There are different types of training for PPDs. Some alert to perceived threats and give an aggressive display. Some are trained to assess threats on their own and bite when they feel it's necessary. Some only bite on command. Some are trained on long bites, others never leave the handler. Some live with just their handler. Some with a family. There is a lot of planning that goes on when building a PPD, whatever version you choose.
First you need the right dog. Totally stable, social, biddable, with balanced drives. Good luck there. You better be getting your dog from a short list of breeders, and hope that you get a great dog.
Second, you need the right socialization and foundation training while they are puppies.
Third, you need a trainer and group of decoys with the equipment and understanding of how to train the dog, the money / time to pay for that training, and the ability to learn to handle a PPD.
And you also need to learn how to assess threats yourself, while in public, while handling the dog.
Soldiers that have been down range, in multiple firefights, who are great leaders of men in combat, fail to become handlers every day.
And to address the "gun is better than a dog" argument; my dog saved my life twice, and I was with 28 guys who all had weapons in their hands and were expecting trouble. Your senses can't begin to compare.
When a dog saves the life of a man, it becomes clear that partnership knows no bounds.
Fama - T.E.D.D. OEF XI-XII (GSD)
Marshall - T.E.D.D. OEF XII-XIII (Lab)
Lucian - Med Alert (Cane Corso)
Pud - the old man (Pit x Lab)