I agree with the 'test needed' stage, but for that, the intended
behaviour should be decided on first. A quick test for checking current
behaviour::

  pythonX.Y -c 'import os; p = os.path; print (os.environ["HOME"],
p.realpath("~"), p.expanduser("~"), p.normpath("~"))'

Run both, with and without HOME set (or set to something unusual).

Current result on Linux for both 2.5 and 2.6::

  ('/home/rank', '/home/rank/~', '/home/rank', '~')

on Mac OS for 2.5, 2.6 and 3.1::

  ('/Users/rank', '/Users/rank/~', '/Users/rank', '~')

(for 3.1 it's not a tuple of course).

Cannot test on Windows at the moment, but I think there's already
something wrong going on here :-).

Adding Macintosh and tested python version tags (there's no Unix tag?).

cheers