如何在python类中定义新属性？

Question

我写了一个类，它有 3 个隐藏属性：

class Car():
    def __init__(self, name, model, brand):
        self.__name = name
        self.__model = model
        self.__brand = brand

是否可以为此类创建一个方法来设置新属性（发布者）？ 我写了这个，但我没有得到结果：

    def set_publisher(self, publisher):
        self.__publisher = publisher


b1 = Car(name="X", model="Y", brand="Z")
publisher="D"
b1.set_publisher(publisher)

错误：

AttributeError: 'Car' object has no attribute 'set_publisher'

Answer 1

如果set_publisher 被正确定义为实例方法，则没有问题。

class Car:
    def __init__(self, name, model, brand):
        self.__name = name
        self.__model = model
        self.__brand = brand

    def set_publisher(self, publisher):
        self.__publisher = publisher

但是，如果 setter 只允许将任何值直接分配给属性，那么为什么还要隐藏它呢？只需将 publisher 设为公共属性：

class Car:
    def __init__(self, name, model, brand, publisher=None):
        self.__name = name
        self.__model = model
        self.__brand = brand
        self.publisher = None  # Define but None is usually better than undefined

c = Car("Dave", "Model T", "Ford")
c.publisher = "Henry"

如果您稍后决定对如何（或是否）设置发布者施加进一步的限制，您可以将其替换为属性而不更改类的接口。

class Car:
    def __init__(self, name, model, brand, publisher=None):
        self.__name = name
        self.__model = model
        self.__brand = brand
        self._publisher = None

    @property
    def publisher(self):
        return self._publisher

    @publisher.setter
    def publisher(self, value):
        # If you want to examine value and reject it,
        # do so and raise a ValueError. You can modify
        # value before assigning it as well.

        if value == "Bob":
            raise ValueError("Bob is not a publisher")
        if value == "Alice":
            # Alice changed her name a while ago
            value = "Carol"
        self._publisher = value

c = Car("Dave", "Model T", "Ford")
c.publisher = "Henry"  # OK
c.publisher = "Alice"  # assert c.publisher == "Carol"
c.publisher = "Bob"  # ValueError

Answer 2

您可以尝试此代码以使用方法（setattr 的包装器）按名称和值添加属性：

class Car:
    def __init__(self, name, model, brand):
        self.__name = name
        self.__model = model
        self.__brand = brand

    def set_attribute(self, attr_name, value):
        setattr(self, attr_name, value)


obj = Car(name="name", model="model", brand="brand")
obj.set_attribute("__publisher", "publisher")
print(obj.__publisher)

>>> publisher