Being Diplomatic is being disciplined

Sometimes being straightforward is more harmful then being diplomatic. Because this is less harming then being so bold and open. Also this's open for the direct and clear interpretation. Being diplomatic is to speak as per the circumstances stand and as per your role and status in any social arena.

It's not lying, it's to use a more modest and acceptable language and tone while conveying your perspective to the others. This's a disciplined approach of reacting to the circumstances. This's to respond with mind and not letting your emotions control your thought process. 

This's an evolving process and is learned through some formal and others available means of training.