All art is propaganda. It is universally and inescabably propaganda; sometimes unconsciously, but often deliberately, propaganda.
All truly great art is optimistic. The individual artist is happy in his creative work. The fact that practically all great art is tragic does not in any way change the above thesis.
The proletarian writer is a writer with a purpose; he thinks no more of art for art's sake than a man on a sinking ship thinks of painting a beautiful picture in the cabin; he thinks of getting ashore - and then there will be time enough for art.