Returns arguments of fn with default=REQUIRED_ARG.

def _required_args(fn):
  """Returns arguments of fn with default=REQUIRED_ARG."""
  spec = getargspec(fn)
  if not spec.defaults:
    return []

  arg_names = spec.args[-len(spec.defaults):]
  return [name for name, val in zip(arg_names, spec.defaults)
          if val is REQUIRED_ARG]