'Retrieve the Asset for a given sid.'

def retrieve_asset(self, sid, default_none=False):

    try:
        asset = self._asset_cache[sid]
        if ((asset is None) and (not default_none)):
            raise SidsNotFound(sids=[sid])
        return asset
    except KeyError:
        return self.retrieve_all((sid,), default_none=default_none)[0]
