How Do I Seed The Random Library Python For Deterministic Tests?

2025-09-03 15:08:45 363

5 Answers

Olivia
Olivia
2025-09-04 15:22:04
I get nerdy about reproducibility sometimes, so here's a deeper take. Start with deterministic seeds for every RNG used in your stack. For Python: random.seed(0) or use random.Random(0) for local control. For NumPy prefer rng = np.random.default_rng(0) (this uses the PCG64 algorithm). For PyTorch do: torch.manual_seed(0); torch.cuda.manual_seed_all(0); torch.backends.cudnn.deterministic = True; torch.backends.cudnn.benchmark = False. For TensorFlow use tf.random.set_seed(0) and also configure inter/intra op parallelism if needed.

Beyond seeding, beware of sources that won’t obey these seeds: the secrets module, os.urandom, and any code that calls into system randomness. Parallel numerical libraries (OpenMP, MKL) can reorder operations across threads and cause tiny floating point differences that cascade; controlling environment variables like OMP_NUM_THREADS=1 and MKL_NUM_THREADS=1 helps. Also, for data-loading workers in tests, set a worker_init_fn that reseeds per worker, e.g., worker_init_fn=lambda wid: np.random.seed(seed + wid). Finally, log the seed used and snapshot RNG state with random.getstate()/setstate if you need exact replay. Even with all this, hardware and library versions can introduce nondeterminism, so treat deterministic tests as a combination of seeding + environment control rather than a single magic call.
Flynn
Flynn
2025-09-05 06:41:00
I usually keep things simple and pragmatic: pick a seed, set it at test start, and use local RNGs for isolation. Example I drop in my test bootstrap: import os, random; os.environ.setdefault('PYTHONHASHSEED', '0'); random.seed(777); rng = random.Random(777). If NumPy is involved I use rng_np = np.random.default_rng(777) instead of global np.random. When I work with ML code I also call the framework-specific seeding helpers and disable benchmark modes — otherwise you’ll chase ghosts.

One nice habit: print or save the seed with each test run. If something fails, rerun with that seed and you can reproduce the exact random trace. It’s not perfect — GPU math and multithreading can still bite — but it cuts the flakiness dramatically and lets me actually enjoy running tests.
Greyson
Greyson
2025-09-08 08:59:55
When I quickly need deterministic behavior I do this: import random; random.seed(1234) or better rng = random.Random(1234) and use rng.randint(...). That avoids global surprises. Note: secrets and os.urandom won’t be affected, so don’t use them if you need reproducibility. For NumPy, prefer rng = np.random.default_rng(1234) so you get a local generator with modern features. Also, if your tests behave differently on different machines, check thread settings and GPU determinism — sometimes floating point parallel ops are the culprit, not the RNG itself. Happy debugging!
Rhett
Rhett
2025-09-08 21:19:05
Okay, practical checklist time — I tend to write tests late at night so I keep this compact and actionable. First line of defence: import random; random.seed(2025) at the beginning of your test session. If you want to avoid global state interference, instantiate your own generator with random.Random(2025) and pass it around. That way each test can have a fresh RNG without surprising cross-test effects.

If your code uses NumPy, prefer np.random.default_rng(seed) for newer code; legacy np.random.seed works but global state is messier. For libraries like PyTorch use torch.manual_seed(seed) and torch.cuda.manual_seed_all(seed), and set deterministic flags (e.g., torch.backends.cudnn.deterministic = True and torch.backends.cudnn.benchmark = False). Remember to set PYTHONHASHSEED in the environment if you’re depending on hash order (export PYTHONHASHSEED=0), and control thread counts with OMP_NUM_THREADS=1 or MKL_NUM_THREADS=1 when numerical libraries introduce parallel nondeterminism. I also save RNG state with random.getstate() and restore with random.setstate(state) in tricky fixtures — it’s saved me a couple times when third-party code mutated global RNG state unexpectedly.
Finn
Finn
2025-09-09 19:50:10
I get a little excited when tests actually stay deterministic — it's such a relief! If you want repeatable sequences from Python's built-in random module, the simplest thing is to set the seed at the start of your test run: import random; random.seed(42). That fixes the global generator so subsequent calls like random.random() or random.randint() produce the same values every run.

For better isolation I usually prefer creating a local generator so tests don't trample each other's state: rng = random.Random(42); x = rng.random(). Put the seed in your test setup (a pytest fixture or a setup function) so every test starts from a known state. Also remember that secrets and anything using os.urandom are not affected by random.seed; they purposely use the OS entropy pool. If you use NumPy, prefer the newer API: rng = np.random.default_rng(12345) rather than np.random.seed(12345). For deep learning frameworks you’ll need to seed their RNGs separately (torch, tensorflow) and often pin deterministic flags — otherwise GPU operations and parallelism can sneak in non-determinism. I like logging the seed (print it or write it to a file) so if a test fails I can reproduce it later.
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