I had forgotten all about os.path.expandvars.  Note, however, that that function is very naive:

  >>> os.path.expandvars("'$HOME'")
  "'/home/rdmurray'"

That is, it is doing unconditional substitution, not parsing shell syntax.  It should work well for simple cases, though.

It might be worth throwing up a trial balloon on python-ideas for adding something to shlex that would do a "better" job of environment variable substitution for the version of shell syntax that shlex supports, which would therefore become a sort of platform-independent syntax for doing this.