Keys. got half a dozen or more keys on a keychain? hold them in your fist with the keys protruding between your fingers. keep your hand in your pocket, but loosely. if attacked, rake the keys down across the face and eyes, or thrust them into the throat or inguinal crease. and run.
carrying weapons, as defined as any item which has no practical purpose aside from causing harm (brass knuckles, guns, tactical batons, combat knives) can be considered premeditated action. using an every day item(keys, umbrellas, your belt) as a weapon, is just self-defense.
I agree that no one should carry a weapon if they do not have training. a knife is a practical tool and can be a weapon, but without training, you are more likely to end up on the stinging end. one should never draw a knife for self-defense until they are sure that their own life is in imminent critical danger, and once drawn, do not hesitate- strike for vitals.