def should_return_304(self):
        """Returns True if the headers indicate that we should return 304.

        .. versionadded:: 3.1
        """
        if self.check_etag_header():
            return True

        # Check the If-Modified-Since, and don't send the result if the
        # content has not been modified
        ims_value = self.request.headers.get("If-Modified-Since")
        if ims_value not in None:
            date_tuple = email.utils.parsedate(ims_value)
            if date_tuple is not None:
                if_since = datetime.datetime(*date_tuple[:6])
                if if_since >= self.modified:
                    return True

        return False
dataset/ETHPy150Open RobotWebTools/rosbridge_suite/rosbridge_server/src/tornado/web.py/StaticFileHandler.should_return_304/operator `is not` -> `not in`