According to "pydoc --help",

pydoc -p <port>
    Start an HTTP server on the given port on the local machine.

The IP address pydoc binds to is not specified, thus I would expect it
to either bind to the local address or to the wildcard address.

Current behaviour of "pydoc -p 1234" is to bind to the IPv4-only 
address 0.0.0.0:1234.

On a IPv4/IPv6 dual-stack machine, I would expect pydoc to listen to
either both 127.0.0.1:1234 and [::1]:<port>, or just [::]:<port> ([::]
also catches accesses to IPv4 addresses such as 127.0.0.1).

Then access to the pydoc webserver is
available both via IPv4 (e.g. http://127.0.0.1:1234/) and IPv6 (e.g.
http://[::1]:1234/), and in both cases via http://localhost:1234/
regardless whether localhost is resolved via IPv6 or IPv4.

Trivial patch attached (tested on Linux x86 machine with local IPv6
networking).

That will break on systems where AF_INET6 doesn't default to
dual-stacked sockets and mapped v4 addresses (e.g. Windows); to make it
work correctly, you'll also have to disable the IPV6_V6ONLY option,
which then breaks on systems which don't implement that option (e.g.
Windows XP).

I understand Windows XP isn’t so important these days. So maybe we just need to disable IPV6_V6ONLY. (Unless we want this for 2.7 maybe?)

Currently pydoc only binds to IPv4 localhost, not the “any-address” 0.0.0.0. See Issue 22421 and Issue 672656.

Apart from satisfying Hans’s expectation, what is the benefit of running pydoc on IPv6? I guess it would be possible to bind two sockets, one to IPv4 localhost (127.1 or ::ffff:127.0.0.1), and the other IPv6 localhost (::1). But why bother?

Agreed with Martin.