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  • Getting an object from a 2d array inside of a class

    - by user36324
    I am have a class file that contains two classes, platform and platforms. platform holds the single platform information, and platforms has an 2d array of platforms. Im trying to render all of them in a for loop but it is not working. If you could kindly help me i would greatly appreciate. void Platforms::setUp() { for(int x = 0; x < tilesW; x++){ for(int y = 0; y < tilesH; y++){ Platform tempPlat(x,y,true,renderer,filename,tileSize/scaleW,tileSize/scaleH); platArray[x][y] = tempPlat; } } } void Platforms::show() { for(int x = 0; x < tilesW; x++){ for(int y = 0; y < tilesH; y++){ platArray[x][y].show(renderer,scaleW,scaleH); } } }

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  • Database Context and Singleton injection with IoC

    - by zaitsman
    All of the below relates to a ASP.NET c# app. I have a Singleton Settings MemoryCache that reads values from database on first access and caches these, then invalidates them using SQL Service Broker message and re-reads as required. For the purposes of standard controllers, i create my Db Context in a request scope. However, this obviously means that i can't use the same context in the Settings Cache class, since that is a singleton and we have a scope collision. At the moment, i ended up with two db contexts - the Controllers get it via IoC container, whereas a Singleton just creates it's own. However, i am not satisfied with this approach (mostly due to the way i feel about two contexts, the cache doesn't set anything on the db hence concurrency is not an issue as much). What is a better way to do it?

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  • Singleton & Multi-threading

    - by ronan
    Friends I have the following class that class Singleton { private: static Singleton *p_inst; Singleton(); public: static Singleton * instance() { if (!p_inst) { p_inst = new Singleton(); } return p_inst; } }; Please do elaborate on precautions taken while implementing Singleton in multi-threaded environment .. Many thanks

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  • Is it possible to apply inheritance to a Singleton class?

    - by Bragaadeesh
    Hi, Today I faced one question in interview. Is it possible to apply inheritance concept on Singleton Classes. I said since the constructor is private, we cannot extend that Singleton class (can someone please validate this). Next thing he asked me is to apply inheritance on that Singleton class. So, I made the Singleton's constructor as protected thinking that child's constructor also has be protected. But I was wrong the child can have a modifier either equal to or higher than that. So, I asked him to give a real world example on such a case. He was not able to give me one and said that I cant ask questions and wanted me to tell whether this scenario is possible or not. I went kind of blank. My question here is, Is this possible? Even if its possible, what is the use of it? What real world scenario would demand such a use. Thanks

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  • C# new class with only single property : derive from base or encapsulate into new ?

    - by Gobol
    I've tried to be descriptive :) It's rather programming-style problem than coding problem in itself. Let's suppose we have : A: public class MyDict { public Dictionary<int,string> dict; // do custom-serialization of "dict" public void SaveToFile(...); // customized deserialization of "dict" public void LoadFromFile(...); } B: public class MyDict : Dictionary<int,string> { } Which option would be better in the matter of programming style ? class B: is to be de/serialized externally. Main problem is : is it better to create new class (which would have only one property - like opt A:) or to create a new class derived - like opt B: ? I don't want any other data processing than adding/removing and de/serializing to stream. Thanks in advance!

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  • DB Object passing between classes singleton, static or other?

    - by Stephen
    So I'm designing a reporting system at work it's my first project written OOP and I'm stuck on the design choice for the DB class. Obviously I only want to create one instance of the DB class per-session/user and then pass it to each of the classes that need it. What I don't know it what's best practice for implementing this. Currently I have code like the following:- class db { private $user = 'USER'; private $pass = 'PASS'; private $tables = array( 'user','report', 'etc...'); function __construct(){ //SET UP CONNECTION AND TABLES } }; class report{ function __construct ($params = array(), $db, $user) { //Error checking/handling trimed //$db is the database object we created $this->db = $db; //$this->user is the user object for the logged in user $this->user = $user; $this->reportCreate(); } public function setPermission($permissionId = 1) { //Note the $this->db is this the best practise solution? $this->db->permission->find($permissionId) //Note the $this->user is this the best practise solution? $this->user->checkPermission(1) $data=array(); $this->db->reportpermission->insert($data) } };//end report I've been reading about using static classes and have just come across Singletons (though these appear to be passé already?) so what's current best practice for doing this?

