Though "Is the glass half empty or half full?" has become the general pop-psychology test to determine whether someone is an optimist or a pessimist, it doesn’t really work, does it?
Pessimism and optimism are only descriptions of a person’expectations not of their perceptions of reality.
The man who expects that, should he try the beverage in said glass, it would make him sick, might be a pessimist.
Further, a man who saw a glass and thought it not half full but half empty, or the other way round, would not be a pessimist, he would be delusional.
All this pop-psy test really tells you is whether the questioned is bright enough to reply "Both", pedantic enough to ask "What glass?" or religious enough to blather "(only) God knows".