code: def commit(self, callback=None):
          '''Close the transaction and commit session to the backend.'''
          if self.executed:
              raise InvalidTransaction('Invalid operation. '
                                       'Transaction already executed.')
          session = self.session
          self.session = None
          self.on_result = self._commit(session, callback)
          return self.on_resulf commit(self, callback=None):
          '''Close the transaction and commit session to the backend.'''
          if self.executed:
              raise InvalidTransaction('Invalid operation. '
                                       'Transaction already executed.')
          session = self.session
          self.session = None
          self.on_result = self._commit(session, callback)
          return self.on_result
docs: Close the transaction and commit session to the backend.