ubuntu 11.10 desktop: paramiko.SSHException: No existing session

I have tried 1.7.7.1, 1.7.4, 1.7.5 ..., they all have same problems:

Python 2.7.2+ (default, Oct 4 2011, 20:03:08)
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import paramiko
>>> paramiko.__version__
'1.7.7.1 (George)'
>>>

yes, i know it. can use "sudo", but i want to know why.:)

You'd have to look at the script to see what it accesses

pass debug info:
DEBUG:paramiko.transport:starting thread (client mode): 0xb714b02cL
INFO:paramiko.transport:Connected (version 2.0, client OpenSSH_4.3)
DEBUG:paramiko.transport:kex algos:['diffie-hellman-group-exchange-sha1', 'diffie-hellman-group14-sha1', 'diffie-hellman-group1-sha1'] server key:['ssh-rsa', 'ssh-dss'] client encrypt:['aes128-ctr', 'aes192-ctr', 'aes256-ctr', 'arcfour256', 'arcfour128', 'aes128-cbc', '3des-cbc', 'blowfish-cbc', 'cast128-cbc', 'aes192-cbc', 'aes256-cbc', 'arcfour', '<email address hidden>'] server encrypt:['aes128-ctr', 'aes192-ctr', 'aes256-ctr', 'arcfour256', 'arcfour128', 'aes128-cbc', '3des-cbc', 'blowfish-cbc', 'cast128-cbc', 'aes192-cbc', 'aes256-cbc', 'arcfour', '<email address hidden>'] client mac:['hmac-md5', 'hmac-sha1', 'hmac-ripemd160', '<email address hidden>', 'hmac-sha1-96', 'hmac-md5-96'] server mac:['hmac-md5', 'hmac-sha1', 'hmac-ripemd160', '<email address hidden>', 'hmac-sha1-96', 'hmac-md5-96'] client compress:['none', '<email address hidden>'] server compress:['none', '<email address hidden>'] client lang:[''] server lang:[''] kex follows?False
DEBUG:paramiko.transport:Ciphers agreed: local=aes128-ctr, remote=aes128-ctr
DEBUG:paramiko.transport:using kex diffie-hellman-group1-sha1; server key type ssh-rsa; cipher: local aes128-ctr, remote aes128-ctr; mac: local hmac-sha1, remote hmac-sha1; compression: local none, remote none
DEBUG:paramiko.transport:Switch to new keys ...
DEBUG:paramiko.transport:Adding ssh-rsa host key for 10.56.117.82: 36a9604e78384576455244088b688c0a
DEBUG:paramiko.transport:userauth is OK
INFO:paramiko.transport:Authentication (password) successful!
DEBUG:paramiko.transport:[chan 1] Max packet in: 34816 bytes
DEBUG:paramiko.transport:[chan 1] Max packet out: 32768 bytes
INFO:paramiko.transport:Secsh channel 1 opened.
DEBUG:paramiko.transport:[chan 1] Sesch channel 1 request ok
out: <paramiko.ChannelFile from <paramiko.Channel 1 (open) window=2097152 -> <paramiko.Transport at 0xb714b02cL (cipher aes128-ctr, 128 bits) (active; 1 open channel(s))>>>
err: <paramiko.ChannelFile from <paramiko.Channel 1 (open) window=2097152 -> <paramiko.Transport at 0xb714b02cL (cipher aes128-ctr, 128 bits) (active; 1 open channel(s))>>>
DEBUG:paramiko.transport:EOF in transport thread

failed debug info:
y136wang@y136wang-VirtualBox:~/work/Eclipse_workspace/python_test/src$ python test.py
DEBUG:paramiko.transport:starting thread (client mode): 0x919a1ccL
INFO:paramiko.transport:Connected (version 2.0, client OpenSSH_4.3)
DEBUG:paramiko.transport:kex algos:['diffie-hellman-group-exchange-sha1', 'diffie-hellman-group14-sha1', 'diffie-hellman-group1-sha1'] server key:['ssh-rsa', 'ssh-dss'] client encrypt:['aes128-ctr', 'aes192-ctr', 'aes256-ctr', 'arcfour256', 'arcfour128', 'aes128-cbc', '3des-cbc', 'blowfish-cbc', 'cast128-cbc', 'aes192-cbc', 'aes256-cbc', 'arcfour', '<email address hidden>'] server encrypt:['aes128-ctr', 'aes192-ctr', 'aes256-ctr', 'arcfour256', 'arcfour128', 'aes128-cbc', '3des-cbc', 'blowfish-cbc', 'cast128-cbc', 'aes192-cbc', 'aes256-cbc', 'arcfour', '<email address hidden>'] client mac:['hmac-md5', 'hmac-sha1', 'hmac-ripemd160', '<email address hidden>', 'hmac-sha1-96', 'hmac-md5-96'] server mac:['hmac-md5', 'hmac-sha1', 'hmac-ripemd160', '<email address hidden>', 'hmac-sha1-96', 'hmac-md5-96'] client compress:['none', '<email address hidden>'] server compress:['none', '<email address hidden>'] client lang:[''] server lang:[''] kex follows?False
DEBUG:paramiko.transport:Ciphers agreed: local=aes128-ctr, remote=aes128-ctr
DEBUG:paramiko.transport:using kex diffie-hellman-group1-sha1; server key type ssh-rsa; cipher: local aes128-ctr, remote aes128-ctr; mac: local hmac-sha1, remote hmac-sha1; compression: local none, remote none
DEBUG:paramiko.transport:Switch to new keys ...
DEBUG:paramiko.transport:Adding ssh-rsa host key for 10.56.117.82: 36a9604e78384576455244088b688c0a
DEBUG:paramiko.transport:Trying SSH agent key 94fb43f4fcab28b992cc3f07ecbd1484
DEBUG:paramiko.transport:userauth is OK
ERROR:paramiko.transport:Exception: key cannot be used for signing
ERROR:paramiko.transport:Traceback (most recent call last):
ERROR:paramiko.transport: File "build/bdist.linux-i686/egg/paramiko/transport.py", line 1559, in run
ERROR:paramiko.transport: self.auth_handler._handler_table[ptype](self.auth_handler, m)
ERROR:paramiko.transport: File "build/bdist.linux-i686/egg/paramiko/auth_handler.py", line 209, in _parse_service_accept
ERROR:paramiko.transport: sig = self.private_key.sign_ssh_data(self.transport.rng, blob)
ERROR:paramiko.transport: File "build/bdist.linux-i686/egg/paramiko/agent.py", line 152, in sign_ssh_data
ERROR:paramiko.transport: raise SSHException('key cannot be used for signing')
ERROR:paramiko.transport:SSHException: key cannot be used for signing
ERROR:paramiko.transport:
Traceback (most recent call last):
  File "test.py", line 19, in <module>
    main()
  File "test.py", line 8, in main
    ssh.connect("10.56.117.82", username="root", password="nokia123")
  File "build/bdist.linux-i686/egg/paramiko/client.py", line 332, in connect
  File "build/bdist.linux-i686/egg/paramiko/client.py", line 493, in _auth
paramiko.SSHException: No existing session

fix method:
ssh.connect(ip_addr, username=user, password=passwd, allow_agent=False, look_for_keys=False)

thank you very much!

Could report a bug or the issue to the author

Thanks actionparsnip, that solved my question.