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  • polymorphism, inheritance in c# - base class calling overridden method?

    - by Andrew Johns
    This code doesn't work, but hopefully you'll get what I'm trying to achieve here. I've got a Money class, which I've taken from http://www.noticeablydifferent.com/CodeSamples/Money.aspx, and extended it a little to include currency conversion. The implementation for the actual conversion rate could be different in each project, so I decided to move the actual method for retrieving a conversion rate (GetCurrencyConversionRate) into a derived class, but the ConvertTo method contains code that would work for any implementation assuming the derived class has overriden GetCurrencyConversionRate so it made sense to me to keep it in the parent class? So what I'm trying to do is get an instance of SubMoney, and be able to call the .ConvertTo() method, which would in turn use the overriden GetCurrencyConversionRate, and return a new instance of SubMoney. The problem is, I'm not really understanding some concepts of polymorphism and inheritance yet, so not quite sure what I'm trying to do is even possible in the way I think it is, as what is currently happening is that I end up with an Exception where it has used the base GetCurrencyConversionRate method instead of the derived one. Something tells me I need to move the ConvertTo method down to the derived class, but this seems like I'll be duplicating code in multiple implementations, so surely there's a better way? public class Money { public CurrencyConversionRate { get { return GetCurrencyConversionRate(_regionInfo.ISOCurrencySymbol); } } public static decimal GetCurrencyConversionRate(string isoCurrencySymbol) { throw new Exception("Must override this method if you wish to use it."); } public Money ConvertTo(string cultureName) { // convert to base USD first by dividing current amount by it's exchange rate. Money someMoney = this; decimal conversionRate = this.CurrencyConversionRate; decimal convertedUSDAmount = Money.Divide(someMoney, conversionRate).Amount; // now convert to new currency CultureInfo cultureInfo = new CultureInfo(cultureName); RegionInfo regionInfo = new RegionInfo(cultureInfo.LCID); conversionRate = GetCurrencyConversionRate(regionInfo.ISOCurrencySymbol); decimal convertedAmount = convertedUSDAmount * conversionRate; Money convertedMoney = new Money(convertedAmount, cultureName); return convertedMoney; } } public class SubMoney { public SubMoney(decimal amount, string cultureName) : base(amount, cultureName) {} public static new decimal GetCurrencyConversionRate(string isoCurrencySymbol) { // This would get the conversion rate from some web or database source decimal result = new Decimal(2); return result; } }

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  • jQuery only firing last class in multiple-class click

    - by user1134644
    I have a set of links like so: <a href="#internalLink1" class="classA">This has Class A</a> <a href="#internalLink2" class="classB">This has Class B</a> <a href="#internalLink3" class="classA classB">This has Class A and Class B</a> And here's the corresponding jQuery: $('.classA').click(function(){ // do class A stuff }); $('.classB').click(function(){ // do class B stuff }); Currently, when I click on the first link with Class A, it does the Class A stuff like it's supposed to. Similarly, when I click on the second link with Class B, it does the Class B stuff like it's supposed to. No worries there. My issue is, when I click on the third link with BOTH classes, it only fires the function for whichever class comes last (in this case, class B. If I put class A at the end instead, it performs class A's function). I want it to fire both. What am I doing wrong? Thanks in advance. EDIT: To those posting fiddles, nearly all of them work, so as many have said, it's most likely not my code, but the way it displays in my file. For a little more clarification, I was teaching myself some jQuery and decided to try making a (very) simple "Choose Your Own Adventure" type game. Here's a jsfiddle containing the opening of my bare-bones-please-don't-laugh game. Click on "Hide in the bushes", then "Examine the victim", then "Take any valuables and leave, he's dead already" <-- THIS is where the issue is. It's supposed to add 98 gold ("hawks") to your inventory, AND tell you that your alignment has shifted 1 point towards Chaotic. At the moment, it only does the chaotic alert, and no gold gets added to your inventory. The other option (refresh the fiddle to restart) that adds money to your inventory, but DOES NOT make you chaotic, works just fine (if you select "Search him for identification" instead of "take the money and run") Sorry this is so long!

