Cautiously Optimistic?

What do you call a person who weighs down his options and thinks of handling outcomes when the intended activity might not go his way? Pessimist/Optimist? Does thinking of possible turns make one a pessimist inspite of the fact, that after so much thought, he will still give it a try no matter what ! Is a person optimist only if they never believe in options unless you are faced with a situation.

This is a point of argument me and my friend always come up with ! My friend's argument is when you weigh down and think of alternatives, you never give your 100%. And I say that knowing where it might go wrong makes me feel sure you can get the job done 100%.