Generate a random length is 12 string that is comprised uppercase and lowercase alpha and numbers

I've been looking for a Python algorithm to Generate a random length is 12 string that is comprised uppercase and lowercase alpha and numbers, unique,not duplicate.

In my situation it would be used as a unique session/key identifier that would "likely" be unique over 500K+ generation.

For example:

837uNNM9abCb

9HFRHcop24Cd

Thanks!