To be truthful and at the same time diplomatic is one of the rarest of combinations, and only a small boy would be equal to it:
Johnny's manners had been improving at home, but at what a cost to his appetite when he had an invitation to dine at a boy friend's house! His hostess said, concernedly, when dessert was reached, "You refuse a second helping of pie? Are you suffering from indigestion, Johnny?" "No, ma'am; politeness."