In Python 2 you can open the standard output and error streams in append mode, despite the fact that they aren't technically seekable. This changed somewhere along the way in (I think) io.open. There's an open bug report about this: https://bugs.python.org/issue27805 I just stumbled on it porting an application from Py2 to Py3 where my default logfile is /dev/stderr. I don't ever want to truncate a user's logfile, so "a" is a nice default, but in case the user doesn't name a logfile, I have to test, just in case /dev/stderr is the candidate. This seems error-prone, and must be done in all applications. The Py2 behavior seems less error-prone. I know explicit is better than implicit, but practicality also beats purity. Also, os.open is fine with O_APPEND on these streams. Any chance of this getting into 3.7 or will a fix have to wait for 3.8 at this point? (I'm guessing "no" as I don't see a patch.) Skip