Errata - 1st paragraph should be:

The code that does not work (python3 example):
...
sock_path = b"/var/tmp/sock.tmp"
server.bind(b"\0" + sock_path)
...
and strace shows:
bind(3, {sa_family=AF_LOCAL, sun_path=@"/var/tmp/sock.tmp"...}, 20) = 0