def METHOD_NAME(self):
    """Test setting a subst"""
    env = PartialConfigEnvironment(self._objdir())
    env.write_vars(config)
    self.assertEqual(env.substs["MOZ_SUBST_1"], "1")
    env.substs["MOZ_SUBST_1"] = "updated"
    self.assertEqual(env.substs["MOZ_SUBST_1"], "updated")
    # A new environment should pull the result from the file again.
    newenv = PartialConfigEnvironment(env.topobjdir)
    self.assertEqual(newenv.substs["MOZ_SUBST_1"], "1")