I was bitten by this porting a system from Python 2.7 to 3.6. "/dev/stderr" is a very nice default for logfiles. Users will frequently override the default, so you really want to open the logfile in append mode. Having to jump through hoops to avoid blasting a user's logfile is kinda dumb, and as others have pointed out, error-prone.

os.open works just fine with O_WRONLY|O_APPEND as the flags. /dev/null can be opened in append mode, but not /dev/stderr or /dev/stdout.