Hey creature
As others have said, unless you have specific training at using a knife in self defence, then stay away from them. It is far to easy to lose controll of the weapon and have it turned against you.
Also pulling a knife has a potential to escalate a bad situation.
A good self defense course which teaches you to escape from grabs and get away will serve you much better, if you definetly need a last ditch weapon,Lying bastard was on the money with the kubotan, it works wonders if you jam it in their eye, neck, you can also have one on your keyring without raising to many eyebrows.
My old Aikido and current kempo club runs short courses aimed specifically at women, I'd bet theres something similar in your area.
I guess my view on self defense weapons is coloured by living in britain where carrying a knife in the manner your suggesting will get your ass thrown in jail, it would be novel over here to be able to own more than a pocket knife without s019 visiting.
edit
I guess I should have read more thoroughly and realised all my points have been made before