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

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

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setter

property

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 subclass42

A

property

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 nothave 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')