You need to split the string:
>>> s = 'http://www.domain.com/?s=some&two=20'
>>> s.split('&')
['http://www.domain.com/?s=some', 'two=20']
That will return a list as you can see so you can do:
>>> s2 = s.split('&')[0]
>>> print s2
http://www.domain.com/?s=some