[英]Python class inheritance: AttributeError: '[SubClass]' object has no attribute 'xxx'
I have the following base class and subclass:我有以下基类和子类:
class Event:
def __init__(self, sr1=None, foobar=None):
self.sr1 = sr1
self.foobar = foobar
self.state = STATE_NON_EVENT
# Event class wrappers to provide syntatic sugar
class TypeTwoEvent(Event):
def __init__(self, level=None):
self.sr1 = level
self.state = STATE_EVENT_TWO
Further on in my code, I am inspecting an instance of a TypeTwoEvent
class, checking for a field I know exists in the base class - I expected it to be defaulted to value None
.在我的代码中,我正在检查TypeTwoEvent
类的实例,检查我知道基类中存在的字段 - 我希望它默认为值None
。 However, my code raises the following exception:但是,我的代码引发了以下异常:
AttributeError: 'TypeTwoEvent' object has no attribute 'foobar' AttributeError:“TypeTwoEvent”对象没有属性“foobar”
I was under the impression that the base class fields would be inherited by the subclass and that creating an instance of a subclass will instantiate the base class (and thus invoke its constructor) ...我的印象是基类字段将由子类继承,并且创建子类的实例将实例化基类(并因此调用其构造函数)......
What am I missing here?我在这里想念什么? Why does TypeTwoEvent
not have a foobar
attribute - when the base class from which it is derived has a foobar
attribute?为什么TypeTwoEvent
没有foobar
属性 - 当派生它的基类具有foobar
属性时?
Your subclass should be:你的子类应该是:
class TypeTwoEvent(Event):
def __init__(self, level=None, *args, **kwargs):
super().__init__(*args, **kwargs)
self.sr1 = level
self.state = STATE_EVENT_TWO
Because you override the __init__
method, so you need to call the parent method if you want the parent behavior to happen.因为您覆盖了__init__
方法,所以如果您希望父行为发生,则需要调用父方法。
Remember, __init__
is not a special method dispite its strange name.请记住, __init__
并不是一个特殊的方法,尽管它的名字很奇怪。 It's just the method automatically called after the object is created.它只是对象创建后自动调用的方法。 Otherwise it's an ordinary method, and ordinary inheritance rules apply.否则它是一个普通的方法,并且应用普通的继承规则。
super().__init__(arguments, that, goes, to, parents)
is the syntax to call the parent version of the method.是调用方法的父版本的语法。
For *args
and **kwargs
, it just ensures we catch all additional arguments passed to __init__
and pass it to the parent method, as you child method signature didn't do it and the parent need these arguments to work.对于*args
和**kwargs
,它只是确保我们捕获传递给__init__
的所有附加参数并将其传递给父方法,因为您的子方法签名没有这样做,而父方法需要这些参数才能工作。
You're overriding the constructor ( __init__
) of the parent class.您正在覆盖父类的构造函数 ( __init__
)。 To extend it, you need to explicitly call the constructor of the parent with a super()
call.要扩展它,您需要使用super()
调用显式调用父级的构造函数。
class TypeTwoEvent(Event):
def __init__(self, level=None, **kwargs):
# the super call to set the attributes in the parent class
super().__init__(**kwargs)
# now, extend other attributes
self.sr1 = level
self.state = STATE_EVENT_TWO
Note that the super
call is not always at the top of the __init__
method in your sub-class.请注意, super
调用并不总是位于子类中__init__
方法的顶部。 Its location depends on your situation and logic.它的位置取决于你的情况和逻辑。
When the instance is created, its __init__
method is called.创建实例时,会调用其__init__
方法。 In this case, that is TypeTwoEvent.__init__
.在这种情况下,即TypeTwoEvent.__init__
。 Superclass methods will not be called automatically because that would be immensely confusing.超类方法不会被自动调用,因为那会非常混乱。
You should call Event.__init__(self, ...)
from TypeTwoEvent.__init__
(or use super
, but if you're not familiar with it, read up on it first so you know what you're doing).您应该从TypeTwoEvent.__init__
调用Event.__init__(self, ...)
(或使用super
,但如果您不熟悉它,请先阅读它,以便您知道自己在做什么)。
I've had the same problem, but in my case I put super().__init__()
on the bottom of my derived class and that's why it doesn't work.我遇到了同样的问题,但就我而言,我将super().__init__()
放在派生类的底部,这就是它不起作用的原因。 Because I tried to use attributes that are not initialized.因为我尝试使用未初始化的属性。
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