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Fixtures for seeding tests and making randomness reproducible

Project description


pytest-rng provides fixtures for ensuring “randomness” in your tests is reproducible from one run to the next. It also allows the seed for all tests to be changed if requested, to help ensure that test successes are not dependent on particular random number seeds.

  • Use the rng fixture to get a pre-seeded random number generator (RNG) that exposes NumPy’s ~numpy.random.mtrand.RandomState interface.
  • Use the seed fixture to get an integer seed that can be used to initialize your own RNG.

The following example prints the same four random numbers every time the test is run.

import numpy as np

def test_rectification(rng, seed):
    print(rng.uniform(-1, 1, size=3))

To use these fixtures, install with

pip install pytest-rng

Once installed, you can use these fixtures like any other fixture: add rng or seed to the arguments of a test function or class.

Seed generation

For the seed fixture, we generate a seed by doing the following:

  1. Concatenate the test’s nodeid and a salt value, if provided.
  2. Hash that string to yield an integer seed.

For the rng fixture, we also add the string "rng" to the salt value before generating the seed as above. The seed is used to instantiate a ~numpy.random.mtrand.RandomState, which is returned.


We add "rng" to the salt to ensure that random numbers are different when using the rng fixture and when manually instantiating a RandomState with the seed fixture.


salt is a string that is added to the test’s nodeid in order to change the seed for all tests. It is advantageous to change seeds regularly to ensure that your test suite is robust to different seeds.

The salt value can be specified in a configuration file like setup.cfg or pytest.ini.


rng_salt = v0.3.0

The salt value can also be specified through the command line.

pytest --rng-salt "v0.4.0"

The salt value passed through the command line takes precedence over the value set in the configuration file so that you can change seeds on-the-fly.

See the full documentation for more details.

Release History

1.0.0 (August 8, 2019)

Initial release of pytest-rng! Thanks to all of the contributors for making this possible!

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