It does not exist
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The weapon being discussed does not exist. Tasers are useful for LE, but they are backed up by a handgun, knives are impractical for the simple fact that you must close with the bad guy, putting you with-in the "7 foot" range that is my limit as well as many professional SD experts. And knife fighting is a skill of a very high level, beleive me.
I try to follow several rules, the first being to always have your handgun with you. Second, and very important, avoid exposure. Quite simply, there are areas that are higher risk, down to where you park when shopping or eating out. Bad guys do not like lighted areas, or anything that increases the time they are exposed. Buy and carry a quality flashlight everywhere you go.
Know and drill with your handgun. Practice, Practice, Practice.
Above all be aware of your surroundings at all times.