Question

如何将实例属性标记为在基类中未实现？（与this问题不同，该问题讨论了将 class 属性标记为未实现，但也许我不能正确理解基类...）

例如我想要类似的东西

const presets = 
[ { id: 0
  , name: "videoFormats"
  , content: ["(avi|mkv|mov|mp4|mpg|wmv)"]
  , hidden: true
  , include: ["imageFormats"]
  }
, { id: 1
  , name: "audioFormats"
  , content: ["(ac3|flac|m4a|mp3|ogg|wav|wma)"]
  , hidden: true
  , include: ["imageFormats"]
  }
, { id: 2
  , name: "imageFormats"
  , content: ["(bmp|gif|jpg|jpeg|png|psd|tif|tiff)"]
  , hidden: true
  }
, { id: 3
  , name: "media"
  , title: "Media"
  , include: ["videoFormats", "audioFormats"]
  , hidden: false
  }
]

const fromArray = (a = []) =>
  a.reduce((r,x) => r.set(x.name, x), new Map)

const traverse = function* (t = new Map, name = "") {
  if (!t.has(name)) return
  yield* traverse1(t, t.get(name))
}

const traverse1 = function* (t = new Map, preset = {}) {
  yield preset
  for (const i of preset.include || [])
    yield* traverse(t, i)
}

const getIncludes = (t = new Map, name = "") =>
{ const r = new Set
  for (const p of traverse(t, name))
    if (r.has(p.name) || p.name === name)
      continue
    else
      r.add(p.name)
  return Array.from(r)
}

const tree =
  fromArray(presets)

console.log(getIncludes(tree, "media"))
// [ "videoFormats", "imageFormats", "audioFormats" ]

console.log(getIncludes(tree, "audioFormats"))
// [ "imageFormats" ]

console.log(getIncludes(tree, "imageFormats"))
// []

或者，是否有更好的方法来强制class Base(): def __init__(self): self.x = NotImplemented class GoodSub(Base): def __init__(self, x): super().__init__() self.x = x #good class BadSub(Base): def __init__(self): super().__init__() #forgot to set self.x good = GoodSub(5) bad = BadSub(-1) good.x #returns 5 bad.x #throws error because x not implemented的所有子类在初始化时设置Base属性？

编辑：link to related question

Answer 1

我会考虑将x设为财产。

class Base():
    def __init__(self):
        self.__x = None
        self.__x_is_set = False
    @property
    def x(self):
        if not self.__x_is_set:
            raise NotImplementedError('Descendents from Base must set x')
        else:
            return self.__x
    @x.setter
    def x(self, value):
        self.__x = value
        self.__x_is_set = True


class GoodSub(Base):
    def __init__(self):
        super().__init__()
        self.x = 5

class BadSub(Base):
    def __init__(self):
        super().__init__()
        pass

class AlsoBad(Base):
    def __init__(self):
        super().__init__()
        self.__x = 5  # sets the attribute, but not through the property

>>> g, a, b = GoodSub(), BadSub(), AlsoBad()
>>> g.x
5
>>> a.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in x
NotImplementedError: Descendents from Base must set x
>>> b.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in x
NotImplementedError: Descendents from Base must set x

Answer 2

具有类装饰器和描述符（__get__方法）的一种解决方案：

def abstract_variables(*args):
    class av:
        def __init__(self, error_message):
            self.error_message = error_message

        def __get__(self, *args, **kwargs):
            raise NotImplementedError(self.error_message)

    def f(klass):
        for arg in args:
            setattr(klass, arg, av('Descendants must set variable `{}`'.format(arg)))
        return klass

    return f


@abstract_variables('x', 'y')
class Base:
    def __init__(self):
        pass

class Derived(Base):
    x = 10


b = Base()
d = Derived()
print(d.x)    # prints 10
print(d.y)    # raises NotImplementedError

打印：

10
Traceback (most recent call last):
  File "main.py", line 28, in <module>
    print(d.y)
  File "main.py", line 7, in __get__
    raise NotImplementedError(self.error_message)
NotImplementedError: Descendants must set variable `y`

实例属性的Python NotImplementedError

2 个答案: