如何在Python中定义惰性变量，它将为抽象代码框架引发NotImplementedError？

Question

我想在类中定义一些常量，这些常量将在类实例（派生的类）中定义-
如果未在子类中重新定义此变量，如何发出错误信号？我想NotImplementedError读一读。

class Parent(object):
   abstract_variable = ?

   # I want achieve same behavior for variable
   def abstract_function(self):
     raise NotImplementedError()

class Child(Parent):
   def __init__():
     # should throw NotImplementedError() on read
     print self.abstract_variable

可以一行完成吗？

abstract_variable = ?

Answer 1

首先，最明显的是不要在父类中做任何事情。然后，在阅读时，您只会得到一个属性错误：

AttributeError: Child instance has no attribute 'abstract_variable'

或者在父类中，您可以在每个子类中都有一个使用getter和setterproperty引发NotImplementedError 并
覆盖它的property。或在子类中将值设置为None在类主体中…

---

但是，如果要提高NotImplementedError，则可以创建一个 非数据 描述符（即， 没有
的描述符类 ，始终具有）。这使您可以在子类中设置值。
____set__``property``__set__

最简单的方法是

class abstract_attribute(object):
    def __get__(self, obj, type):
        raise NotImplementedError("This attribute was not set in a subclass")

你像这样使用它

class Parent(object):
    variable = abstract_attribute()

class Child(Parent):
    def __init__(self):
        try:
            print(self.variable)
        except Exception as e:
            print("Got error %s" % e)

        self.variable = 42
        print(self.variable)

Child()

哪个输出

Got error This attribute was not set in a subclass
42

Aproperty不能像my那样容易地设置值abstract_attribute。

---

但是，等等，我们可以使其更加神奇：描述符可以找出从 哪个 属性访问它：

class abstract_attribute(object):
    def __get__(self, obj, type):   
        # Now we will iterate over the names on the class,
        # and all its superclasses, and try to find the attribute
        # name for this descriptor
        # traverse the parents in the method resolution order
        for cls in type.__mro__:
            # for each cls thus, see what attributes they set
            for name, value in cls.__dict__.items():
                # we found ourselves here
                if value is self:
                    # if the property gets accessed as Child.variable,
                    # obj will be done. For this case
                    # If accessed as a_child.variable, the class Child is 
                    # in the type, and a_child in the obj.
                    this_obj = obj if obj else type

                    raise NotImplementedError(
                         "%r does not have the attribute %r "
                         "(abstract from class %r)" %
                             (this_obj, name, cls.__name__))

        # we did not find a match, should be rare, but prepare for it
        raise NotImplementedError(
            "%s does not set the abstract attribute <unknown>", type.__name__)

使用此代码，访问self.variable会引发异常提示信息：

NotImplementedError: <__main__.Child object at 0x7f7c7a5dd860> does not
have the attribute 'variable' (abstract from class 'Parent')

并Child.variable给

NotImplementedError: <class '__main__.Child'> does not have the 
attribute 'variable' (abstract from class 'Parent')