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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 key(self, *args, _prefix=None, **kwargs):
 """Get the cache key for the given function args. Kwargs: prefix: A constant to prefix to the key. """

        if kwargs:
            raise NotImplementedError(
                'kwarg cache keys not implemented')
        return (_prefix,) + tuple(args)