A bug with file.tell()?

Nick Craig-Wood nick at craig-wood.com
Fri Sep 10 10:30:02 CEST 2004


Nick Jacobson <nicksjacobson at yahoo.com> wrote:
>  Is this a bug?
> 
>  This code involves the file methods "seek" and "tell".  Even though
>  the file pointer is in the middle of the file, "tell" returns a
>  position at the end of the file!

Its something to do with buffering when using the file() as an
iterator.

>  fp = file("test.txt")
>  #read in some of the file:
>  for line in fp:
>      if line == "blah3\n":
>          break
>  fpos = fp.tell() #save the current position...
> 
>  for line in fp:
>      print line #prints "asdf", so it wasn't at the end of the file
> 
>  fp.seek(fpos) #rewind?
>  for line in fp:
>      print line #prints nothing, because it's at the end of the file!
> 
> 
>  The test.txt file is:
>  blah1
>  blah2
>  blah3
>  asdf

If you re-write your loop using this equivalent code :-

#...
while 1:
    line = fp.readline()
    if line == "": # EOF
        break
    if line == "blah3\n":
        break
#...

You'll find it works fine.

IMHO this is a bug - even with buffering tell() should give where the
file returned data to the user not where its read data from the file.

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick



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