Glass Half-Full
I think that the majority of PhD candidates can be broadly optimistic for their viva.
Believing that you’ll succeed doesn’t come from simply hoping though. Hope is good, but generally hope is what we have left when we have no meaningful actions we can take. A PhD candidate, on the other hand, can build up their capability and their confidence as they work through their PhD journey.
A candidate can do a lot more than hope that they will succeed: they can feel sure of it, optimistic that whatever happens they can engage well at their viva.