TypeError: coercing to Unicode: need string or buffer, User found

i have to crawl last.fm for users (university exercise). I'm new to python and get following error:

 Traceback (most recent call last):
  File "crawler.py", line 23, in <module>
    for f in user_.开发者_开发问答get_friends(limit='200'):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pylast.py", line 2717, in get_friends
    for node in _collect_nodes(limit, self, "user.getFriends", False):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pylast.py", line 3409, in _collect_nodes
    doc = sender._request(method_name, cacheable, params)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pylast.py", line 969, in _request
    return _Request(self.network, method_name, params).execute(cacheable)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pylast.py", line 721, in __init__
    self.sign_it()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pylast.py", line 727, in sign_it
    self.params['api_sig'] = self._get_signature()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pylast.py", line 740, in _get_signature
    string += self.params[name]
TypeError: coercing to Unicode: need string or buffer, User found

i use the pylast lib for crawling. what i want to do:

i want to get a users friends and the friends of the users friends. the error occurs, when i have a for loop in another for loop. here's the code:

network = pylast.get_lastfm_network(api_key = API_KEY, api_secret = API_SECRET, username = username, password_hash = password_hash)
user = network.get_user("vidarnelson")

friends = user.get_friends(limit='200')

i = 1

for friend in friends:
 user_ = network.get_user(friend)
 print '#%d %s' % (i, friend)
 i = i + 1

 for f in user_.get_friends(limit='200'):
  print f

any advice?

thanks in advance. regards!

---

It looks like get_friends will return a list of User objects, so you don't need to call get_user on its entries. Just use:

for friend in friends:
    for f in friend.get_friends(limit='200'):
        ...