I've let my snake play in the grass when I'm filming or photographing them, but otherwise i keep them inside. They do seem to like being in the grass, but i wouldn't leave them unsupervised. If you feel like letting your snake play around in your backyard, then do it! As long as you're watching them, they can't go far. And although there is a chance they may escape, it's unlikely. After i take my snakes in the backyard, i soak them in warm water for around 15 minutes, just to get the dirt off of them. After that they are fine. So as long a you watch your snake, and you rinse the dirt off of them afterwards (if any is visible) you should have no problem!