"Corn snakes are found throughout the southeastern and central United States. Their docile nature, reluctance to bite, moderate adult size 3.9â€“6.0 feet (1.2â€“1.8 m), attractive pattern, and comparatively simple care make them popular pet snakes". http://en.wikipedia.org/wiki/Corn_snake
With that information, I would decide if I would allow that pet. I would probably require that the tenant gets renters insurance to cover any liability if the snake were to bite.
In this particular case, I would not require a pet deposit because I am not worried about the corn snake doing any damage to the unit considering it would be living in a confined environment or waterless tank.
But, it is something you have to negotiate with and convince your landlord in reference to the pros and cons with your specific pet.
Besides guiding dogs for the blinds, there are tenants who claim to have a dog prescribed by their physicians for distressing from their medical conditions; they must provide medical proof though.