dirname in sys.path.  This means that you can create a symbolic link
foo in /usr/local/bin pointing to /usr/yourname/src/foo/foo.py, and
then invoking foo will insert /usr/yourname/src/foo in sys.path, not
/usr/local/bin.  This makes it easier to have multifile programs
(before, the program would have to do an os.readlink(sys.argv[0])
itself and insert the resulting directory in sys.path -- Grail does
this).

Note that the expansion is only used for sys.path; sys.argv[0] is
still the original, unadorned filename (/usr/local/bin/foo in the
example).