設置為1000")
            self._defence = 1000
        else:
            self._defence = value

    @property
    def age(self):
        return (datetime.datetime.now() - self._obtain_time).seconds

a = PK("鎧甲",defence=10000)
print(a.defence)
a.defence = -100
print(a.age)
time.sleep(1)
print(a.age)

@property 是把方法變為屬性，_one_underline 標識這個是私有不應該直接被訪問，不屬於API，但可以通過defence 這個實例屬性來訪問  

@property 用法