设置描述符的默认（初始）值

Question

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)      <-----                 ------ | | ------

如何分别为fingerCount和eyesCount指定默认（初始）值

这样我就不必做：

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

，但我们只能做到：

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

Answer 1

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

Answer 2

将默认值传递给__init__方法，该方法将保存该值，以便在instance.__dict__[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

class Person:
    fingerCount = NonNegative(10)
    eyesCount = NonNegative(2)
    def __init__(self, name):
        self.name = name

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)

如果需要属性的每个实例起始值，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