def _extract_referenced_vars(self, s):
        d = {}
        while True:
            try:
                s.format(**d)
            except KeyError as exc:
                # exc.args[0] contains the name of the key that was not found;
                # 0 is used because it appears to work with all types of placeholders.
                d[exc.args[0]] = 0
            else:
                break
        return d.keys()
Helper for ``url`` with host=SERVICES_URL.
dataset/ETHPy150Open hozn/stravalib/stravalib/protocol.py/ApiV3._extract_referenced_vars--crossover with--dataset/ETHPy150Open mozilla/addons-server/src/olympia/amo/helpers.py/services_url