Python 属性和变量赋值

Question

下面是来自 this tutorial 的一个类旨在演示 Python 中属性的功能。

该类设置和获取摄氏温度，并转换为华氏温度:

class Celsius:
    def __init__(self, current_temp = 25):
        self.temperature = current_temp

    def to_fahrenheit(self):
        return (self.temperature * 1.8) + 32

    def get_temperature(self):
        print("Getting value")
        return self._temperature

    def set_temperature(self, value):
        if value < -273:
            raise ValueError("Temperature below -273 is not possible")
        print("Setting value")
        self._temperature = 50

    temperature = property(get_temperature,set_temperature)

问题:self.temperature(一个实例变量)是否真的引用了类变量 温度？如果是，那为什么？

例子:

obj = Celsius()
print(obj._temperature, obj.temperature)

返回:

Setting valueGetting value50 50

我只是不明白具有指定值 (current_temp) 的实例变量如何实际引用类变量。对不起，如果我有什么错误。

Answer 1

Is it true that self.temperature (an instance variable) is actually referencing the class variable temperature?

你的意思是属性，不是变量。但是是的。

If so, then why?

因为类的实例没有temperature属性(它们只有_temperature属性，所以在Python找不到之后实例中的属性，它也会检查类。

也许您被抛弃了，因为 __init__ 方法分配给了 self.temperature。但是，这不会在实例上创建 temperature 属性，因为首先找到该属性。

有complicated rules属性查找的工作原理；它们旨在让您可以做您想做的事 - 例如，让 property 可以工作。

I just don't understand how an instance variable with an assigned value (current_temp) could be actually referencing a class variable.

我在你的代码中没有看到 current_temp，所以我无法帮助你。

---

代码令人困惑的部分原因是它以非常不标准的方式做事。 Normally , property 在它的装饰器形式中使用，看起来像这样:

class Celsius:
    def __init__(self, current_temp = 25):
        # set the underlying value directly; don't use the property
        # although you *can*, but it's arguably clearer this way
        self._temperature = current_temp

    def to_fahrenheit(self):
        return (self.temperature * 1.8) + 32

    # the getter is given the name that the property should have.
    @property
    def temperature(self):
        print("Getting value")
        return self._temperature

    # The setter is "attached" to the getter with this syntax
    @temperature.setter
    # It doesn't matter that this has the same name, because of how the
    # internal attachment logic works.
    def temperature(self, value):
        if value < -273:
            raise ValueError("Temperature below -273 is not possible")
        print("Setting value")
        self._temperature = value # good idea to actually use the value!

好处:可以说，更好的设计是具有单独的 celcius 和 fahrenheit 属性，并且能够从摄氏度或华氏度值创建实例(使用 code>staticmethod 创建工厂方法)。看起来像:

class Temperature:
    # Secret implementation detail: we store the value in celcius.
    # We could have chosen fahrenheit instead, and do the conversions
    # the other way around. Users should not call this directly.
    def __init__(self, c):
        # Actually, I lied. One good reason to use the property in
        # the init method is to enforce bounds checking.
        self.celcius = c

    # We offer factory methods that let the user be explicit about
    # how the initial value is provided.
    @staticmethod
    def in_celcius(c):
        return Temperature(c)

    @staticmethod
    def in_fahrenheit(f):
        return Temperature((f - 32) * 5/9)

    # Now we define properties that let us get and set the value
    # as either a celcius value or a fahrenheit one.
    @property
    def celcius(self):
        return self._celcius

    @celcius.setter
    def celcius(self, c):
        if c < -273:
            raise ValueError("below absolute zero")
        self._celcius = c

    @property
    def fahrenheit(self):
        return (self._celcius * 9/5) + 32

    @fahrenheit.setter
    def fahrenheit(self, f):
        # use the other property to reuse the bounds-checking logic.
        self.celcius = (f - 32) * 5/9