Many landlords opt to simply put a "no pets/animals" clause into their lease, because they've been burned or know someone who was.
Pets/Animals can cause a LOT of very expensive damage. One cat can cause you having to remove most of the flooring, trim, even part of a wall. If that cat "sprays" there are no cleaners in this world who will truly eradicate the stink. You have to remove everything that has come into contact, down to the bare concrete or studs. Which have to be painted with a barrier, before replacing the wall/trim/carpet.
Dog pee isn't much lovelier and when they start chewing trim/doors/walls..well, you get the pictures. Fish tanks can cause enormous water damage. To that apartment and possibly some underneath of it.
Rodents stink and chew.
Etc etc etc.
Its just easier for a landlord to simply not allow pets then to go through that headache.
Now, all that said, there IS something you can try. Prepare your case. Print out a short but thorough information sheet on Ball Pythons as pets. One which goes over how small and harmless they stay. That there is no "mess". That they do not cause damage. How docile and shy they are.
Then add information about the steps YOU take to house is correctly, meaning NO ESCAPES POSSIBLE !!! Make a big point of that and then follow them as well. A landlord has to think of the other tenants as well. Can you imagine a snake being "lost" and other tenants throwing a fit and moving to hotels, etc ? Expecting the landlord to foot the bill? Those are the things the landlords worry about.
Add information about the feeding and cleaning. Present your case and paperwork.
Show how responsible you are.
End it with how much you are actually attached to this animal and that you are a very responsible owner/keeper.
Offer to show your setup and snake and how "its run".
Ask for an exception and mention that if none can be had, you will have to find another place. Make sure you let the landlord know that you do not want to put them on the spot, but you love your animal and want to do right by it.
Good luck!