What am I doing wrong here?

[Bill C. Wong]

################################
def func( s ):
    s1, s2 = s.split( ' ', 1 )
    try:
        raise s1
    except "aaaa":
        print 'except "aaaa"'
    except:
        print 'except:'

s = "aaaa "
func( s )
################################
If I manually assign s1 with s1 = "aaaa", then it works fine! What am I
doing wrong here?