def METHOD_NAME(self):
    path = "/tmp/this_file_does_not_exists_known_hosts"
    key = 'example.com ssh-rsa AAAAetc\n'
    diff = compute_diff(path, found_line=None, replace_or_add=False, state='present', key=key)
    self.assertEqual(diff, {
        'before_header': '/dev/null',
        'after_header': path,
        'before': '',
        'after': 'example.com ssh-rsa AAAAetc\n',
    })