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  • Hints to properly design UML class diagram

    - by mic4ael
    Here is the problem. I have just started learning UML and that is why I would like to ask for a few cues from experienced users how I could improve my diagram because I do know it lacks a lot of details, it has mistakes for sure etc. Renovation company hires workers. Each employee has some kind of profession, which is required to work on a particular position. Workers work in groups consisting of at most 15 members - so called production units, which specializes in a specified kind of work. Each production unit is managed by a foreman. Every worker in order to be able to perform job tasks needs proper accessories. There are two kind of tools - light and heavy. To use heavy tools, a worker must have proper privileges. A worker can have at most 3 light tools taken from the warehouse.

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  • Isn't class scope purely for organization?

    - by Di-0xide
    Isn't scope just a way to organize classes, preventing outside code from accessing certain things you don't want accessed? More specifically, is there any functional gain to having public, protected, or private-scoped methods? Is there any advantage to classifying method/property scope rather than to, say, just public-ize everything? My presumption says no simply because, in binary code, there is no sense of scope (other than r/w/e, which isn't really scope at all, but rather global permissions for a block of memory). Is this correct? What about in languages like Java and C#[.NET]?

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  • What class structure allows for a base class and mix/match of subclasses? (Similar to Users w/ roles)

    - by cdeszaq
    I have a set of base characteristics, and then a number of sub-types. Each instance must be one of the sub-types, but can be multiple sub-types at once. The sub-types of each thing can change. In general, I don't care what subtype I have, but sometimes I do care. This is much like a Users-Roles sort of relationship where a User having a particular Role gives the user additional characteristics. Sort of like duck-typing (ie. If my Foo has a Bar, I can treat it like a ThingWithABar.) Straight inheritance doesn't work, since that doesn't allow mix/match of sub-types. (ie. no multi-inheritance). Straight composition doesn't work because I can't switch that up at runtime. How can I model this?

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  • Constructor and Destructor of a singleton object called twice

    - by Bikram990
    I'm facing a problem in singleton object in c++. Here is the explanation: Problem info: I have a 4 shared libraries (say libA.so, libB.so, libC.so, libD.so) and 2 executable binary files each using one another shared library( say libE.so) which deals with files. The purpose of libE.so is to write data into a file and if the executable restarts or size of file exceeds a certain limit it is zipped and a new file is created with time stamp in name. It is using singleton object. It exports a handler class for getting and using singleton. Compressing only happens in the above said two cases. The user/loader executable can specify the starting name of file only no other control is provided by handler class. libA.so, libB.so, libC.so and libD.so have almost same behavior. They all have a class and declare and object of an handler which gets the instance of the singleton in libE.so and uses it for further purpose. All these libraries are linked to two executable binary files. If only one of the two executable runs then its fine, But if both executable runs one after other then the file of the first started executable gets compressed. Debug info: The constructor and destructor of the singleton object is called twice.(for each executable) The object of singleton is a static object and never deleted. The executable is not able to exit/return gives: glibc detected * (exe1 or exe2): double free or corruption (!prev): some_addr * Running with binaries valgrind gives that the above error is due to the destructor of the singleton object. Thanks

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  • how can i select first second or third element with given class name using CSS?

    - by Tumharyyaaden
    ie. i have the following: <div class="myclass">my text1</div> some other code+containers... <div class="myclass">my text2</div> some other code+containers... <div class="myclass">my text3</div> some other code+containers... i have the css class div.myclass {doing things} that applies to all obviously but i also wanted to be able to select the first, second or third like this: div.myclass:first {color:#000;} div.myclass:second {color:#FFF;} div.myclass:third {color:#006;} almost like the jQuery index selection .eq( index ) which is what i am using currently but need a noscript alternative. Thanks in advance!

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  • Accessing every child class of parent class in Java

    - by darkie15
    Hi All, I have to implement a logic whereby given a child class, I need to access its parent class and all other child class of that parent class, if any. I did not find any API in Java Reflection which allows us to access all child classes of a parent class. Is there any way to do it? Ex. class B extends class A class C extends class A Now using class B, I can find the superclass by calling getSuperClass(). But is there any way to find all the child classes once I have the parent class i.e. class B and class C?? Regards, darkie

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  • Some confusion with a class variable, but with a twist...

