Stateful Modules in Python

TIL that random.seed persists across imports, aka modules with state. (aka unintentional MonkeyPatching?)

Let's say we have "moduleA.py" (WARNING:  Do not rely on "random" for cryptographic purposes, use "os.urandom" instead)

import random


def get_random_numbers():
    s = random.sample(range(1000), 10)
    return s

And another file named "main.py"

from moduleA import get_random_numbers

import random


if __name__ == '__main__':
    for i in range(3):
        random.seed(1234)
        # do things with random
        
        # code from another module that uses random
        print(get_random_numbers())

The side effect may be obvious here (prints the same random numbers on each iteration).

Perhaps .seed should be treated as a runtime context? (instead of associating state with the module)

with random.seed(1337):
    _ = random.random()

To avoid this, one can seed random with os.urandom before each use.

import random
import os


def get_random_numbers():
    random.seed(os.urandom(256))
    s = random.sample(range(1000), 10)
    return s