Python in a Nutshell, 2nd Edition

Random and Pseudorandom Numbers

The random module of the standard Python library generates pseudorandom numbers with various distributions. The underlying uniform pseudorandom generator uses the Mersenne Twister algorithm, with a period of length 2**19937-1. (Older versions of Python used the Whichmann-Hill algorithm, with a period of length 6,953,607,871,644.)

Physically random and cryptographically strong random numbers

Pseudorandom numbers provided by module random, while very good, are not of cryptographic quality. If you want higher-quality random numbers (ideally, physically generated random numbers rather than algorithmically generated pseudorandom numbers), in Python 2.4, you can call os.urandom (from module os, not random).

urandom

urandom(n) Returns n random bytes, read from physical sources of random bits such as /dev/urandom on recent Linux releases or from cryptographical-strength sources such as the CryptGenRandom API on Windows. If no suitable source exists on the current system, urandom raises NotImplementedError.

For an alternative source of physically random numbers, see http://www.fourmilab.ch/hotbits.

The random module

All functions of module random are methods of one hidden global instance of class random.Random. You can instantiate Random explicitly to get multiple generators that do not share state. Explicit instantiation is advisable if you require random numbers in multiple threads (threads are covered in Chapter 14). This section documents the most frequently ...