Query regarding set([])?

[vox]

On Jul 10, 4:17 pm, Dave Angel <da... at ieee.org> wrote:
> vox wrote:
> > On Jul 10, 2:04 pm, Peter Otten <__pete... at web.de> wrote:
>
> >> You are probably misinterpreting len(s3). s3 contains lines occuring in
> >> "file1" but not in "file2". Duplicate lines are only counted once, and the
> >> order doesn't matter.
>
> >> So there are 119 lines that occur at least once in "file2", but not in
> >> "file1".
>
> >> If that is not what you want you have to tell us what exactly you are
> >> looking for.
>
> >> Peter
>
> > Hi,
> > Thanks for the answer.
>
> > I am looking for a script that compares file1 and file2, for each line
> > in file1, check if line is present in file2. If the line from file1 is
> > not present in file2, print that line/write it to file3, because I
> > have to know what lines to add to file2.
>
> > BR,
> > Andy
>
> There's no more detail in that response.  To the level of detail you
> provide, the program works perfectly.  Just loop through the set and
> write the members to the file.
>
> But you have some unspecified assumptions:
>     1) order doesn't matter
>     2) duplicates are impossible in the input file, or at least not
> meaningful.  So the correct output file could very well be smaller than
> either of the input files.
>
> And a few others that might matter:
>     3) the two files are both text files, with identical line endings
> matching your OS default
>     4) the two files are ASCII, or at least 8 bit encoded, using the
> same encoding  (such as both UTF-8)
>     5) the last line of each file DOES have a trailing newline sequence

Thanks all for the input!
I have guess I have to think it through a couple times more. :)

BR,
Andy