Python 设置描述符的默认（初始）值

假设我们得到了以下代码：

class NonNegative():
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value

class Person:
    fingerCount = NonNegative()
    eyesCount = NonNegative()
    def __init__(self, name):
        self.name = name

p = Person("name")
print(p.fingerCount)    <----- I want a default value printed here, instead of a KeyError
print(p.eyesCount)      <-----                 ------ | | ------

，但我们只能做到：

p = Person("name")
print(p.fingerCount)  # output: 10   
print(p.eyesCount)    # output: 2

---

将默认值传递给uuu init uuuu方法，该方法将保存该值，以便在instance.uuu dict_uuu[self.name]尚不存在时使用

class NonNegative():
    def __init__(self, v=0):  # You may or may not want a default default value
        self.dflt = v

    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__.get(self.name, self.dflt)

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value

如果愿意，可以将NonNegative子类化，以提供特定于用例的默认值

class FingerCount(NonNegative):
    def __init__(self, v=10):
        super().__init__(v)


class EyeCount(NonNegative):
    def __init__(self, v=2):
        super().__init__(v)


class Person:
    fingerCount = FingerCount()
    eyecount = EyeCount()


class Cyclops(Person):
    eyecount = EyeCount(1)

如果要为属性指定每个实例的起始值，请像任何其他属性一样，通过_init__中的setter初始化该属性

class Person:
    fingerCount = NonNegative()
    eyesCount = NonNegative()

    def __init__(self, fcount=10, ecount=2):
        self.fingerCount = fcount
        self.eyesCount = ecount

---

对于更静态的硬编码解决方案

class NonNegative:
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, instance, owner):
        if instance is None:
            return self
        if isinstance(instance, Person) and instance.__dict__.get('fingerCount') is None:
            instance.fingerCount = 10
        if isinstance(instance, Person) and instance.__dict__.get('eyesCount') is None:
            instance.eyesCount = 2
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value

class Person:
    fingerCount = NonNegative()
    eyesCount = NonNegative()
    def __init__(self, name):
        self.name = name

p = Person("name")
print(p.fingerCount)
print(p.eyesCount)
# 10
# 2

---

你知道，我确实考虑过这个问题，但我不认为它会起作用，因为Person类的所有实例都共享同一个非负描述符实例。问题：既然Person类的所有实例都共享同一个非负描述符实例，那么怎么可能有不同的self.dflt值？！你没有，这不是一个问题，只要你处理的是不可变的默认值。默认值仅在基础属性在实例字典中尚不存在时使用，根据定义，它是属性的一部分，而不是访问属性的实例。

class Person:
    fingerCount = NonNegative()
    eyesCount = NonNegative()

    def __init__(self, fcount=10, ecount=2):
        self.fingerCount = fcount
        self.eyesCount = ecount

class NonNegative:
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, instance, owner):
        if instance is None:
            return self
        if isinstance(instance, Person) and instance.__dict__.get('fingerCount') is None:
            instance.fingerCount = 10
        if isinstance(instance, Person) and instance.__dict__.get('eyesCount') is None:
            instance.eyesCount = 2
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value

class Person:
    fingerCount = NonNegative()
    eyesCount = NonNegative()
    def __init__(self, name):
        self.name = name

p = Person("name")
print(p.fingerCount)
print(p.eyesCount)
# 10
# 2