Possible read()/readline() bug?

[pruebauno at latinmail.com]

On Oct 22, 3:44 pm, prueba... at latinmail.com wrote:
> On Oct 22, 2:54 pm, Mike Kent <mrmak... at cox.net> wrote:
>
>
>
> > Before I file a bug report against Python 2.5.2, I want to run this by
> > the newsgroup to make sure I'm not being stupid.
>
> > I have a text file of fixed-length records I want to read in random
> > order.  That file is being changed in real-time by another process,
> > and my process want to see the changes to the file.  What I'm seeing
> > is that, once I've opened the file and read a record, all subsequent
> > seeks to and reads of that same record will return the same data as
> > the first read of the record, so long as I don't close and reopen the
> > file.  This indicates some sort of buffering and caching is going on.
>
> > Consider the following:
>
> > $ echo "hi" >foo.txt  # Create my test file
> > $ python2.5              # Run Python
> > Python 2.5.2 (r252:60911, Sep 22 2008, 16:13:07)
> > [GCC 3.4.6 20060404 (Red Hat 3.4.6-9)] on linux2
> > Type "help", "copyright", "credits" or "license" for more information.
>
> > >>> f = open('foo.txt')  # Open my test file
> > >>> f.seek(0)                # Seek to the beginning of the file
> > >>> f.readline()             # Read the line, I get the data I expected
> > 'hi\n'
> > >>> # At this point, in another shell I execute 'echo "bye" >foo.txt'.  'foo.txt' now has been changed
> > >>> # on the disk, and now contains 'bye\n'.
> > >>> f.seek(0)                # Seek to the beginning of the still-open file
> > >>> f.readline()             # Read the line, I don't get 'bye\n', I get the original data, which is no longer there.
> > 'hi\n'
> > >>> f.close()                 # Now I close the file...
> > >>> f = open('foo.txt') # ... and reopen it
> > >>> f.seek(0)               # Seek to the beginning of the file
> > >>> f.readline()            # Read the line, I get the expected 'bye\n'
> > 'bye\n'
>
> > It seems pretty clear to me that this is wrong.  If there is any
> > caching going on, it should clearly be discarded if I do a seek.  Note
> > that it's not just readline() that's returning me the wrong, cached
> > data, as I've also tried this with read(), and I get the same
> > results.  It's not acceptable that I have to close and reopen the file
> > before every read when I'm doing random record access.
>
> > So, is this a bug, or am I being stupid?
>
> This has to do how the OS file-system operates. This is equivalent of
> doing:
>
> echo "hi" >foo.txt
> vi foo.txt
> in another session type: echo "bye" > foo.txt
>
> the text in the vi session doesn't change.
>
> you can even type 'rm foo.txt' and vi will still have the text there.

Actually disregard what I said. vi loads everything in memory. You
might want to try:

f = open('foo.txt','r',0)

and see if that fixes your problem.