继承缺失属性

Question

我收到一条错误消息说 class 吉他没有属性 "type"，但它应该继承自超级 class 设备。

class Equipment(object):
    __id = 0
    __type = 0
    __brand = 0
    __model = 0
    __serialNumber = 0
    __purchaseCost = 0
    __hireCost = 0
    __available = 0

    def __init__(self, type, brand, model, serialNumber, purchaseCost, hireCost, available):
        self.__type = type
        self.__brand = brand
        self.__model = model
        self.__serialNumber = serialNumber
        self.__purchaseCost = purchaseCost
        self.__hireCost = hireCost
        self.__available = available

    def get_type(self):
        return self.__type
    def set_type(self,type):
        self.__type = type

    def get_brand(self):
        return self.__brand
    def set_brand(self,brand):
        self.__brand = brand

    def get_model(self):
        return self.__brand
    def set_model(self,model):
        self.__model = model

    def get_serialNumber(self):
        return self.__serialNumber
    def set_serialNumber(self,serialNumber):
        self.__serialNumber = serialNumber

    def get_purchaseCost(self):
        return self.__purchaseCost
    def set_purchaseCost(self,purchaseCost):
        self.__purchaseCost = purchaseCost

    def get_hireCost(self):
        return self.__hireCost
    def set_hireCost(self,hireCost):
        self.__hireCost = hireCost

    def get_available(self):
        return self.__available
    def set_available(self,available):
        self.__available = available

    def toString(self):
        return "type : {}, brand : {}, model : {}, serial number : {}, purchase cost : {}, hire cost : {}, available : {}".format(self.__type,
                                                        self.__brand,
                                                        self.__model,
                                                        self.__serialNumber,
                                                        self.__purchaseCost,
                                                        self.__hireCost,
                                                        self.__available)           
class Guitar(Equipment):
    __subtype = 0
    __bodyType = 0
    __pickupType = 0
    __tremSystem = 0

    def __init__(self, type, brand, model, serialNumber, purchaseCost, hireCost, available, subtype, bodyType, pickupType, tremSystem):
        super(Guitar, self).__init__(type, brand, model, serialNumber, purchaseCost, hireCost, available)
        self.__subtype = subtype
        self.__bodyType = bodyType
        self.__pickupType = pickupType
        self.__tremSystem = tremSystem


    def get_subtype(self):
        return self.__subtype
    def set_subtype(self, subtype):
        self.__subtype = subtype

    def get_bodyType(self):
        return self.__bodyType
    def set_bodyType(self, bodyType):
        self.__bodyType = bodyType

    def get_pickupType(self):
        return self.__pickupType
    def set_pickupType(self, pickupType):
        self.__pickupType = pickupType

    def get_tremSystem(self):
        return self.__tremSystem
    def set_tremSystem(self, tremSystem):
        self.__tremSystem = tremSystem

    def toString(self):
        return "type : {}, subtype : {}, brand : {}, model : {}, serial number : {}, purchase cost : {}, hire cost : {}, available : {}, body type : {}, pickup type : {}, trem system : {}".format(self.__type,
                                self.__subtype,
                                self.__brand,
                                self.__model,
                                self.__serialNumber,
                                self.__purchaseCost,
                                self.__hireCost,
                                self.__available,
                                self.__bodyType,
                                self.__pickupType,
                                self.__tremSystem)  

def main():
    type = input("please enter type: ")
    brand = input("please enter brand: ")
    model = input("please enter model: ")
    serialNumber = input("please enter serial number: ")
    purchaseCost = input("please enter purchase cost: ")
    hireCost = input("please enter hire cost: ")
    available = input("is item available: ")

    if type == "guitar":
        subtype = input("please enter subtype: ")
        bodyType = input("please enter body type: ")
        pickupType = input("please enter pickup config: ")
        tremSystem = input("please enter trem system: ")


    guitar = Guitar(type, brand, model, serialNumber, purchaseCost, hireCost, available, subtype, bodyType, pickupType, tremSystem)

    print(guitar.toString())

main()

我对装备class也做了同样的事情，所以我认为这一定是一个继承的东西。

Answer 1

当你在 class 中定义一个变量时，变量前面有双下划线，最后最多有一个下划线，它会导致 python 执行 Name Mangling在定义的时候class。

就是这样的名字 - __type 被转换为 _<className>__type 。从 documentation -

Since there is a valid use-case for class-private members (namely to avoid name clashes of names with names defined by subclasses), there is limited support for such a mechanism, called name mangling. Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped.

显示此行为的示例 -

>>> class CA:
...     def __init__(self):
...             self.__type = 1
...
>>> c = CA()
>>> c.__type
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'CA' object has no attribute '__type'
>>> c._CA__type
1

举个例子 - __type，之后你不能使用 __type 访问 class 之外的变量，当它被继承时，它将被继承为 _<ClassName>__type.

即使在继承期间也适用相同的规则，

>>> class BA(CA):
...     pass
...
>>> b = BA()
>>> b._CA__type
1

但从表面上看，不确定您是否真的需要在类中定义所有名称，这样就会发生 名称重整。如果您不需要进行名称重整，请删除起始双下划线。

此外，当你直接在class中定义一个变量时，它们变成了class变量，即所有实例共享相同的引用，直到你将每个实例的引用设置为不同的对象（通过赋值）。在您的情况下，您不需要将所有变量都定义为 class 变量。

Answer 2

您在所有属性名称前加上了两个下划线。这会触发名称修改机制，将 class 名称添加到属性名称中，因此子 classes 无法覆盖它们。这是设计使然，旨在避免意外覆盖（有点像 Java 中的 "private final"）

所以这里的解决方案是去掉这些下划线。 Python 没有真正的访问限制机制（更接近的是上面解释的名称修改）并且依赖于一个简单的命名约定：带有单个前导下划线的名称被视为 "not part of the public api"（等同于其他语言中的 "protected"）所以这可能就是您想要的。

话虽这么说，你在这里甚至不需要这个。 Python 对计算属性有很强的支持，因此您也可以删除所有 getters/setters 并改用普通属性，如果需要，您可以随时选择将它们转换为计算属性。

继承缺失属性

Inheritance missing attribute

python

inheritance

super

superclass