Notebook type, acceptable values are 'jupyter' as well as any names
        defined by external notebook hooks that have been installed.

def notebook_type(self, notebook_type):
        ''' Notebook type, acceptable values are 'jupyter' as well as any names
        defined by external notebook hooks that have been installed.

        '''
        if notebook_type is None or not isinstance(notebook_type, string_types):
            raise ValueError("Notebook type must be a string")
        self._notebook_type = notebook_type.lower()