Try and find a mid point between you and your dad. See if you can get one and only keep it for a year, and work your way down, 6 months, 4 months etc. If he finally gives in, and thats a big IF, once you have one and he sees that they are not messy. not aggressive etc, than he may possibly let you keep it permanently.
This is what i did with my wife, she allowed me to buy a snake and was only allowed to keep it for a year, only took her a month or so to say that i can now keep it permanently once she saw how docile, un-aggressive and completely not dangerous they really are.
You just need to show those who have had that uneducated fear built into them that that fear is unfounded and it's not all deadly, messy, dangerous stuff that the media and 95% of the population make it out to be.
As above, don't just buy one and hide it and hope that when he finds out, or you tell him, he will be ok with it. He won't and will be even more angry and determined to never let you get one again.