2 Answers
+ 10
I would recommend looking it up further, even if it leads to non-python math stuff. But the way I would do it is make a list of both text files and check to see if each character matches. Each time there's a match, add a point. If the points are a certian amount of the total len of one of them, then its safe to say its plagiarized. This is not a perfect way to do it, but it's a start.
+ 3
To make a really good checker it is harder than it sounds. You have to check for every quasi-sentence from the file being checked in a file to compare with. Unless you only want to check for exact fragments' occurrence - that's really simple to write (but might be exhausting to execute if the files are really large).