New submission from Micah R Ledbetter <m...@micahrl.com>: When using the random.Random class, using the .seed() method with version=1 does not always reproduce the same results as the .seed() method did in Python 2.
>From the docs, I did expect this, but on closer inspection, I can't tell >whether I made a bad assumption or whether there is a bug in the module. The docs state an intention of compatibility with older versions of Python: https://docs.python.org/3.9/library/random.html#notes-on-reproducibility > Most of the random module’s algorithms and seeding functions are subject to > change across Python versions, but two aspects are guaranteed not to change: > > If a new seeding method is added, then a backward compatible seeder will be > offered. > > The generator’s random() method will continue to produce the same sequence > when the compatible seeder is given the same seed. It's not clear from the docstring in the code whether this is intended to cover Python 2.7 behavior: https://github.com/python/cpython/blob/3.9/Lib/random.py#L134 > For version 2 (the default), all of the bits are used if *a* is a str, > bytes, or bytearray. For version 1 (provided for reproducing random > sequences from older versions of Python), the algorithm for str and > bytes generates a narrower range of seeds. But the results I've spot checked sometimes do match the Python 2 results, and sometimes are the Python 2 result +1. I wrote a python script that calls the .seed() method with version=1 under Python 3, and without a version= argument under Python 2. It uses a wordlist I happen to have in /usr/share/dict that I copied to $PWD. #!/usr/bin/env python import os, random, sys mydir = os.path.dirname(os.path.abspath(__file__)) r = random.Random() maxidx = None with open('{}/web2'.format(mydir)) as webdict: for idx, raw_word in enumerate(webdict.readlines()): word = raw_word.strip() if sys.version_info[0] == 2: r.seed(word) elif sys.version_info[0] == 3: r.seed(word, version=1) else: raise Exception("Unexpected python version") print("{}: {}".format(word, r.randrange(0, 65535, 1))) if maxidx != None and idx >= maxidx: break I also wrote a shell script to run my Python script with the Python versions I happen to have installed locally, along with Python 2.7 and 3.4-3.9 in the ci-image Docker container linked from the Python download page. #!/bin/sh set -eux mkdir -p results /usr/bin/python test.py > results/macos10.15.4.system.python2.7.16 /Library/Frameworks/Python.framework/Versions/3.8/bin/python3 test.py > results/macos10.15.4.system.python3.8.2 docker run -v $PWD:/testpy:rw -u root -it --rm quay.io/python-devs/ci-image sh -c 'python3.9 /testpy/test.py > /testpy/results/ci-image.python3.9' docker run -v $PWD:/testpy:rw -u root -it --rm quay.io/python-devs/ci-image sh -c 'python3.8 /testpy/test.py > /testpy/results/ci-image.python3.8' docker run -v $PWD:/testpy:rw -u root -it --rm quay.io/python-devs/ci-image sh -c 'python3.7 /testpy/test.py > /testpy/results/ci-image.python3.7' docker run -v $PWD:/testpy:rw -u root -it --rm quay.io/python-devs/ci-image sh -c 'python3.6 /testpy/test.py > /testpy/results/ci-image.python3.6' docker run -v $PWD:/testpy:rw -u root -it --rm quay.io/python-devs/ci-image sh -c 'python3.5 /testpy/test.py > /testpy/results/ci-image.python3.5' docker run -v $PWD:/testpy:rw -u root -it --rm quay.io/python-devs/ci-image sh -c 'python2.7 /testpy/test.py > /testpy/results/ci-image.python2.7' I've made a github repo that contains both scripts and the results: https://github.com/mrled/random-Random-seed-version-testing I ran the script on my Mac, which means I used the system installed Python binaries that came with macOS x86_64, but the ci-image Python versions are running under an x86_64 Linux virtual machine (because of how Docker for Mac works). To summarize the results: * The Python 2.7 on my Mac works the same as the Python 2.7 on the ci-image * The Python 3.8 on my Mac works the same as Pythons 3.5-3.9 on the ci-image * Python 3.4 is different from both (although it is now unsupported anyway) A sample of the results. I haven't programmatically analyzed them, but from my spot checks, they all appear to be like this: > head results.ci-image.python2.7 | > head results.ci-image.python3.9 A: 8866 | A: 8867 a: 56458 | a: 56459 aa: 29724 | aa: 29724 aal: 11248 | aal: 11248 aalii: 16623 | aalii: 16623 aam: 62302 | aam: 62303 Aani: 31381 | Aani: 31381 aardvark: 6397 | aardvark: 6397 aardwolf: 32525 | aardwolf: 32526 Aaron: 32019 | Aaron: 32019 ---------- components: Library (Lib) messages: 369356 nosy: mrled priority: normal severity: normal status: open title: random.Random.seed() with version=1 does not consistently match Python 2 behavior versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue40682> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
