'Lookup playlist with given URI in both the set of playlists and in any
other playlist source.
Returns the playlists or :class:`None` if not found.
*MUST be implemented by subclass.*
:param uri: playlist URI
:type uri: string
:rtype: :class:`mopidy.models.Playlist` or :class:`None`'

def lookup(self, uri):

    raise NotImplementedError
