Python 过滤重复单词

描述：一个英文句子仅由单词、逗号、句号、空格组成。要求过滤句子中的重复单词，例如：输入where there is a will, there is a way.

输出：where there is a will way

str = raw_input("");
a = []
str = str.replace(",", "").replace(".", "")
a = str.split(" ")
b = sorted(set(a),key=a.index)
for x in b:
    if x !="":
        print x,