If you don't need heavy querying of the data after you persist it to file (as in, you usually read the data that you write) why not use a python technology like:
http://docs.python.org/library/pickle.html or anything equivalent.
Avoid XML and YAML like the plague. Why? Think of it this way? XML really buys you nothing but makes you pay for extra baggage especially when you know the structure of your persisted data.
Of course if you require projecting your persisting data into another dataset, SQL is the tested and polished gun.