Old post but adding for info
A specific element on a page will only have 1 xpath (Or possible a subset of these), but it is very definitely finite. This is because of the way xpath is inherently structured. It represents the DOM.
CSS Selectors can be defined in a multitude of ways, and as such it can be considered than a single element could have infinite css selectors.
If you want to find out the specific class property of an element, that is possible by doing my_page.my_element['class']
- However this will return a space delimited string of just the class properties of the current element.