on the prng behind random.random()

[Peter Otten]

Robert Girault wrote:

> Looking at its source code, it seems the PRNG behind random.random() is
> Mersenne Twister, but I'm not sure.  It also seems that random.random()
> is using /dev/urandom.  Can someone help me to read that source code?
> 
> I'm talking about CPython, by the way.  I'm reading
> 
>   https://github.com/python/cpython/blob/master/Lib/random.py
> 
> The initial comment clearly says it's Mersenne Twister, but the only
> random() function there seems to call _urandom(), which I suppose is an
> interface to /dev/urandom.
> 
> What am I missing here?

There's a class random.Random which is instantiated at the end of the file, 
and random() is bound to the corresponding method:

_inst = Random()
...
random = _inst.random

The Random class inherits from _random.Random which is implemented in C and 
does most of the actual work. If you can read C:

https://github.com/python/cpython/blob/master/Modules/_randommodule.c

The most relevant part seems to be genrand_int32() which is wrapped by 
random_random() that actually implenents the _random.Random.random() method.