Turn a function into a decorator that can be called with or without
    arguments.

def dual_use_decorator(fn):
    """Turn a function into a decorator that can be called with or without
    arguments."""
    @functools.wraps(fn)
    def decorator(*args, **kw):
        if len(args) == 1 and not kw and callable(args[0]):
            return fn()(args[0])
        else:
            return fn(*args, **kw)
    return decorator