def METHOD_NAME(self):
    """Test to overwrite existing key"""
    o = ScmOverride({ 'set' : { 'branch' : "master" } })
    match, scm = o.mangle(self.scm, Env())
    self.assertEqual(scm, {
        'scm' : "git",
        'url' : "git@git.com:foo/bar.git",
        'branch' : "master"
    })
    # test substitution
    o = ScmOverride({'set' : { 'branch' : "${BRANCH}" } })
    e = Env({"BRANCH" : "master"})
    match, scm = o.mangle(self.scm, e)
    self.assertEqual(scm, {
        'scm' : "git",
        'url' : "git@git.com:foo/bar.git",
        'branch' : "master"
    })