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  • What are the common programming mistakes in Python?

    - by Paul McGuire
    I was about to tag the recent question in which the OP accidentally shadowed the builtin operator module with his own local operator.py with the "common-mistakes" tag, and I saw that there are a number of interesting questions posted asking for common mistakes to avoid in Java, Ruby, Scala, Clojure, .Net, jQuery, Haskell, SQL, ColdFusion, and so on, but I didn't see any for Python. For the benefit of Python beginners, can we enumerate the common mistakes that we have all committed at one time or another, in the hopes of maybe steering a newbie or two clear of them? (In homage to "The Princess Bride", I call these the Classic Blunders.) If possible, a little supporting explanation on what the problem is, and the generally accepted resolution/workaround, so that the beginning Pythoner doesn't read your answer and say "ok, that's a mistake, how do I fix it?"

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  • LINQ query null reference exception

    - by user289082
    Hi! I have the next query: var bPermisos = from b in ruc.Permisos where b.IdUsuario == cu.Id select new Permisos(){ Id=b.Id, IdUsuario=b.Id, IdPerfil=b.Id, Estatus=b.Estatus }; var qSisPer = from perm in bPermisos select new { perm.IdPerfil,perm.Cat_Perfil.Nivel,perm.Cat_Perfil.Nombre,Nombre_Sistem=perm.Cat_Perfil.Cat_Sistema.Nombre}; And is throwing me an exception, plz help!

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  • Groovy: stub typed reference

    - by Don
    Hi, I have a Groovy class similar to class MyClass { Foo foo } Under certain circumstances I don't want to initialize foo and want to stub out all the calls to it. Any methods that return a value should do nothing. I could do it like this: Foo.metaClass.method1 = {param -> } Foo.metaClass.method2 = { -> } Foo.metaClass.method3 = {param1, param2 -> } While this will work, it has a couple of problems Tedious and long-winded, particularly if Foo has a lot of methods This will stub out calls to any instance of Foo (not just foo) Although Groovy provides a StubFor class, if I do this: this.foo = new groovy.mock.interceptor.StubFor(Foo) I get a ClassCastException at runtime. Although this would work if I could redefine foo as: def foo But for reasons I won't go into here, I can't do that. Thanks, Don

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  • SQLite join selection from the same table using reference from another table

    - by daikini
    I have two tables: table: points |key_id | name | x | y | ------------------------ |1 | A |10 |20 | |2 | A_1 |11 |21 | |3 | B |30 |40 | |4 | B_1 |31 |42 | table: pairs |f_key_p1 | f_key_p2 | ---------------------- |1 | 2 | |3 | 4 | Table 'pairs' defines which rows in table 'points' should be paired. How can I query database to select paired rows? My desired query result would be like this: |name_1|x_1|x_2|name_2|x_2|y_2| ------------------------------- |A |10 |20 |A_1 |11 |21 | |B |30 |40 |B_1 |31 |41 |

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  • Beginning OS X & iPhone Development: Reference texts

    - by twinbornJoint
    My programming experience is limited to xhtml, css, php, sql and some javascript. The books I have are: Programming in Objective-C 2nd Edition Beginning iPhone Development Cocoa(R) Programming for Mac(R) OS X (Paperback) Is there anything else I will need to get started on my journey into iPhone/OS X development?

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  • Swig C++ Lua Pass class by reference

    - by Jeremy
    I don't know why I'm having a hard time with this. All I want to do is this: class foo { public: foo(){} ~foo(){} float a,b; }; class foo2 { public: foo2(){} foo2(const foo &f){*this = f;} ~foo2(){} void operator=(const foo& f){ x = f.a; y = f.b; } float x,y; }; /* Usage(cpp): foo f; foo2 f2(f); //or using the = operator f2 = f; */ The problem I'm having is that, after swigging this code, I can't figure out how to make the lua script play nice. /* Usage(lua) f = example.foo() f2 = example.foo2(f) --error */ The error I get is "Wrong arguments for overloaded function 'new_Foo2'": Possible c/c++ prototypes are: foo2() foo2(foo const &) The same thing happens if I try and use do f2 = f. As I understand it everything is stored as a pointer so I did try adding an additional constructor that took a pointer to foo but to no avail.

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  • ASP.NET control in one content area needs to reference a control in another content area

