Python singleton pattern

example.py

class Catalog(object):
    """Represents the mapping between queues and shard drivers."""

    __slots__ = ['_shards']

    _singleton = None

    def __init__(self):
        self._shards = {}

    def __new__(cls):
        if cls._singleton is None:
            cls._singleton = super(Catalog, cls).__new__(cls)

        return cls._singleton