In some versions of John Goergen's program offlineimap, he includes a
copy of imaplib.py with the attached changes.  It results in a speedup
of more than 50% compared to using the stock imaplib.py.

This patch buffers data inside imaplib, so if anything else wants to use
the socket it may generate surprises.  Piers?

Sean, could you please explain a bit more what are your concerns about this patch?

Anyhow, the code has slightly changed and currently John is not maintaining offlineimap anymore, but what seems the most recent version can be found at: https://github.com/nicolas33/offlineimap/

Let me clarify. Offlineimap used to ship a modified version of imaplib in its distribution, but eventually the author decided he no longer wanted to maintain his imaplib fork, so he dropped it and went with stock imaplib (at a significant performance penalty). The patch I submitted here is the difference between the forked imaplib circa 2007 and the upstream version it was forked from.  The current version of offlineimap is not relevant to this issue, because it no longer contains any imaplib code.

The part which is replaced by that patch (the readline() method in IMAP4_SSL) doesn't exist anymore in current imaplib code. Instead the file-like object created by socket.makefile() is used; its readline() method is probably quite faster than any pure Python implementation.