Message 219926 - Python tracker

Looking back at this I think a "send_blocksize" argument is necessary after all. shutil.copyfileobj() has it, so is ftplib.FTP.storbinary() and  httplib (issue 13559) which will both be using socket.sendfile() once it gets included.
Updated patch is in attachment. If there are no other objections I'd be for committing this next week or something as I'm pretty confident with the current implementation.