Some traditions, particularly those that advocate early morning practice, like Ashtanga, advise that you do your asanas on a completely empty stomach (and after bathing and moving your bowels). B.K.S. Iyengar, in his classic book, Light on Yoga, writes that if this is difficult for you, you can have coffee, cocoa, or milk before yoga, which, at least in the case of the later two, probably doesn't sound too appealing. As with most things yogic, you know your body best so you should decide which pre-yoga snack works for you.
After class, you can eat whenever you feel hungry, though you may notice that you're inclined to keep it light and healthy, which is one of the ways yoga can help you lose weight.