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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 get_default_driver(self):
 """ Get the default cache driver name. :rtype: str :raises: RuntimeError """

        if 'default' in self._config:
            return self._config['default']

        if len(self._config['stores']) == 1:
            return list(self._config['stores'].keys())[0]

        raise RuntimeError('Missing "default" cache in configuration.')