class Value:
    def __init__(self):
        self.value = None

    def __get__(self, obj, obj_type):
        return self.value

    def __set__(self, obj, value):
        self.value = value - value * obj.commission

    def __delete__(self, obj):
        pass


class MyAccount:
    amount = Value()

    def __init__(self, commission):
        self.commission = commission
