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为什么我在尝试urlencode此字符串时收到此错误

 >>> callback = "http://localhost/application/authtwitter?twitterCallback"
 >>> urllib.urlencode(callback)
 Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python2.7/urllib.py", line 1261, in urlencode
      raise TypeError
 TypeError: not a valid non-string sequence or mapping object