I’ve never used sendfile() nor shutil.copyfile(), but my immediate reaction is maybe we need a backup plan if os.sendfile() is available but not supported in the circumstances. E.g. if it is practical to use copyfile() to copy from a named socket in the filesystem, the Linux man page <http://man7.org/linux/man-pages/man2/sendfile.2.html> says it will raise EINVAL in this case. Maybe a test case would be good to prove this is still handled.