be whistling in the dark
To be sure of a particular outcome when all evidence points to the contrary. Based on the polls, he's whistling in the dark if he thinks he's going to unseat the incumbent. He never pays any attention to you, so you're whistling in the dark if you think he's really going to ask you out on a date.
be whistling in the dark
If someone is whistling in the dark, they are trying to make themselves feel less afraid. Then I waited, trying not to feel as if I were whistling in the dark, but I experienced no easing of my fear and anxiety.