    - by Omega
    I have a class called MyPackage.WebServer and it has a property called DBEngine. I am also dynamically loading a module and class using load_module. Inside this class, it attempts to reference MyPackage.WebServer. When it does though, DBEngine is not set to the value given when WebServer is instantiated. It's the default (None). Would the fact that I'm using load_module cause a different object graph to be created and thus isolate my dynamically loaded class from the rest of my python app?

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  • Alternatives to the Singleton Design Pattern

    The Singleton Design Pattern is one of the simplest and most widely known design patterns in use in software development. However, despite its simplicity, it is very easy to get wrong and the consequences of its use even when properly implemented can outweigh its benefits. It turns out there are other ways to achieve the goals of the Singleton pattern which will often prove to be simpler, safer, and more maintainable.

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  • "Public" nested classes or not

    - by Frederick
    Suppose I have a class 'Application'. In order to be initialised it takes certain settings in the constructor. Let's also assume that the number of settings is so many that it's compelling to place them in a class of their own. Compare the following two implementations of this scenario. Implementation 1: class Application { Application(ApplicationSettings settings) { //Do initialisation here } } class ApplicationSettings { //Settings related methods and properties here } Implementation 2: class Application { Application(Application.Settings settings) { //Do initialisation here } class Settings { //Settings related methods and properties here } } To me, the second approach is very much preferable. It is more readable because it strongly emphasises the relation between the two classes. When I write code to instantiate Application class anywhere, the second approach is going to look prettier. Now just imagine the Settings class itself in turn had some similarly "related" class and that class in turn did so too. Go only three such levels and the class naming gets out out of hand in the 'non-nested' case. If you nest, however, things still stay elegant. Despite the above, I've read people saying on StackOverflow that nested classes are justified only if they're not visible to the outside world; that is if they are used only for the internal implementation of the containing class. The commonly cited objection is bloating the size of containing class's source file, but partial classes is the perfect solution for that problem. My question is, why are we wary of the "publicly exposed" use of nested classes? Are there any other arguments against such use?

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  • Is It "Wrong"/Bad Design To Put A Thread/Background Worker In A Class?

    - by Jetti
    I have a class that will read from Excel (C# and .Net 4) and in that class I have a background worker that will load the data from Excel while the UI can remain responsive. My question is as follows: Is it bad design to have a background worker in a class? Should I create my class without it and use a background worker to operate on that class? I can't see any issues really of creating my class this way but then again I am a newbie so I figured I would make sure before I continue on. I hope that this question is relevant here as I don't think it should be on stackoverflow as my code works, this just a design issue.

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  • VB.NET class inherits a base class and implements an interface issue (works in C#)

    - by 300 baud
    I am trying to create a class in VB.NET which inherits a base abstract class and also implements an interface. The interface declares a string property called Description. The base class contains a string property called Description. The main class inherits the base class and implements the interface. The existence of the Description property in the base class fulfills the interface requirements. This works fine in C# but causes issues in VB.NET. First, here is an example of the C# code which works: public interface IFoo { string Description { get; set; } } public abstract class FooBase { public string Description { get; set; } } public class MyFoo : FooBase, IFoo { } Now here is the VB.NET version which gives a compiler error: Public Interface IFoo Property Description() As String End Interface Public MustInherit Class FooBase Private _Description As String Public Property Description() As String Get Return _Description End Get Set(ByVal value As String) _Description = value End Set End Property End Class Public Class MyFoo Inherits FooBase Implements IFoo End Class If I make the base class (FooBase) implement the interface and add the Implements IFoo.Description to the property all is good, but I do not want the base class to implement the interface. The compiler error is: Class 'MyFoo' must implement 'Property Description() As String' for interface 'IFoo'. Implementing property must have matching 'ReadOnly' or 'WriteOnly' specifiers. Can VB.NET not handle this, or do I need to change my syntax somewhere to get this to work?

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  • Hide or Show singleton?

