关于继承：Python NotImplementedError for instance attributes

Python NotImplementedError for instance attributes

如何将实例属性标记为未在基类中实现？ (与讨论将类属性标记为未实现的问题不同，但也许我没有正确理解基类……)

例如我想要类似

的东西

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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

或者，有没有更好的方法来强制 Base 的所有子类在初始化时设置 self.x 属性？

编辑：相关问题的链接

一个带有类装饰器和描述符的解决方案(__get__ 方法)：

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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：

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7

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`

我会考虑将 x 设为属性。

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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

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>>> 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

原创文章，作者：ItWorker，如若转载，请注明出处：https://blog.ytso.com/267867.html

关于继承：Python NotImplementedError for instance attributes

Python NotImplementedError for instance attributes

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