    - by harrije
    I have a master page that divides the main content into two areas. There are two asp:ContentPlaceHolder controls in the body section of the master page with IDs cphMain and cphSideBar respectively. One of the corresponding content pages has a control in cphMain that needs to refer to a control in cphSideBar. Specifically, a SqlDataSource in cphMain references a TextBox in cphSideBar to use as a parameter in the select command. When the content page loads the following run-time error occurs: Could not find control 'TextBox1' in ControlParameter 'date_con'. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.InvalidOperationException: Could not find control 'TextBox1' in ControlParameter 'date_con'. Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. Stack Trace: [InvalidOperationException: Could not find control 'TextBox1' in ControlParameter 'date_con'.] System.Web.UI.WebControls.ControlParameter.Evaluate(HttpContext context, Control control) +1753150 System.Web.UI.WebControls.Parameter.UpdateValue(HttpContext context, Control control) +47 System.Web.UI.WebControls.ParameterCollection.UpdateValues(HttpContext context, Control control) +114 System.Web.UI.WebControls.SqlDataSource.LoadCompleteEventHandler(Object sender, EventArgs e) +43 System.EventHandler.Invoke(Object sender, EventArgs e) +0 System.Web.UI.Page.OnLoadComplete(EventArgs e) +8698566 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +735 I kinda know what the problem is... ASP.NET does not like the fact that the SqlDataSource and TextBox are in different asp:Content controls within the content page. As a workaround, I have another TextBox in cphMain with the SqlDataSource which has Visible=False. Then in the Page_Load() event handler the contents of the TextBox in cphSideBar is copied into the contents of the non-visible TextBox in cphMain. I get the results I want with the work around I've come up with, but it seems like such a hack. I was wondering if there is a better solution I'm missing. Please advise.

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  • How to pass interface type/GUID reference to an automation method in Delphi

    - by Alan Clark
    In Delphi you can pass class references around to compare the types of objects, and to instantiate them. Can you do the same with interface references being passed to a COM automation server? For example, you can define a method taking a GUID parameter using the type library editor: function ChildNodesOfType(NodeType: TGUID): IMBNode; safecall; In this function I would like to return automation types that support the interface specified by NodeType, e.g. if Supports(SomeNode, NodeType) then result := SomeNode; But the Supports call always fails, I tried passing in the Guids defined in the type library but none of the different types (Ixxx, Class_xxxx, IId_Ixxxx) seem to work.

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  • Passing expression as a parameter in Call by reference

    - by darkie15
    All, When we are passing an expression as a parameter, how does the evaluation occur? Here is a small example. This is just a pseudocode kind of example: f (x,y) { y = y+1; x = x+y; } main() { a = 2; b = 2; f(a+b, a) print a; } When accessing variable x in f, does it access the address of the temp variable which contains the result of a+b or will it access the individual addresses of a and b and then evaluate the value of a+b Please help. Regards, darkie15

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  • linq NullReferenceException while checking for null reference

    - by Odrade
    I have the following LINQ query: List<FileInputItem> inputList = GetInputList(); var results = from FileInputItem f in inputList where ( Path.GetDirectoryName(f.Folder).ToLower().Trim() == somePath || Path.GetDirectoryName(f.Folder).ToLower().Trim() == someOtherPath ) && f.Expression == null select f; Every time this query is executed, it generates a NullReferenceException. If I remove the condition f.Expression == null or change it to f.Expression != null, the query executes normally (giving the wrong results, of course). The relevant bits of FileInputItem look like this: [Serializable] public class FileInputItem { [XmlElement("Folder")] public string Folder { get; set; } [XmlElement("Expression")] public string Expression { get; set; } /*SNIP. Irrelevant properties */ } I'm new to LINQ to objects, so I'm probably missing something fundamental here. What's the deal?

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  • Exporting Eclipse project with a reference to native library

    - by TacB0sS
    I have an Eclipse project, that uses JMF, I found out I could skip the JMF installation process and still to use the CaptureDeviceManager of the JMF, and to receive the list of devices if I could point my project to the native lib of the JMF. I've managed to add the native lib to the IDE run/debug, but once I export the application to an external runnable Jar, the application cannot find the native lib. the files are located in c:\JMF*.dll I tried to add the folder path to the environment variable in windows - didn't work. I tried to add them into another Jar and add it to the project - didn't work. I tried to add the files into the project - didn't work. I tried to add the path to the class path - didn't work. I tried to add the path to the library path - didn't work. does someone have any sort of a solution? Thanks in advance, Adam Zehavi.

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  • WCFExtras SOAP headers support returns null reference exception

    - by jlp
    I use WCFExtras to add headers to my service. I set up everything according to http://wcfextras.codeplex.com/wikipage?title=HowToUse&referringTitle=Home My WCF client works but throws NullReferenceException on both: client.InnerChannel.SetHeader("myHeader", new local.Header()); and var header = client.InnerChannel.GetHeader<local.Header>("myHeader"); I wonder if web.config element <wsdlExtensions location="noidea" singleFile="True"/> might cause a problem because I don't know what type in location attribute. I simply tried to type service address but it didn't fix anything.

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  • How to quote and reference MS SQL table and field names

    - by artvolk
    Good day! Despite the fact LINQ2SQL and ADO.NET Entity Framework exists there were situations when I need to revert to plain old DataSet (not typed) and friends. While writing SQL for SqlCommand: Is it needed to quote field and table names with []? Is it good to prefix table names with [dbo] I use to use this syntax: SqlCommand command = new SqlCommand("SELECT [Field1], [Field2] FROM [dbo].[TableName]", connection); May be there is a better way? Thanks in advance!

