Python – constructing absolute path with os.path.join()

I'd like to construct an absolute path in python, while at the same time staying fairly oblivious of things like path-separator.

edit0: for instance there is a directory on the root of my filesystem /etc/init.d (or C:\etc\init.d on w32), and I want to construct this only from the elements etc and init.d (on w32, I probably also need a disk-ID, like C:)

In order to not having to worry about path-separators, os.join.path() is obviously the tool of choice. But it seems that this will only ever create relative paths:

 print("MYPATH: %s" % (os.path.join('etc', 'init.d'),)
 MYPATH: etc/init.d

Adding a dummy first-element (e.g. '') doesn't help anything:

 print("MYPATH: %s" % (os.path.join('', 'etc', 'init.d'),)
 MYPATH: etc/init.d

Making the first element absolute obviously helps, but this kind of defeats the idea of using os.path.join()

 print("MYPATH: %s" % (os.path.join('/etc', 'init.d'),)
 MYPATH: /etc/init.d

edit1: using os.path.abspath() will only try to convert a relative path into an absolute path.
e.g. consider running the following in the working directory /home/foo:

 print("MYPATH: %s" % (os.path.abspath(os.path.join('etc', 'init.d')),)
 MYPATH: /home/foo/etc/init.d

So, what is the standard cross-platform way to "root" a path?

 root = ??? # <--
 print("MYPATH: %s" % (os.path.join(root, 'etc', 'init.d'),)
 MYPATH: /etc/init.d

edit2: the question really boils down to: since the leading slash in /etc/init.d makes this path an absolute path, is there a way to construct this leading slash programmatically?
(I do not want to make assumptions that a leading slash indicates an absolute path)