How to return an "not string' error in function?

[breakfastea at gmail.com]

first of all I have to claim that I'm a noob so please help me don't
blame me:)

for example:

def test(s):
       if type(s) != ? :
          return
#So here I want establish a situation about that if <s> is not string
#then <return>, but how should write the <?> ?
#Or is there any other way to do it?

Any suggestion would be appreciate

Peace