Switching to a documentation issue is fine to me. Indeed I can just cache the result of StringIO.getvalue(), although it feels a bit crude.

I won't be able to create a documentation patch since English is not my primary language. While you're at it, the doc says about SSLSocket.write:

Returns the number of bytes written.

It actually either returns 0 or len(data), at least as long as we don't have SSL partial writes. That's a different behaviour from regular sockets, and I had to look in _ssl.c to figure out why I never had values inbetween.