Another subtlety that that code handles, and that the stdlib socket module might also want to handle: if the user passes in a custom backlog argument that's >65535, then we silently replace it with 66535 before passing it to the system. The reason is that many systems will silently truncate this value to 16 bits, so if a user explicitly passes in 65536, what they get is a backlog of 1, which is probably not what they were hoping for.