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  • What is the typical method to separate connected letters in a word using OCR

    - by Maysam
    I am very new to OCR and almost know nothing about the algorithms used to recognize words. I am just getting familiar to that. Could anybody please advise on the typical method used to recognize and separate individual characters in connected form (I mean in a word where all letters are linked together)? Forget about handwriting, supposing the letters are connected together using a known font, what is the best method to determine each individual character in a word? When characters are written separately there is no problem, but when they are joined together, we should know where every single character starts and ends in order to go to the next step and match them individually with a letter. Is there any known algorithm for that?

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  • Core Data: Deleting causes 'NSObjectInaccessibleException' from NSOperation with a reference to a deleted object

    - by Bryan Irace
    My application has NSOperation subclasses that fetch and operate on managed objects. My application also periodically purges rows from the database, which can result in the following race condition: An background operation fetches a bunch of objects (from a thread-specific context). It will iterate over these objects and do something with their properties. A bunch of rows are deleted in the main managed object context. The background operation accesses a property on an object that was deleted from the main context. This results in an 'NSObjectInaccessibleException', reason: 'CoreData could not fulfill a fault' Ideally, the objects that are fetched by the NSOperation can be operated on even if one is deleted in the main context. The best way I can think to achieve this is either to: Call [request setReturnsObjectsAsFaults:NO] to ensure that Core Data won't try to fulfill a fault for an object that no longer exists in the main context. The problem here is I may need to access the object's relationships, which (to my understanding) will still be faulted. Iterate through the managed objects up front and copy the properties I will need into separate non-managed objects. The problem here is that (I think) I will need to synchronize/lock this part, in case an object is deleted in the main context before I can finish copying. Am I missing something obvious? It doesn't seem like what I'm trying to accomplish is too out of the ordinary. Thanks for your help.

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  • return by reference - which of the two is the correct one

    - by eriks
    i have the function: const A& f(...) {...} a. const A a1 = f(..); b. const A &a2 = f(...); which of the is the better one to use? in both cases, if i understand correctly, i prevent the possibility of modifying the returned object. in the first option, the copy constructor of A will be called - am i correct?

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  • reference dll not copying to bin with deployment project causing error

    - by g.foley
    We have several external dlls being referenced in our Web Application Project. We have a deployment project for installing on the hosting servers. When we were using .net 3.5 and vs2008 the dll were being copied to the bin folder. Since we have upgraded to .net 4 vs2010 this no longer happens and we are getting servers errors since the references cannot be found. CopyLocal is set to true, and i cannot find anything inside the web.config which suggest this is being set else where.

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  • How to reference using Entity Framework and Asp.Net Mvc 2

    - by Picflight
    Tables CREATE TABLE [dbo].[Users]( [UserId] [int] IDENTITY(1,1) NOT NULL, [UserName] [varchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, [Email] [varchar](255) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, [BirthDate] [smalldatetime] NULL, [CountryId] [int] NULL, CONSTRAINT [PK_Users] PRIMARY KEY CLUSTERED ([UserId] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY] CREATE TABLE [dbo].[TeamMember]( [UserId] [int] NOT NULL, [TeamMemberUserId] [int] NOT NULL, [CreateDate] [smalldatetime] NOT NULL CONSTRAINT [DF_TeamMember_CreateDate] DEFAULT (getdate()), CONSTRAINT [PK_TeamMember] PRIMARY KEY CLUSTERED ([UserId] ASC, [TeamMemberUserId] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY] dbo.TeamMember has both UserId and TeamMemberUserId as the index key. My goal is to show a list of Users on my View. In the list I want to flag, or highlight the Users that are Team Members of the LoggedIn user. My ViewModel public class UserViewModel { public int UserId { get; private set; } public string UserName { get; private set; } public bool HighLight { get; private set; } public UserViewModel(Users users, bool highlight) { this.UserId = users.UserId; this.UserName = users.UserName; this.HighLight = highlight; } } View <%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<MvcPaging.IPagedList<MyProject.Mvc.Models.UserViewModel>>" %> <% foreach (var item in Model) { %> <%= item.UserId %> <%= item.UserName %> <%if (item.HighLight) { %> Team Member <% } else { %> Not Team Member <% } %> How do I toggle the TeamMember or Not If I add dbo.TeamMember to the EDM, there are no relationships on this table, how will I wire it to Users object? So I am comparing the LoggedIn UserId with this list(SELECT TeamMemberUserId FROM TeamMember WHERE UserId = @LoggedInUserId)

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  • Linux - specific API reference

    - by Goofy
    Hello! Where can I find centralized and complete documentation aboput Linux - specific API? I'm preparing Linux port of my application and i want to use as much Linux - specific features as it's possible. So far I found that Linux provide epoll and inotify API, which are great news for me, because my program works as network server and monitor local file systems.

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  • C# WPF Apllication class

    - by hans
    my applicationen consists of App.cs/App.xaml and some other files. Everything is compiled to an exe file. However, I want a 2. application, which should reference the first one. In this Application is a class derived from App. But when I call the the Run() function I get an very stragne error in a xaml code of the first application, saying that some image files could not be loaded. I had a similar architecture when i was using Windows Forms and it worked fine.

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