Do you HAVE to pit in a race?

As title suggests. I know it's doubtful getting away without pitting, but ya never know. Do you have to, like in real-life F1, and if you do have to and you don't, do you get penalised/excluded?

No. Simple

Yes, if you want a chance to win. Also simple. ;-)

You do not have to pit if you think you can get round on one set of tyres.
There are no penalties.

if your only running a 25% race depends on track malaysia for e.g. would only be 14 laps, and hard tyres will last close to 25 laps round there so i wouldnt be pitting

50%+ theres no chance tyres would last long enough

Its much more a question of lap times to me. You can easily motor a whole race on one set of wets, when its raining, but your lap times are...well...you should have taken the bus instead. Finally that would have saved you money.

25% races....? I don't know anything about to admit.