How python random module returns a different number each time?

I know how PRNGs works (and I know most of them). Function random.seed() gives every time the same sequence (and this is clear), but for example random.choice(seq) - not.

So, how it works? Where does a different result come from? At first glance there are two possibilities - either the module draws randomness from some system data, e.g. a clock, or it iterates successive numbers of the underlying generator after each call (so we get something different every time).

How exactly does it work?