class Test:
    W = 0
 
    def __init__(self, l, A):
        self.l = l
        self.A = A
 
    @property
    def A(self):
        try:
            return self._A
        except AttributeError:
            return 0
 
    @A.setter
    def A(self, value):
        Test.W += (value - self.A) * self.l
        self._A = value
 
instance1 = Test(5, 10)
instance2 = Test(3, 7)
instance3 = Test(6, 13)
 
print(Test.W)
 
instance1.A = 20
instance2.A = 30
instance3.A = 40
 
print(Test.W)

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