io.StringIO is quite slower than ''.append() when used for mass concatenation (around 5x slower). This patch brings it to similar performance by deferring construction of the internal buffer until needed.

The problem is that it's very easy to disable the optimization by calling a method other than write() and getvalue().

It would be interesting to see how often the "bad" case triggers, i.e. that a write-only stringio sees any of the other methods invoked at all.
As a special case, you may consider that .truncate(0) doesn't really need to realize the buffer first.

I also wonder how much StringIO will be used in praxis, as opposed to BytesIO.

Yes, these are things I've been wondering about. The use-case for an append-only StringIO is obviously overlapping with the use-case for using ''.join(). However, the implementation I'm proposing is better than ''.join() when writing very small strings, since there's a periodic consolidation.

> As a special case, you may consider that .truncate(0) doesn't really
> need to realize the buffer first.

True. Also, seek(0) then read() could use the same optimization.

Like parts of the Python test suite, I use StringIO to capture print/write output for testing in an output...output/getvalue/reset(seek(0),truncate(0)) cycle. While this enhancement would not currently affect me (as I only do a few prints each cycle), I can easily imagine other cases where it would.

New changeset 8d9a869db675 by Antoine Pitrou in branch 'default':
Issue #13149: Speed up append-only StringIO objects.
http://hg.python.org/cpython/rev/8d9a869db675

I've committed an improved version (which also optimizes seek(0); read()).