Warning to Washington hostesses: If you invite David Petrou to your parties during the next six months you may find yourselves in his novel next year -- probably in a compromising position.

Petrou has written, so far, one-third of a novel called "The Hostess," and he is in town to gather more material and finish his writing. The subject is Washington social life and the style and approach, in the author's words, are "commercial -- either classy trash or trashy class; you can take your pick."

What if Petrou gave a party and nobody came? It happened last night -- almost. Among those who had been invited were Sargent Shriver (Eunice was out ot town) and the Saudi Arabian ambassador, who David met recently in the Virgin Islands. Neither made it to last night's cocktail reception at the Four Seasons.

"It's just like my bar mitzvah," said Petrou, not at all bothered at the turnout. "My mother and father are here, my girlfriend and her parents, my sister . . . My rabbi would have been here, but he's vacationing in St. Thomas." Actually, there were quite a few others, including Gen. Robert Montague, director of the Special Olympics, with whom Petrou works three days a week at the John F. Kennedy Foundation because his advance for the novel from Fawcett is "astronomically less than seven figures."

None of the people who will be in the novel seemed to be at the party -- except for the fictional heroine, Katherine Payne McNaulty, who was to be seen in a corner of the Dunbarton Room. Actually, only part of her was to be seen -- the part from just above the lips to a little below the navel, sketchilly covered by a near-transparent flowered peignoir, which a pair of males hands was busily trying to remove. She was set on an easel, engulfed in print, emblazoned on the cover painting for the book.

Petrou stood looking at it appraisingly: "I think they've captured that balance of elegance and sleaze; now all I have to do is write something to fill in the middle."

In the opposite corner, Peggy Lacey Craig at the harpsichord and Alan Fendler on the recorder were playing baroque music -- beautifully and in exquisitely authentic style (though a gamba or cello to reinforce the bass sound would not have been amiss).

Between these two extremes stretched a buffet as elegant as those at any Washington party, including baked oyster, large shrimp, smoked salmon, superbly pickled herring and many other delicacies. A veteran book-partygoer, surveying the scene and recalling that Petrou's checkered past included a stint as a paperback publicist, might have concluded that he was running a parody of the industry's traditional rites of passage for a new publication. If so, he did it with a straight face -- almost.

He did admit readily, however, that "I've done this for so many other people -- I thought I'd like to do it for myself."

His novel's heroine, he revealed, also has a background in public relations; she is a self-made woman, the head of her own public-relations firm, who has reached the top by less-than-honest means and then marries big money and becomes (it has to be said in capital letters) The Hostess.

"The book will not only examine the seamier side of Washington society, but the glitter and pomp," says Petrou. "We will look at the cave-dweller, green-book Washington society, and there will be a mythical, Middle-Eastern oil potentate who gives really spectacular parties. But the Washington story is not just parties and pomp -- it's power, which is more corrupting and more sexy.

"The book will have sex and politics and adventure and sex and intrigue and sex and glitter and glamour and sex. Kinky sex and straight, gay and bi -- all the 31 flavors." Whatever his writing talents may be, Petrou clearly has a flair for publicity.

Besides book publicity, his career has included a variety of writing jobs in Hollywood -- "I have written several novels as movie tie-ins," he says, "but this is my first real book." He was part of the "Superman" team (as assistant producer for literary development), and his last Hollywood job was as executive director of promotion for "Time after Time" which he now calls "a totally forgettable motion picture.

"After that," he says, "I decided to go back to the novel I was tinkering with for the last five years. I did a 30-page outline and the first four chapters, sent it to my agents, Arthur Pine Associates, and they sold it in three weeks.

Petrou is a native Washingtonian who has been away for six years (including three in London) and has clearly been suffering from homesickness. It may be, in fact, that his Washington epic was a sympton of that homesickness -- perhaps even part of an elaborate scheme to find a pretext for coming back to Washington. His parents, John and Bebe Petrou, have a 15-room home adjoining the Burning Tree Country Club, but he is looking for a permanent base downtown. "I am a great believer," he says, "that you can't go home again at the age of 30. I now have my eye on a house on O St. in Georgetown, and I have convinced Fawcett that I should be living in the middle of the scene, making my presence felt, observing. But frankly, my advance would not buy the front door of a house in Georgetown."

Two Hollywood producers have taken first and second options on the partially-written book, and Petrou insists that if a film is made he will want to be involved in the production. "I didn't spend all that time out on the coast learning to speak Californian for nothing. I am a mogul-in-waiting."

If the picture is made, he says, "I've promised all my parents' society friends that they will definitely be extras in the party scenes. By the time it goes into production, we may be able to rent the Iranian Embassy for next to nothing."