import math class Square: # 正方形 def __init__(self, l): self.length = l # 邊長 def __setattr__(self, key, value): s = "呼叫__setattr__, key={}, value={}".format( key, value) print(s) if key == "length" and value > 0: self.__dict__["length"] = value self.__dict__["perimeter"] = value * 4 self.__dict__["area"] = value ** 2 if key == "perimeter" and value > 0: self.__dict__["length"] = value / 4 self.__dict__["perimeter"] = value self.__dict__["area"] = (value / 4) ** 2 if key == "area" and value > 0: self.__dict__["length"] = math.sqrt(value) self.__dict__["perimeter"] = math.sqrt(value) * 4 self.__dict__["area"] = value sq = Square(10) print("length =", sq.length) print("perimeter =", sq.perimeter) print("area =", sq.area) print("-------") sq.perimeter = 12 print("length =", sq.length) print("perimeter =", sq.perimeter) print("area =", sq.area) print("########") sq.area = 25 print("length =", sq.length) print("perimeter =", sq.perimeter) print("area =", sq.area) # 執行結果 呼叫__setattr__, key=length, value=10 length = 10 perimeter = 40 area = 100 ------- 呼叫__setattr__, key=perimeter, value=12 length = 3.0 perimeter = 12 area = 9.0 ######## 呼叫__setattr__, key=area, value=25 length = 5.0 perimeter = 20.0 area = 25