If it's just a substring search you can use string.find("substring").
You do have to be a little careful with find, index, and in though, as they are substring searches. In other words, this:
s = "This be a string"
if s.find("is") == -1:
print "No 'is' here!"
print "Found 'is' in the string."
It would print Found 'is' in the string. Similarly, if "is" in s: would evaluate to True. This may or may not be what you want.