'`relative_to` is the name of a sibling value that is
being validated at the same time.
`in_app_dir` indicates whether the path should be resolved
inside the application\'s config directory (even when the setting
does not come from a file).'

def __init__(self, default=REQUIRED, cwd=None, relative_to=None, in_app_dir=False):

    super(Filename, self).__init__(default)
    self.cwd = cwd
    self.relative_to = relative_to
    self.in_app_dir = in_app_dir
