Reverse Word Order

rev_words.py

def rev_words(s):
    '''Returns the string `s` with its words in the reverse order.
       Example: rev_words("Reverse this!") #"this! Reverse"
    '''
    def helper(s, word_acc, str_acc):
        # Base case: Return `word_acc` plus whatever words
        # you have in the string acc.
        if len(s) == 0:
            return word_acc + str_acc
        # This is the end of a word. Clear `word_acc`, and start with
        # the next word.
        elif s[0] == " ":
            return helper(s[1:], "", " " + word_acc + str_acc)
        # In the middle of a word. Add the letter to `word_acc`, and continue
        else:
            return helper(s[1:], word_acc + s[0], str_acc)
    return helper(s, "", "")

if __name__=="__main__":
    assert rev_words("") == ""
    assert rev_words(" ") == " "
    assert rev_words("a") == "a"
    assert rev_words("Captain Kirk") == "Kirk Captain"
    assert rev_words(" Captain Kirk ") == " Kirk Captain "
    assert rev_words("THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG") == "DOG LAZY THE OVER JUMPS FOX BROWN QUICK THE"
    assert rev_words("Reverse this string.") == "string. this Reverse"
    assert rev_words("   many spaces    many spaces") == "spaces many    spaces many   "