(OT) python file-truncate on fc10 zero-fills file: who to ask?

[John Pilkington]

Hi:  I'm asking here because this is the most fc10-oriented list that I 
use and I thought someone might know the answer.  I'd be happy to ask 
elsewhere but don't know where might be the best place.

I'm trying to do shrink-to-fit with video files on fc10 after tcrequant 
was deprecated.  vamps looks as if it will do the job but I think I need 
to chop off a few kb from the end of the input file first to avoid a 
failure exit.  I can do this with dd but that's slow and file truncation 
would be better.  I can't see a command-line truncate but since I am 
hacking the mythburn.py script I thought I had found the answer here, in 
truncate([size]) :

http://www.python.org/doc/2.5.2/lib/bltin-file-objects.html

I've tried that.  The file size shrinks as required but the result is 
zero-filled.  Here's some stripped-down test code:

         vobsize = os.path.getsize(source)
         write("Initial vobsize is %s bytes" % vobsize)

         vobsize -= 2048
         f=open(source,'wb')
         f.truncate( vobsize )
         f.close()

         vobsize = os.path.getsize(source)
         write("vobsize after truncation is %s bytes" % vobsize)

Is this a known problem with python, or in its fc10 implementation? 
Where would be the best place to ask how to do this job?

TIA

John Pilkington

[Patrick O'Callaghan]

On Thu, 2009-03-05 at 15:32 +0000, John Pilkington wrote:
>  Hi:  I'm asking here because this is the most fc10-oriented list
> that I 
>  use and I thought someone might know the answer.  I'd be happy to ask 
>  elsewhere but don't know where might be the best place.
>  
>  I'm trying to do shrink-to-fit with video files on fc10 after tcrequant 
>  was deprecated.  vamps looks as if it will do the job but I think I need 
>  to chop off a few kb from the end of the input file first to avoid a 
>  failure exit.  I can do this with dd but that's slow and file truncation 
>  would be better.  I can't see a command-line truncate but since I am 
>  hacking the mythburn.py script I thought I had found the answer here, in 
>  truncate([size]) :
>  
>  http://www.python.org/doc/2.5.2/lib/bltin-file-objects.html
>  
>  I've tried that.  The file size shrinks as required but the result is 
>  zero-filled.  Here's some stripped-down test code:
>  
>           vobsize = os.path.getsize(source)
>           write("Initial vobsize is %s bytes" % vobsize)
>  
>           vobsize -= 2048
>           f=open(source,'wb')
>           f.truncate( vobsize )
>           f.close()
>  
>           vobsize = os.path.getsize(source)
>           write("vobsize after truncation is %s bytes" % vobsize)
>  
>  Is this a known problem with python, or in its fc10 implementation? 
>  Where would be the best place to ask how to do this job? 
I've no idea, but I suspect you could probably write a tiny C program to
do the truncation is less time than it would take to get an answer.

poc

[John Pilkington]

Patrick O'Callaghan wrote:
>  On Thu, 2009-03-05 at 15:32 +0000, John Pilkington wrote:
> > Hi:  I'm asking here because this is the most fc10-oriented list that I 
> > use and I thought someone might know the answer.  I'd be happy to ask 
> > elsewhere but don't know where might be the best place.
> >
> > I'm trying to do shrink-to-fit with video files on fc10 after tcrequant 
> > was deprecated.  vamps looks as if it will do the job but I think I need 
> > to chop off a few kb from the end of the input file first to avoid a 
> > failure exit.  I can do this with dd but that's slow and file truncation 
> > would be better.  I can't see a command-line truncate but since I am 
> > hacking the mythburn.py script I thought I had found the answer here, in 
> > truncate([size]) :
> >
> > http://www.python.org/doc/2.5.2/lib/bltin-file-objects.html
> >
> > I've tried that.  The file size shrinks as required but the result is 
> > zero-filled.  Here's some stripped-down test code:
> >
> >          vobsize = os.path.getsize(source)
> >          write("Initial vobsize is %s bytes" % vobsize)
> >
> >          vobsize -= 2048
> >          f=open(source,'wb')
> >          f.truncate( vobsize )
> >          f.close()
> >
> >          vobsize = os.path.getsize(source)
> >          write("vobsize after truncation is %s bytes" % vobsize)
> >
> > Is this a known problem with python, or in its fc10 implementation? 
> > Where would be the best place to ask how to do this job?
>  
>  I've no idea, but I suspect you could probably write a tiny C program to
>  do the truncation is less time than it would take to get an answer.
>  
>  poc
>   
For the record - ie in case someone comes to this by Google - the 
problem was solved by using f=open(source,'ab'), which opens the binary 
file in append mode.

John P