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I added a doctest that was missing, converted a few text examples to doctests.
additionally, i also changed your random_string function to utilize os.urandom, which is supposed to be better.
From the python docs (as we discussed some time ago):
Almost all module functions depend on the basic function random(), which generates a random float uniformly in the semi-open range [0.0, 1.0). Python uses the Mersenne Twister as the core generator. It produces 53-bit precision floats and has a period of 2**19937-1. The underlying implementation in C is both fast and threadsafe. The Mersenne Twister is one of the most extensively tested random number generators in existence. However, being completely deterministic, it is not suitable for all purposes, and is completely unsuitable for cryptographic purposes.
Anyway, pull if you wish.