Many tradesmen will carry tools that can easily be used as a weapon to protect yourself, even tools in your toolbox in the boot of you car could easily be used/mistaken as weapons.
You can carrying a folding pocket knife with a blade of less than 3 inches in length when going camping, but doubt you would get away with it taking it to the pub, unless you have a very good reason.
A set of car keys can be used as a weapon when held correctly to defend oneself, I've taught my wife and daughters this. Also a pair of women's stiletto heel shoes can do some serious damaged to a person, many women already know this.