Well, that certainly depends on the determination and nerve of the attacker. I would think most attackers would realize, "Hey, I've just been shot" they don't know exactly what they've been shot with, nor the extent of the damage, that is, they don't know whether it's an injury then can walk to the ER with, or if it's one that gives them mere minutes left to live, so they'll realize the more pressing concern would be to get the wound taken care of and give up before another wound is inflicted, rather than carry on with an attempted crime. Even a largely superficial wound is liable to be lethal if not properly and promptly treated.
Of course, there may be those who by natural or chemical means, would be less fazed by a scary-looking yet largely superficial wound, I think such people are much fewer and farther between.
But, a pocket pistol could achieve the same compactness as a derringer, with a greater ammo capacity, greater accuracy, in a package that's more comfortable to shoot, so I'd think that would be a better choice if you're more worried about bad people rather than snakes.