os.sendfile() documentation states:
"""
On Solaris, out may be the file descriptor of a regular file or the file descriptor of a socket. On all other platforms, out must be the file descriptor of an open socket.
"""

This isn't true for Linux anymore (changed somewhere around 2.6.33), so it would probably be better to rephrase to something like "all platforms support sockets as out file descriptor, and some platforms allow other types (e.g. regular files) as well".