'Group a list of sids by asset type.
Parameters
sids : list[int]
Returns
types : dict[str or None -> list[int]]
A dict mapping unique asset types to lists of sids drawn from sids.
If we fail to look up an asset, we assign it a key of None.'

def group_by_type(self, sids):

    return invert(self.lookup_asset_types(sids))
