Three problems:
1) You only have one FTP account, and it has whatever permissions it has. You can limit what a script does, but if it's in an interpreted language like Python, you need to release the source code to the user and they can easily modify things at their end, allowing them to completely take over your site, add malware, etc. Remember, you only have one account, and they'll be logged in as you, so directory-level permissions can't be applied.
2) Users would only be able to run the script if they have the appropriate Python libraries installed locally.
3) Public uploads are in violation of the terms of service, so if you do manage to get it going your account will be suspended. Exceptions are normally made for innocuous things like small avatar images/signatures for forums and so on, but conspicuous growth on the filesystem will be checked out. And if anyone uploads anything at all that is in violation of the TOS or the acceptable use policy and does that with your credentials you have no plausible deniability. (In the extreme, and let's step outside of the confines of the user/host relationship, you could become criminally liable for what is uploaded. All it takes is one child porn image, and you could be toast.)
