How does the file.seek() work ?

[Graham Dumpleton]

On Aug 25, 5:37 am, Tim Chase <python.l... at tim.thechases.com> wrote:
> > I want the file pointer set to 100 and overwrite everything from there
> [snip]
> > def application(environ, response):
> >     query=os.path.join(os.path.dirname(__file__),'teeeeeeeeeemp')
> >     range=environ.get('HTTP_RANGE','bytes=0-').replace
> > ('bytes=','').split(',')
> >     offset=[]
> >     for r in range: offset.append(r.split('-'))
> >     with open(query,'w+') as f:
> >          f.seek(int(offset[0][0]))
> >          while True:
> >              chunk=environ['wsgi.input'].read(8192).decode('latin1')
> >              if not chunk: break
> >              f.write(chunk)
> >     f=open(query)
> >     l=str(os.fstat(f.fileno()).st_size)
> >     response('200 OK', [('Content-Type', 'text/plain'), ('Content-
> > Length', str(len(l)))])
> >     return [l]
>
> A couple items of note:
>
> - you don't open the file in binary mode -- seek is more reliable
> in binary mode :)

If my memory is right, if file is opened in binary mode, also wouldn't
need to be decoding the WSGI input stream as latin-1 to get a string.
Instead can just deal with bytes and write bytes to file.

Graham

> - if you want to lop off the rest of the file, use f.truncate()
>
> An example:
>
> # create the initial file
>  >>> f = file('zzz.zzz', 'wb+')
>  >>> f.write('abcdefghijklmnop')
>  >>> f.close()
>
>  >>> f = file('zzz.zzz', 'ab+')
>  >>> f.read() # show the existing content
> 'abcdefghijklmnop'
>  >>> f.seek(5) # seek to the desired offset
>  >>> f.truncate() # throw away everything after here
>  >>> f.write('zyx') # write the new data at pos=5
>  >>> f.close()
>
> # demonstrate that it worked
>  >>> f = file('zzz.zzz', 'rb')
>  >>> f.read()
> 'abcdezyx'
>  >>> f.close()
>
> > also why must I open the file a second time to know how big it is ?
>
> Likely the output has been buffered.  You can try using
>
>    f.flush() # write all the data to the disk first
>    size = os.fstat(f.fileno()).st_size
>
> which seems to do the trick for me.
>
> -tkc