> It would probably be simpler, safer and more efficient to simply call
> the POSIX function instead (but it most be exposed somewhere, by
> posixmodule.c I suppose).

Indeed.

> Would it be a good idea to use the full docstring of posixpath.realpath
> to os.realpath and then just do “realpath = os.realpath” in posixpath?

Yes. That would also be a way to add the '.' default argument.

> An initial patch.

I'm actually concerned because realpath doesn't seem as portable as I first thought. Here's what my man page says:
"""
OSIX.1-2001 says that the behavior if resolved_path is NULL is implementation-defined.  POSIX.1-2008 specifies the behavior described in this page.
"""

So I'm not sure that all implementations support passing a NULL buffer as argument.

Also:
"""
Solaris may return a relative pathname when the path argument is relative.
"""

Well, I guess we can also commit this patch and see if a buildbot breaks...