THAT SWEEPING ethics in government law, conceived by the Carter administration and enacted by Congress in 1978, is about to get its first major workout. The Republicans who will be rolling into town face something that wasn't here when their party last occupied the White House: full financial disclosure. The Democrats who are rolling -- or being rolled -- out face the new restraints on what they can do after they leave the government payroll.

No doubt there will be some cries of anguish and maybe even some public protests. Few new presidential appointees can be expected to like the fact that their family finances must be laid out in great detail for public scrutiny. Fewer still will find any pleasure in complying with the stock divestiture or blind trust requirements to which they will be subjected. Among the presidential appointees leaving office, there are some already discovering that they are not as free to capitalize on their government service as they might prefer to be; and all are learning that complying with this new law takes time and effort as well as a constant awareness of potential conflicts of interest.

Is the whole exercise worthwhile? Do government officials make different (or more honest) decisions because they are subjected to these rigors? Or is it all a show, designed to convince the public that government officials are honest and to remove any shadows that might otherwise be cast on their integrity?

Well, to begin with, no government can legislate or decree ethical conduct. Incoming and outgoing officials who want to cheat will find ways to do it, even though the new law puts some pretty big obstacles in their paths. Other officials and prospective officials who wouldn't dream of cheating may still find those obstacles so formidable that they will either refuse to come to Washington or end up wishing they hadn't.

The Reagan administration can provide one of its first useful services by tracking the problems that the new entrance and exit requirements create. If these requirements turn out to be too expensive in terms of the quantity and quality of people who are prevented by them from working for the government, the rules can be changed. After all, the first set of post-government employment restrictions imposed by the Carter administration and Congress had to be eased substantially last year when it became clear they were a serious impediment to recruitment of qualified officials. The same may turn out to be true of the sweeping financial disclosure provisions and the instant stock divestiture procedures.

There is nothing especially sacred about any aspect of the current laws, executive orders and congressional practices concerning ethics. All of them are, at best, stabs in the dark at an illusive target. If it is any consolation to those incoming and outgoing officials who are having problems with the requirements imposed on them, the story is an old one in this town. It happens every time the presidency changes hands. And it will continue to happen because no one can figure out a system of screening the profiteers out of government without causing some inconvenience and hardship for everyone else.