[R] Checking on closed file connections

[Duncan Murdoch]

On 21/07/2009 8:13 PM, Jim Nemesh wrote:
> On Jul 21, 2009, at 6:40 PM, Duncan Murdoch wrote:
> 
>> On 21/07/2009 5:03 PM, Jim Nemesh wrote:
>>> Hi!  I'm wondering if there's a smart way around this:
>>> fileName= (some valid file on your system)
>>>> 	fileCon=file(fileName, open="rt")
>>>> 	l<-readLines (fileCon, n= 1)
>>>>
>>>> isOpen(fileCon)
>>> [1] TRUE
>>>> 	close(fileCon)
>>>> isOpen(fileCon)
>>> Error in isOpen(fileCon) : invalid connection
>>> How do you test for a file being closed if isOpen gives you an  
>>> error  message?
>> You could use try().
>>
>> But you shouldn't be doing that.  Whoever opens the connection  
>> should close it.  The isOpen() test is designed to be used on newly  
>> created connections, being passed to some other function and  
>> guaranteed to be valid.  Was it created with file(fileName,  
>> open="rt") or file(fileName)?  That's what isOpen() is there to  
>> determine.  (The first one was opened, and should be closed by  
>> whoever opened it; the second one was not, and the function that  
>> uses it shouldn't leave it open.)
>>
>> Duncan Murdoch
>>
> 
> The story is of course slightly more complicated.  I'm writing a  
> streaming caching file input class.  When you instantiate the class,  
> you open a connection (as shown in the sample code.)  I wanted to  
> close the file input when either a) I run out of lines in the file b)  
> when the class is garbage collected.  There's something fishy going on  
> with the connection, as it always claims to be invalid when I try to  
> close it, yet it reads data out perfectly fine.
> 
> Essentially, I'm always getting warnings about not closing the file,  
> but I can't close the file!
> 
> Some paired down "real world" code (using R.oo Library)

I don't have anything against R.oo, but I really don't know it at all. 
So you're on your own with this one.

Duncan Murdoch

> 
> setConstructorS3("StreamingFileReader", function(fileName) {
> 	if (missing (fileName)) fileName =NA
> 	extend(Object(), "StreamingFileReader",
> 	.fileName=fileName,
> 	.fileCon=NA,
> 	.initialized=F)
> })
> 
> setMethodS3("init", "StreamingFileReader", function(this, ...){
> 	if (this$.initialized==F) {
> 		this$.fileCon=file(this$.fileName, open="rt")
> 		#read header
> 		if (this$.hasHeader) {
> 			l<-readLines (this$.fileCon, n= 1)
> 			l<-strsplit (l, '\t')[[1]]
> 			this$.header=l
> 		}	
> 		this$.initialized=T
> 	}
> })
> 
> setMethodS3("getHeader", "StreamingFileReader", function(this,  
> numLines=1, ...) {
> 	this$init()
> 	return (this$.header);
> })
> 
> #lots of attempts to remove, cleanup, etc the connection I'm holding  
> on to.
> setMethodS3("finalize", "StreamingFileReader", function(this, ...) {
> 	#if (this$.hasMoreData==F) return()
> 	#if (is.na(this$.fileCon)) return()
> 	#rm (this$.fileCon)
> 	#print (isOpen(this$.fileCon))
> 	#close(this$.fileCon)
> 	#close(this$.fileCon)
> })