code: def from_transport(cls, t):
          
          chan = t.open_session()
          if chan is None:
              return None
          chan.invoke_subsystem('sftp')
          return cls(chan)
docs: Create an SFTP client channel from an open L{Transport}.
  
          @param t: an open L{Transport} which is already authenticated
          @type t: L{Transport}
          @return: a new L{SFTPClient} object, referring to an sftp session
              (channel) across the transport
          @rtype: L{SFTPClient}