code: def start_client(self, event=None):
          
          self.active = True
          if event is not None:
              # async, return immediately and let the app poll for completion
              self.completion_event = event
              self.start()
              return
  
          # synchronous, wait for a result
          self.completion_event = event = threading.Event()
          self.start()
          Random.atfork()
          while True:
              event.wait(0.1)
              if not self.active:
                  e = self.get_exception()
                  if e is not None:
                      raise e
                  raise SSHException('Negotiation failed.')
              if event.isSet():
                  break
docs: Negotiate a new SSH2 session as a client.  This is the first step after
          creating a new L{Transport}.  A separate thread is created for protocol
          negotiation.
  
          If an event is passed in, this method returns immediately.  When
          negotiation is done (successful or not), the given C{Event} will
          be triggered.  On failure, L{is_active} will return C{False}.
  
          (Since 1.4) If C{event} is C{None}, this method will not return until
          negotation is done.  On success, the method returns normally.
          Otherwise an SSHException is raised.
  
          After a successful negotiation, you will usually want to authenticate,
          calling L{auth_password <Transport.auth_password>} or
          L{auth_publickey <Transport.auth_publickey>}.
  
          @note: L{connect} is a simpler method for connecting as a client.
  
          @note: After calling this method (or L{start_server} or L{connect}),
              you should no longer directly read from or write to the original
              socket object.
  
          @param event: an event to trigger when negotiation is complete
              (optional)
          @type event: threading.Event
  
          @raise SSHException: if negotiation fails (and no C{event} was passed
              in)