super | 易学教程

How to access a superclass's class attributes in Python?

阅读更多关于 How to access a superclass's class attributes in Python?

问题 Have a look at the following code: class A(object): defaults = {'a': 1} def __getattr__(self, name): print('A.__getattr__') return self.get_default(name) @classmethod def get_default(cls, name): # some debug output print('A.get_default({}) - {}'.format(name, cls)) try: print(super(cls, cls).defaults) # as expected except AttributeError: #except for the base object class, of course pass # the actual function body try: return cls.defaults[name] except KeyError: return super(cls, cls).get

Does any magic happen when I call `super(some_cls)`?

阅读更多关于 Does any magic happen when I call `super(some_cls)`?

问题 While investigating this question, I came across this strange behavior of single-argument super : Calling super(some_class).__init__() works inside of a method of some_class (or a subclass thereof), but throws an exception when called anywhere else. Code sample: class A(): def __init__(self): super(A).__init__() # doesn't throw exception a = A() super(A).__init__() # throws exception The exception being thrown is Traceback (most recent call last): File "untitled.py", line 8, in <module> super

In Java type arguments, does <? extends E> mean strictly subtypes only? or would E also suffice?

阅读更多关于 In Java type arguments, does

问题 In Java type arguments, does mean strictly subtypes only? or would E also suffice? 回答1: Yes, super and extends gives inclusive lower and upper bounds respectively. Here's a quote from Angelika Langer's Generics FAQ: What is a bounded wildcard? A wildcard with an upper bound looks like ? extends Type and stands for the family of all types that are subtypes of Type , type Type being included . Type is called the upper bound . A wildcard with a lower bound looks like ? super Type and stands for

how does metaclass work with the MRO list when super() is called?

阅读更多关于 how does metaclass work with the MRO list when super() is called?

I'm really confused by the following code sample: class Meta_1(type): def __call__(cls, *a, **kw): # line 1 print("entering Meta_1.__call__()") print(cls) # line 4 print(cls.mro()) # line 5 print(super(Meta_1, cls).__self__) # line 6 rv = super(Meta_1, cls).__call__(*a, **kw) # line 7 print("exiting Meta_1.__call__()") return rv class Car(object, metaclass=Meta_1): def __new__(cls, *a, **kw): print("Car.__new__()") rv = super(Car, cls).__new__(cls, *a, **kw) return rv def __init__(self, *a, **kw): print("Car.__init__()") super(Car,self).__init__(*a, **kw) if __name__ == '__main__': c = Car()

Java wildcards and generics ? super T and ? extends T

阅读更多关于 Java wildcards and generics ? super T and ? extends T

问题 when dealing with wildcards such as setting/adding a generic item to a certain container is it suggested to use something like this? void add(List<? super T> someList,someitem){ someList.add(someItem); } and when retrieving an item it is suggested to use something like this <T> void f1(List<? extends T> obj, T item) { obj.add(item); } What is the principle behind this? and when will I know if I should use this ? 回答1: you should have a look at the explanation of PECS principle What is PECS

Dart inheritance and super constructor

阅读更多关于 Dart inheritance and super constructor

I am trying to define an abstract parent class Event and a concrete child PlaceChangeEvent : abstract class Event { String name; DateTime occurredAt; int handleCount; Event(this.name, this.occurredAt, [this.handleCount = 0]); } class PlaceChangeEvent implements Event { Place place; PlaceChangeEvent(Place place) : super(place.name, DateTime.now()); } The PlaceChangeEvent is giving me 2 compiler warnings: The first is on the class name itself: Missing inherited members: 'Event.name', 'Event.handleCount' and 'Event.occurredAt' The second is on the call to super : 0 positional arguments expected,

Android RecyclerView: Super Fast ListView 超级快速的Lis

阅读更多关于 Android RecyclerView: Super Fast ListView 超级快速的Lis

Android RecyclerView: Super Fast ListView 原文来自我的微信公众号: longkai_1991 先上图，看效果：前几天刚release完公司的一个项目，有了点时间，于是就想找一些有意思的东西学习一下，顺便运用在项目之中。看到iOS的同事们在谈论iOS8的xx特性时，我突然也有想在公司项目的下一个版本中添加Android L版本的特性。六月底的时候收看Google io时，当时对Android新的设计语言，Material Design，没什么太大的好感，感觉色彩一坨一坨的，好难看的样子，当时觉得亮点就是新的ART运行时环境和一些酷炫的动画效果。再后来，8月初的时候，自己出于好奇真的拿Nexus 5安装了一个L的预览版，体验很差...好多软件都还是holo的，反正觉得不是很期待就是啦。回到重点，下载好最新的SDK，你会发现在 ANDROID_HOME/extras/android/m2repository/com/android/support 下面多了不少兼容库， cardview , support-annotations , recyclerview-v7 ，眼前一亮吧~这回，Google真的是拿出了好多东西呀，赞，尤其是cardview和recyclerview这两个新的控件，这个在Google最新的Material

demystify super in python?

阅读更多关于 demystify super in python?

问题 I was trying to understand how super works in python and tried the following example: class A(object): def __init__(self): print "in A's init" class B(object): def __init__(self): print "in B's init" class C(A,B): def __init__(self): super(C,self).__init__() print "In C" if __name__=="__main__": c=C() fairly simple.. And I tried the following super calls(displayed with the results here): >>> super(B,c).__init__() >>> super(B,c).__init__() >>> super(A,c).__init__() in B's init >>> super(A,c)._

Explicit passing of Self when calling super class's init in python

阅读更多关于 Explicit passing of Self when calling super class's __init__ in python

This question is in relation to posts at What does 'super' do in Python? , How do I initialize the base (super) class? , and Python: How do I make a subclass from a superclass? which describes two ways to initialize a SuperClass from within a SubClass as class SuperClass: def __init__(self): return def superMethod(self): return ## One version of Initiation class SubClass(SuperClass): def __init__(self): SuperClass.__init__(self) def subMethod(self): return or class SuperClass: def __init__(self): return def superMethod(self): return ## Another version of Initiation class SubClass(SuperClass):

Java set super instance of instance

阅读更多关于 Java set super instance of instance

I might just be unable to google for the right words, but I can't find an answer to the following question. Is it possible to explicitly set the superclass of a new class instance. E.g. I have a SuperClazz instance and want to create a new instance of Clazz which extends SuperClazz . Can I just do something like this (the code is just what I want to do, it doesn't compile and is not correct): class Clazz extends SuperClazz{ Clazz(SuperClazz superInstance){ this.super = superInstance; } } The super class is always instantiated implicitly, so you cannot do it — "plant" the super class inside an