How does Python seed the Mersenne twister
python, random
Solution
The seed is based on the clock or (if available) an operating system source. The `random` module creates (and hence seeds) a shared `Random` instance when it is imported, not when first used.
References
Python docs for random.seed:
random.seed(a=None, version=2)
Initialize the random number generator.
If a is omitted or None, the current system time is used. If randomness sources are provided by the operating system, they are used instead of the system time (see the os.urandom() function for details on availability).
Source of random.py (heavily snipped):
from os import urandom as _urandom
class Random(_random.Random):
def __init__(self, x=None):
self.seed(x)
def seed(self, a=None, version=2):
if a is None:
try:
a = int.from_bytes(_urandom(32), 'big')
except NotImplementedError:
import time
a = int(time.time() * 256) # use fractional seconds
# Create one instance, seeded from current time, and export its methods
# as module-level functions. The functions share state across all uses
#(both in the user's code and in the Python libraries), but that's fine
# for most programs and is easier for the casual user than making them
# instantiate their own Random() instance.
_inst = Random()
The last line is at the top level, so it is executed when the module is loaded.
Problem
How does Python seed its Mersenne twister pseudorandom number generator used in the built-in random library if no explicit seed value is provided? Is it based on the clock somehow? If so, is the seed found when the random module is imported or when it is first called? Python's documentation does not seem to have the answer.