CSV Issues

[Rohan]

Hello,
I'm working on a script which collects some data and puts into a csv
file which could be exported to excel.
so far so good, I'm able to do what I described.
When I run the script for the second time after a certain period of
time the results should appear next to the results of the last run,
I'm unable to make a new column when the script is run after the first
time.
Ideally I would like to have an output which looks like this.
1/20       1/27
we.py    we.py
gh.py    gj.py   <- Indicating tht the file has changed
fg.py    fg.py

Please help me out.
Thanks