I have a QDialogBox where there is an option to upload a file.
I can upload files and save them to a folder. It works fine.
But if in case there is a file that already exists in the folder, I am not sure how to handle that scenario.
I want to warn the user that the file with same name already exists.
Is there a Windows API that I can use in this case? (because when we manually save an existing file, we get a warning, how can I use that?)
If someone can point me to that documentation, it will be great.
If you are using a QFileDialog, confirmOverwrite is activated by default, so, if getSaveFileName() returned a non-empty QString, then that means the user accepted to overwrite the file. Other way, you get an empty QString.
Then, you can check if the file exists, and remove it in that case, but you know that the user was Ok with that.
There is always a potential race condition when saving files. Checking to see if the file exists first is not safe, because some other process could create a file with the same name in between the check and when you actually write the file.
To avoid problems, the file must be opened with exclusive access, and in such a way that it immediately fails if it already exists.
If you want to do things properly, take a look at these two answers:
How do I create a file in python without overwriting an existing
file
Safely create a file if and only if it does not exist with
python
You can use QDir::entryList() to get the file names in a directory if you're not using a QFileDialog.
QDir dir("/path/to/directory");
QStringList fileNames = dir.entryList();
Then iterating through file names, you can see if there's a file with the same name. If you need it, I can give an example for that too. It'd be C++, but easily adaptable to Python.
Edit: Smasho just suggested that using QDir::exists() method. You can check if the file name exists in the directory with this method instead of iterating like I suggested.
if(dir.exists(uploadedFileName))
Related
How do I see if a file exists without exceptions using Julia? I want to make sure that my program does not crash if for some reason the file I am trying to open is not accessible, has been deleted, or does not exist.
There are two simple ways of doing so.
First:
println(isfile("Sphere.jl"))
false
This isfile() function will simply check if the file exists. Note: if Sphere.jl is not in your current file path, you would need to provide the absolute path to get to that file.
Second (more of a trial by fire example):
try
open("Sphere.jl", "w") do s
println(s, "Hi")
end
catch
#warn "Could not open the file to write."
end
The second example utilizes the try-catch schema. It is always best for your program to not have to deal with errors so it's recommended that you use isfile() unless you have to use try-catch for your use case.
It's worth noting that there may be some cases where the file exists, but writing to it is not possible (i.e. it's locked by the os). In that case, using try-catch is a great option when attempting to write.
A passage in the file documentation caught my eye:
## We can do the same thing with an anonymous file.
Tfile <- file()
cat("abc\ndef\n", file = Tfile)
readLines(Tfile)
close(Tfile)
What exactly is this anonymous file? Does it exist on disk, or only in memory? I'm interested in this as I'm contemplating a program that will potentially need to create/delete thousands of temporary files, and if this happens only in memory it seems like it would have a much lesser impact on system resources.
This linux SO Q appears to suggest this file could be a real disk file, but I'm not sure how relevant to this particular example that is. Additionally, this big memory doc seems to hint at a real disk based storage (though I'm assuming the file based anonymous file is being used):
It should also be noted that a user can create an “anonymous” file-backed big.matrix by specifying "" as the filebacking argument. In this case, the backing resides in the temporary directory and a descriptor file is not created. These should be used with caution since even anonymous backings use disk space which could eventually fill the hard drive. Anonymous backings are removed either manually, by a user, or automatically, when the operating system deems it appropriate.
Alternatively, if textConnection is appropriate for use for this type of application (opened/closed hundreds/thousands of times) and is memory only that would satisfy my needs. I was planning on doing this until I read the note in that function's documentation:
As output text connections keep the character vector up to date line-by-line, they are relatively expensive to use, and it is often better to use an anonymous file() connection to collect output.
My C is very rusty, so hopefully more experienced people can correct me, but I think the answer to your question "What exactly is this anonymous file? Does it exist on disk, or only in memory?" is "It exists on disk".
Here is what happens at C level (I'm looking at the source code at http://cran.r-project.org/src/base/R-3/R-3.0.2.tar.gz):
A. Function file_open, defined in src/main/connections.c:554, has the following logic related to anonymous file (with an empty description), lines 565-568:
if(strlen(con->description) == 0) {
temp = TRUE;
name = R_tmpnam("Rf", R_TempDir);
} else name = R_ExpandFileName(con->description);
So a new temporary filename is generated if no file name was supplied to file.
B. If the name of the file is not equal to stdin, the call R_fopen(name, con->mode) happens at line 585 (there some subtleties with Win32 and UTF8 names, but we can ignore them now).
