How can I append outpu to a file??

[akhar]

I am trying to  build a bot that will read links from a given page and
output them into a file but it has to be able to append to it!! I tried
rebol first for this project it was very easy but it was not reading some
sites properly, so I went back to Python now I have coded everything except
for appending the links to a file: when I tried to use append it gives me
name error for ex:
a = "adsaddsadasd"
s = "sadsdasdsad"
a.append(s)
Traceback (...)
...
nameError: append
what am I doing wrong?? Also I am trying to have each link on a line but
they are all outputed to one line, how can I change this? I am using re and
I am finding to be to unpresice compared to rebol for ex:
for the string http://www.python.com/er/  In rebol I was able to extract
www.python.com whereas in python it gives me the whole string.. I am lost in
python's power!!!
help!
regards
Stephane