Query regarding set([])?

[Dave Angel]

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