How do i convice my parents to let me get a snake?

Well i have a 20 gallon tank with everything i need, i used to have a leopard gecko, i also currently have a leopard gecko. I have done some research on some snakes like the ball python, cornsnake, rosy boa, and milk snake. also if you have any ideas on what kind of snakes can live their whole lifes in a 20 gallon tank ( its a standard 20 gallon) i really have room to upgrade .... Also dont suggest the Garder snake because they are illegal in my city ... thank you