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[QUOTE="Anthony Tolle, post: 4017415"]
My guess is that there are a few possible problems:

1) In this case, writing to file opened with 'r+' without an explicit
f.seek is probably not a good idea.  The file iterator (for line in f)
uses a readahead buffer, which means you can't guarantee what the
current file position will be.

2) It may be necessary to do an explicit f.flush or f.close when
writing to an 'r+' file.  In your case, the close should automatically
happen when the f object falls out of scope, which tells me that were
still looking at some other problem, like not using f.seek

3) It is possible that f.tell implicitly flushes buffers used by the
file object.  That would explain why uncommenting the f.tell causes
the writes to show up.

What are you trying to accomplish?  Overwrite the original file, or
append to it?  If you want to overwrite the file, it may be better to
generate a new file, delete the old one, then rename the new one.  If
you want to append, then it would be better to open the file with
append mode ('a')
[/QUOTE]

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