reverse words

def reverse_words(s):
    s.reverse()
    start = 0
    for end in range(len(s)):
        if end == len(s)-1 or s[end+1] == ' ':
            for shift in range((end+1-start) // 2):
                s[start+shift], s[end-shift] = s[end-shift], s[start+shift]
            start = end + 2


s = list('abc f')
reverse_words(s)
assert s == list('f abc')

s = list('x bla')
reverse_words(s)
assert s == list('bla x')

s = list('')
reverse_words(s)
assert s == list('')