def __init__(self, uuid, params, version=<NUM_LIT:2>):
<EOL>if version not in self._version_to_prefix:<EOL><INDENT>raise ValueError("<STR_LIT>" % version)<EOL><DEDENT>self.uuid = uuid<EOL>self.params = params<EOL>self.version = version<EOL>self._cache = None<EOL>
Initialize a BucketKey.

:param uuid: The UUID of the limit the bucket corresponds to.
:param params: A dictionary of the request parameters
               corresponding to the bucket.
:param version: The version of the bucket.  Optional; defaults
                to 2.  Most callers should use the default.