class A:
    def __init__(self):
        self._bill = 1

    @property
    def bill(self):
        return self._bill
    
    @bill.setter
    def bill(self,value):
        self._bill = value
        # raise PermissionError("You can't change the bill")

class B(A):
    def __init__(self):
        super().__init__()
        self._bill = 2

# b = B()
# print(b.bill)
a=A()
a.bill=3
print(a.bill)