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The current implementation already delays concatenation and typically only uses a single ''.join() to build the final buffer, so it's reasonably efficient.

What I was musing about in my previous post is a way to entierely avoid the copy overhead and let all operations deal directly with a list of strings as the basic data structure (or maybe a tree of strings, or some more advanced structure).  Repeated writes of very small strings should probably still be consolidated into larger strings (e.g. of a few KBs) but no single huge string (or array) needs to be built at all.

But this is just musing aloud, and this thread is about a bug in StringIO.py, so we should probably focus on fixing it...