The closest snake I can find that matches your explanation is the "Pigmy Rattlesnake" (or in particular the "Dusky Pigmy Rattlesnake"). It can grow up to two feet long and they usually eat frogs. They have 3 rows of black spots with a whitish ring around each spot. Their underside is light grey and their rattle is very small. Their bite, if left untreated, can be fatal.

You can find images of the dusky pigmy rattlesnake here, here and here.

Personally, I wouldn't be keeping them around as a pet! But, each to their own I suppose.

 Oh, how lovely...did occur to you to mention exactly where in the world, you have encountered these reptiles, Madam?