    - by Sinker
    Singleton is a common pattern implemented in both native libraries of .NET and Java. You will see it as such: C#: MyClass.Instance Java: MyClass.getInstance() The question is: when writing APIs, is it better to expose the singleton through a property or getter, or should I hide it as much as possible? Here are the alternatives for illustrative purposes: Exposed(C#): private static MyClass instance; public static MyClass Instance { get { if (instance == null) instance = new MyClass(); return instance; } } public void PerformOperation() { ... } Hidden (C#): private static MyClass instance; public static void PerformOperation() { if (instance == null) { instance = new MyClass(); } ... } EDIT: There seems to be a number of detractors of the Singleton design. Great! Please tell me why and what is the better alternative. Here is my scenario: My whole application utilises one logger (log4net/log4j). Whenever, the program has something to log, it utilises the Logger class (e.g. Logger.Instance.Warn(...) or Logger.Instance.Error(...) etc. Should I use Logger.Warn(...) or Logger.Warn(...) instead? If you have an alternative to singletons that addresses my concern, then please write an answer for it. Thank you :)

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  • Accessing parent class attribute from sub-class body

    - by warwaruk
    I have a class Klass with a class attribute my_list. I have a subclass of it SubKlass, in which i want to have a class attribute my_list which is a modified version of the same attribute from parent class: class Klass(): my_list = [1, 2, 3] class SubKlass(Klass): my_list = Klass.my_list + [4, 5] # this works, but i must specify parent class explicitly #my_list = super().my_list + [4, 5] # SystemError: super(): __class__ cell not found #my_list = my_list + [4, 5] # NameError: name 'my_list' is not defined print(Klass.my_list) print(SubKlass.my_list) So, is there a way to access parent class attribute without specifying its name?

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  • Objective-C Class Question?

    - by tarnfeld
    Hey, My head is about to explode with this logic, can anyone help? Class A #imports Class B. Class A calls Method A in Class B. This works great Class B wants to send a response back to Class A from another method that is called from Method A. If you #import Class A from Class B, it is in effect an infinite loop and the whole thing crashes. Is there a way to do this properly, like a parent type thing? BTW, I'm developing for iPhone.

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  • Followup: Python 2.6, 3 abstract base class misunderstanding

