isOpen on closed connections

As far as I can tell, 'isOpen' cannot return FALSE in the case when 'rw = ""'. 
If the connection has already been closed by 'close' or some other function, 
then isOpen will produce an error.  The problem is that when isOpen calls 
'getConnection', the connection cannot be found and 'getConnection' produces an 
error.  The check to see if it is open is never actually done.

This came up in some code where I'm trying to clean up connections after 
successfully opening them.  The problem is that if I try to close a connection 
that has already been closed, I get an error (because 'getConnection' cannot 
find it).  But then there's no way for me to find out if a connection has 
already been closed.  Perhaps there's another approach I should be taking?  The 
context is basically,

con <- file("foo", "w")

tryCatch({
	## Do stuff that might fail
	writeLines(stuff, con)
	close(con)

	file.copy("foo", "bar")
}, finally = {
	close(con)
})

"Roger D. Peng" <rpeng at jhsph.edu> writes:

As far as I can tell, 'isOpen' cannot return FALSE in the case when 'rw = ""'.
If the connection has already been closed by 'close' or some other function,
then isOpen will produce an error.  The problem is that when isOpen calls
'getConnection', the connection cannot be found and 'getConnection' produces an
error.  The check to see if it is open is never actually done.

I see this too with R-devel (r43376) {from Nov 6th}.

   con = file("example1", "w")
   isOpen(con)

   [1] TRUE

   showConnections()

     description class  mode text   isopen   can read can write
   3 "example1"  "file" "w"  "text" "opened" "no"     "yes"

   close(con)
   isOpen(con)

   Error in isOpen(con) : invalid connection

   ## printing also fails
   con
   Error in summary.connection(x) : invalid connection

This came up in some code where I'm trying to clean up connections after
successfully opening them.  The problem is that if I try to close a connection
that has already been closed, I get an error (because 'getConnection' cannot
find it).  But then there's no way for me to find out if a connection has
already been closed.  Perhaps there's another approach I should be taking?  The
context is basically,

con <- file("foo", "w")

tryCatch({
	## Do stuff that might fail
	writeLines(stuff, con)
	close(con)

	file.copy("foo", "bar")
}, finally = {
	close(con)
})

This doesn't address isOpen, but why do you have the call to close
inside the tryCatch block?  Isn't the idea that finally will always be
run and so you can be reasonably sure that close gets called once?

If your real world code is more complicated, perhaps you can make use
of a work around like:

myIsOpen = function(con) tryCatch(isOpen(con), error=function(e) FALSE)

You could do similar with myClose and "close" a connection as many
times as you'd like :-)

+ seth

I think the confusion here is over close(): that closes *and destroys* a 
connection, so it no longer exists.

isOpen applies to existing connections: you cannot close but not destroy 
them at R level, but C code can (and does).  You will see it in use in 
the utils package.


On Wed, 14 Nov 2007, Seth Falcon wrote:

"Roger D. Peng" <rpeng at jhsph.edu> writes:

As far as I can tell, 'isOpen' cannot return FALSE in the case when 
'rw = ""'.
If the connection has already been closed by 'close' or some other 
function,
then isOpen will produce an error.  The problem is that when isOpen 
calls
'getConnection', the connection cannot be found and 'getConnection' 
produces an
error.  The check to see if it is open is never actually done.

I see this too with R-devel (r43376) {from Nov 6th}.

   con = file("example1", "w")
   isOpen(con)

   [1] TRUE

   showConnections()

     description class  mode text   isopen   can read can write
   3 "example1"  "file" "w"  "text" "opened" "no"     "yes"

   close(con)
   isOpen(con)

   Error in isOpen(con) : invalid connection

   ## printing also fails
   con
   Error in summary.connection(x) : invalid connection

This came up in some code where I'm trying to clean up connections after
successfully opening them.  The problem is that if I try to close a 
connection
that has already been closed, I get an error (because 'getConnection' 
cannot
find it).  But then there's no way for me to find out if a connection 
has
already been closed.  Perhaps there's another approach I should be 
taking?  The
context is basically,

con <- file("foo", "w")

tryCatch({
    ## Do stuff that might fail
    writeLines(stuff, con)
    close(con)

    file.copy("foo", "bar")
}, finally = {
    close(con)
})

This doesn't address isOpen, but why do you have the call to close
inside the tryCatch block?  Isn't the idea that finally will always be
run and so you can be reasonably sure that close gets called once?

If your real world code is more complicated, perhaps you can make use
of a work around like:

myIsOpen = function(con) tryCatch(isOpen(con), error=function(e) FALSE)

You could do similar with myClose and "close" a connection as many
times as you'd like :-)

+ seth