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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def keys(self, pattern=None):
 """Returns a list of keys matching ``pattern``. By default return all keys. ['l0', 's0'] [] :param pattern: key pattern :type pattern: str :return: list of keys in db :rtype: list of str """

        logger.debug('call pop %s', pattern)
        if pattern is None:
            pattern = '*'
        return self._redis.keys(pattern=pattern)