I'm -1 on this approach; I don't think yet another container type is the right solution, given that we have already plenty of them.

If you want to avoid creating large lists, then the StringIO type should already provide that. So I wonder why these functions couldn't be rewritten to use StringIO.

If you really want to use this approach, I'd try to avoid allocating the large list if there are only few substrings. I.e. allocate it only when flushing, and only if the flush is not the final flush.