code: def start_session(self,
                        causal_consistency=True,
                        default_transaction_options=None):
          
          return self.__start_session(
              False,
              causal_consistency=causal_consistency,
              default_transaction_options=default_transaction_options)
docs: Start a logical session.
  
          This method takes the same parameters as
          :class:`~pymongo.client_session.SessionOptions`. See the
          :mod:`~pymongo.client_session` module for details and examples.
  
          Requires MongoDB 3.6. It is an error to call :meth:`start_session`
          if this client has been authenticated to multiple databases using the
          deprecated method :meth:`~pymongo.database.Database.authenticate`.
  
          A :class:`~pymongo.client_session.ClientSession` may only be used with
          the MongoClient that started it.
  
          :Returns:
            An instance of :class:`~pymongo.client_session.ClientSession`.
  
          .. versionadded:: 3.6