    - by Aaron
    I asked a question at Python 2.6, 3 abstract base class misunderstanding. My problem was that python abstract base classes didn't work quite the way I expected them to. There was some discussion in the comments about why I would want to use ABCs at all, and Alex Martelli provided an excellent answer on why my use didn't work and how to accomplish what I wanted. Here I'd like to address why one might want to use ABCs, and show my test code implementation based on Alex's answer. tl;dr: Code after the 16th paragraph. In the discussion on the original post, statements were made along the lines that you don't need ABCs in Python, and that ABCs don't do anything and are therefore not real classes; they're merely interface definitions. An abstract base class is just a tool in your tool box. It's a design tool that's been around for many years, and a programming tool that is explicitly available in many programming languages. It can be implemented manually in languages that don't provide it. An ABC is always a real class, even when it doesn't do anything but define an interface, because specifying the interface is what an ABC does. If that was all an ABC could do, that would be enough reason to have it in your toolbox, but in Python and some other languages they can do more. The basic reason to use an ABC is when you have a number of classes that all do the same thing (have the same interface) but do it differently, and you want to guarantee that that complete interface is implemented in all objects. A user of your classes can rely on the interface being completely implemented in all classes. You can maintain this guarantee manually. Over time you may succeed. Or you might forget something. Before Python had ABCs you could guarantee it semi-manually, by throwing NotImplementedError in all the base class's interface methods; you must implement these methods in derived classes. This is only a partial solution, because you can still instantiate such a base class. A more complete solution is to use ABCs as provided in Python 2.6 and above. Template methods and other wrinkles and patterns are ideas whose implementation can be made easier with full-citizen ABCs. Another idea in the comments was that Python doesn't need ABCs (understood as a class that only defines an interface) because it has multiple inheritance. The implied reference there seems to be Java and its single inheritance. In Java you "get around" single inheritance by inheriting from one or more interfaces. Java uses the word "interface" in two ways. A "Java interface" is a class with method signatures but no implementations. The methods are the interface's "interface" in the more general, non-Java sense of the word. Yes, Python has multiple inheritance, so you don't need Java-like "interfaces" (ABCs) merely to provide sets of interface methods to a class. But that's not the only reason in software development to use ABCs. Most generally, you use an ABC to specify an interface (set of methods) that will likely be implemented differently in different derived classes, yet that all derived classes must have. Additionally, there may be no sensible default implementation for the base class to provide. Finally, even an ABC with almost no interface is still useful. We use something like it when we have multiple except clauses for a try. Many exceptions have exactly the same interface, with only two differences: the exception's string value, and the actual class of the exception. In many exception clauses we use nothing about the exception except its class to decide what to do; catching one type of exception we do one thing, and another except clause catching a different exception does another thing. According to the exception module's doc page, BaseException is not intended to be derived by any user defined exceptions. If ABCs had been a first class Python concept from the beginning, it's easy to imagine BaseException being specified as an ABC. But enough of that. Here's some 2.6 code that demonstrates how to use ABCs, and how to specify a list-like ABC. Examples are run in ipython, which I like much better than the python shell for day to day work; I only wish it was available for python3. Your basic 2.6 ABC: from abc import ABCMeta, abstractmethod class Super(): __metaclass__ = ABCMeta @abstractmethod def method1(self): pass Test it (in ipython, python shell would be similar): In [2]: a = Super() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Super with abstract methods method1 Notice the end of the last line, where the TypeError exception tells us that method1 has not been implemented ("abstract methods method1"). That was the method designated as @abstractmethod in the preceding code. Create a subclass that inherits Super, implement method1 in the subclass and you're done. My problem, which caused me to ask the original question, was how to specify an ABC that itself defines a list interface. My naive solution was to make an ABC as above, and in the inheritance parentheses say (list). My assumption was that the class would still be abstract (can't instantiate it), and would be a list. That was wrong; inheriting from list made the class concrete, despite the abstract bits in the class definition. Alex suggested inheriting from collections.MutableSequence, which is abstract (and so doesn't make the class concrete) and list-like. I used collections.Sequence, which is also abstract but has a shorter interface and so was quicker to implement. First, Super derived from Sequence, with nothing extra: from abc import abstractmethod from collections import Sequence class Super(Sequence): pass Test it: In [6]: a = Super() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Super with abstract methods __getitem__, __len__ We can't instantiate it. A list-like full-citizen ABC; yea! Again, notice in the last line that TypeError tells us why we can't instantiate it: __getitem__ and __len__ are abstract methods. They come from collections.Sequence. But, I want a bunch of subclasses that all act like immutable lists (which collections.Sequence essentially is), and that have their own implementations of my added interface methods. In particular, I don't want to implement my own list code, Python already did that for me. So first, let's implement the missing Sequence methods, in terms of Python's list type, so that all subclasses act as lists (Sequences). First let's see the signatures of the missing abstract methods: In [12]: help(Sequence.