C. Finally, the file name is unlinked at line 607. The documentation for unlink says:
The unlink() function removes the link named by path from its
directory and decrements the link count of the file which was
referenced by the link. If that decrement
reduces the link count of the file to zero, and no process has the file open, then all resources associated with the file are
reclaimed. If one or more process have the
file open when the last link is removed, the link is removed, but the removal of the file is delayed until all references to it have
been closed.
So in effect the directory entry is removed but file exists as long as it's being open by R process.
D. Finally, R_fopen is defined in src/main/sysutils.c:135 and just calls fopen internally.
I am after the one which is most used. A number of ones I have come across are:
CurDir
Environment.CurrentDirectory()
AppDomain.CurrentDomain.BaseDirectory
Application.StartupPath (this one doesn't work for me, missing a library?)
I am using it to save a file, for argument sake, "test.txt"
I may be oversimplifying, but if you want to save something in the folder where your app is running, just omit the path.
call MyObj.SaveTo("test.txt")
I am creating an xml file. I need to check first if the file exists or not. If the file does not exist, create it and add the data cmg from a .cs file.
If the file exists, don't create the file just add the data cmg from a .cs file.
My code looks like this:
string filename="c:\\employee.xml";
XmlTextWriter tw=new XmlTextWriter(filename,null);//null represents
the Encoding Type//
tw.Formatting=Formatting.Indented; //for xml tags to be indented//
tw.WriteStartDocument(); //Indicates the starting of document (Required)//
tw.WriteStartElement("Employees");
tw.WriteStartElement("Employee","Genius");
tw.WriteStartElement("EmpID","1");
tw.WriteAttributeString("Name","krishnan");
tw.WriteElementString("Designation","Software Developer");
tw.WriteElementString("FullName","krishnan Lakshmipuram Narayanan");
tw.WriteEndElement();
tw.WriteEndElement();
tw.WriteEndDocument();
tw.Flush();
tw.Close();
so next time we add data to file we need to check if the file exits and add data to xml file
and as we have made empID as a primary key, if user tries to make duplicate entry we need to avoid
Is this possible to do?
if (!File.Exists(filename))
{
// create your file
}
or
if (File.Exists(filename))
{
File.Delete(filename);
}
// then create your file
File class is in System.IO namespace (add using System.IO; to your file)
You can't append records to an XML file, you have to read the file and then rewrite it.
So, just check if the file exists, and read the records from it. Then write the file including all previous records and the new record.
Have a look at the File.Exists method here
Testing for existance of a file before attempting to create it inherently is subject to a "things change after check" race condition. Who can guarantee you that your application isn't preempted and put to sleep for a moment after you checked, someone else creates/deletes that file, your app gets to run again and does exactly the opposite of what you intended ?
Windows (as well as all UN*X variants) supports file open/create modes that allow to perform that create-if-nonexistant/open-if-existant operation as a single call.
As far as .NET goes, this means for your task (create an XML file) you'd first create a System.IO.FileStream with the appropriate modes, see http://msdn.microsoft.com/en-us/library/system.io.filemode.aspx and then pass that stream to the XmlWriter constructor. That's safer than simply performing an "exists" check and hoping for the best.
As part of a Classic ASP Project the user should be able to download a file - which is dynamicly extracted from a zip archive and sent via Response.BinaryWrite() - by simply calling "document.asp?id=[some id here]".
Extracting and sending is not the problem but I need to delete the extracted file after the download finished. I never did any ASP or VBA before and I guess that's why I stuck here.
I tried deleting the file right after Response.WriteBinary() using FileSystemObject.DeleteFile() but this results in a 404-Error on the client-side.
How can I wait till the download finished and then do additional actions?
Edit: This is how my code looks like:
'Unzip a specified file from an archive and put it's path in *document*
set stream = Server.CreateObject("ADODB.Stream")
stream.Open
stream.Type = 1 ' binary
stream.LoadFromFile(document)
Response.BinaryWrite(stream.Read)
'Here I want to delete the *document*
I suspect that the point you are calling the DeleteFile method the file you are trying delete is currently locked by something else, the question is what?
Try including:-
stream.Close()
after your BinaryWrite. Also make sure you've done a similar thing to the component you've used to extract the file. If the component doesn't offer any obviouse "close" methods they trying assigning Nothing to the variables referencing them.
Is it not possible to stream the file into memory, then binary write the stream to the browser, this way the file is never created on the server and there is no need to delete it.
I found a solution: The extracted files are saved in a special directory and everytime a user runs the document.asp it checks this directory for files older than one hour and deletes them.
I think it's the simplest way to manage, but furthermore I would prefer a solution where the document is deleted after downloading.