import math

class Square:  # 正方形

    def __init__(self, l):

        self.length = l  # 边长

    def __setattr__(self, key, value):

        s = "调用__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)

# 执行结果

调用__setattr__, key=length, value=10

length = 10

perimeter = 40

area = 100

-------

调用__setattr__, key=perimeter, value=12

length = 3.0

perimeter = 12

area = 9.0

########

调用__setattr__, key=area, value=25

length = 5.0

perimeter = 20.0

area = 25