Wow, I've been mentioned a few times on this board. :3 XD

I wish you luck convincing your mom to let you have one.
As others have said, I was able to convince my dad to let me have one, but his problem wasn't that he thought they were dangerous, but he was concerned that they'd stink and that my mom would have to take care of it. The last part was confusing since I got my first hamster when I was 7 or 8 and took care of him by myself, got my first lizard at 9 after I was told I could never have one in this house and now I have three lizards and just got my ball python a couple days ago.
Dad even said, it was the paper on the ball pythons that convinced him to let me get one. Knowledge really is the key I believe. Let your parents know you know a lot about them, and that they aren't dangerous. I personally haven't heard of any ball pythons killing anyone.
If the size is what makes your mom think they're dangerous, go for a male. They're smaller than the females. I was going to get a male until I saw an adult female at the reptile expo. I made sure with dad that it was okay I got one that big and he told me: It's your snake.
Mom wasn't too crazy about the fact I got a snake that got bigger, but she seemed to enjoy her once she held her.
And since she changed her no to a maybe, that's a GREAT start! On fourth of July dad flat out told me no. It annoyed me for a bit, but I was determined. I researched a lot more and spent hours and hours on this site learning everything I could to construct a letter he couldn't say no to. He read the letter and I didn't get a no. Of course, I couldn't tell he said yes. He said you convinced me and I found out a day before the reptile expo that it was firm I could have one, even about 5 months before I originally planned to get one. But I have her no and I'm so excited and happy about her.
But I was told by another member on here to write a letter, give it to him, and wait.
So yes, do your research, give her a report she can't say no to, and don't be afraid to ask like me. (It took me so long to build up the courage to actually ask Dad and I only could when mom told him there was something I wanted to ask him.)

I hope I was a little bit of help. I don't know if you found my thread with my letter of pros/cons and info on them, but if you want you can read my letter to see what I did.
It was already suggested to go look at the board about me asking how to convince him, but here it is in case you want to read it: http://ball-pythons.net/forums/showt...onvince-Dad.-3

I wish you luck! Hopefully you'll be allowed to get a great, almost life long companion!
Good luck again, and hopefully I was some help. XD