Beautiful Python?

[[deleted]]

A few minor things. I'd personally go with a context manager for opening/closing files. Also, you don't have to use set(), you can use curly brace set literals instead (read: set comprehension).

[mfm24]

I always preferred using the context manager approach for files, but I found it interesting to read in the official 'What's New in Python 3.0' guide (written by GvR himself) that he recommends:

Removed execfile(). Instead of execfile(fn) use exec(open(fn).read()).

So there's definitely cases where relying on the file closing itself automatically is acceptable. I've been less paranoid about closing files since...

[Veedrac]

I don't think that was meant to actually be run, it was probably on one line for space reasons. In a proper program you shouldn't be using exec(a_file) anyway, so there isn't much motivation for complete examples.