I want to have a function that returns wether the user is in tree or form view. How do i get the current view mode?

    def action_button_get_view(self, cr, uid, ids, context=None):
        if context is None:
            context = {}
        mode = context.get('view_mode')
        _logger = logging.getLogger('test_func')
        _logger.info("view mode is  %s" % mode)
return True

my function just returns None.