__getitem__) Help on method __getitem__ in module _abcoll: __getitem__(self, index) unbound _abcoll.Sequence method (END) In [14]: help(Sequence.__len__) Help on method __len__ in module _abcoll: __len__(self) unbound _abcoll.Sequence method (END) __getitem__ takes an index, and __len__ takes nothing. And the implementation (so far) is: from abc import abstractmethod from collections import Sequence class Super(Sequence): # Gives us a list member for ABC methods to use. def __init__(self): self._list = [] # Abstract method in Sequence, implemented in terms of list. def __getitem__(self, index): return self._list.__getitem__(index) # Abstract method in Sequence, implemented in terms of list. def __len__(self): return self._list.__len__() # Not required. Makes printing behave like a list. def __repr__(self): return self._list.__repr__() Test it: In [34]: a = Super() In [35]: a Out[35]: [] In [36]: print a [] In [37]: len(a) Out[37]: 0 In [38]: a[0] --------------------------------------------------------------------------- IndexError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() /home/aaron/projects/test/test.py in __getitem__(self, index) 10 # Abstract method in Sequence, implemented in terms of list. 11 def __getitem__(self, index): ---> 12 return self._list.__getitem__(index) 13 14 # Abstract method in Sequence, implemented in terms of list. IndexError: list index out of range Just like a list. It's not abstract (for the moment) because we implemented both of Sequence's abstract methods. Now I want to add my bit of interface, which will be abstract in Super and therefore required to implement in any subclasses. And we'll cut to the chase and add subclasses that inherit from our ABC Super. from abc import abstractmethod from collections import Sequence class Super(Sequence): # Gives us a list member for ABC methods to use. def __init__(self): self._list = [] # Abstract method in Sequence, implemented in terms of list. def __getitem__(self, index): return self._list.__getitem__(index) # Abstract method in Sequence, implemented in terms of list. def __len__(self): return self._list.__len__() # Not required. Makes printing behave like a list. def __repr__(self): return self._list.__repr__() @abstractmethod def method1(): pass class Sub0(Super): pass class Sub1(Super): def __init__(self): self._list = [1, 2, 3] def method1(self): return [x**2 for x in self._list] def method2(self): return [x/2.0 for x in self._list] class Sub2(Super): def __init__(self): self._list = [10, 20, 30, 40] def method1(self): return [x+2 for x in self._list] We've added a new abstract method to Super, method1. This makes Super abstract again. A new class Sub0 which inherits from Super but does not implement method1, so it's also an ABC. Two new classes Sub1 and Sub2, which both inherit from Super. They both implement method1 from Super, so they're not abstract. Both implementations of method1 are different. Sub1 and Sub2 also both initialize themselves differently; in real life they might initialize themselves wildly differently. So you have two subclasses which both "is a" Super (they both implement Super's required interface) although their implementations are different. Also remember that Super, although an ABC, provides four non-abstract methods. So Super provides two things to subclasses: an implementation of collections.Sequence, and an additional abstract interface (the one abstract method) that subclasses must implement. Also, class Sub1 implements an additional method, method2, which is not part of Super's interface. Sub1 "is a" Super, but it also has additional capabilities. Test it: In [52]: a = Super() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Super with abstract methods method1 In [53]: a = Sub0() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Sub0 with abstract methods method1 In [54]: a = Sub1() In [55]: a Out[55]: [1, 2, 3] In [56]: b = Sub2() In [57]: b Out[57]: [10, 20, 30, 40] In [58]: print a, b [1, 2, 3] [10, 20, 30, 40] In [59]: a, b Out[59]: ([1, 2, 3], [10, 20, 30, 40]) In [60]: a.method1() Out[60]: [1, 4, 9] In [61]: b.method1() Out[61]: [12, 22, 32, 42] In [62]: a.method2() Out[62]: [0.5, 1.0, 1.5] [63]: a[:2] Out[63]: [1, 2] In [64]: a[0] = 5 --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: 'Sub1' object does not support item assignment Super and Sub0 are abstract and can't be instantiated (lines 52 and 53). Sub1 and Sub2 are concrete and have an immutable Sequence interface (54 through 59). Sub1 and Sub2 are instantiated differently, and their method1 implementations are different (60, 61). Sub1 includes an additional method2, beyond what's required by Super (62). Any concrete Super acts like a list/Sequence (63). A collections.Sequence is immutable (64). Finally, a wart: In [65]: a._list Out[65]: [1, 2, 3] In [66]: a._list = [] In [67]: a Out[67]: [] Super._list is spelled with a single underscore. Double underscore would have protected it from this last bit, but would have broken the implementation of methods in subclasses. Not sure why; I think because double underscore is private, and private means private. So ultimately this whole scheme relies on a gentleman's agreement not to reach in and muck with Super._list directly, as in line 65 above. Would love to know if there's a safer way to do that.

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  • Singleton Properties

    - by coffeeaddict
    Ok, if I create a singleton class and expose the singleton object through a public static property...I understand that. But my singleton class has other properties in it. Should those be static? Should those also be private? I just want to be able to access all properties of my singleton class by doing this: MySingletonClass.SingletonProperty.SomeProperty2 Where SingletonProperty returns me the single singleton instance. I guess my question is, how do you expose the other properties in the singleton class..make them private and then access them through your public singleton static property?

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