🎲 Pytest plugin to randomly order tests and control random.seed
Clone or download

README.rst

pytest-randomly

Randomness power.

Pytest plugin to randomly order tests and control random.seed. (Also available for nose).

Features

All of these features are on by default but can be disabled with flags.

  • Randomly shuffles the order of test items. This is done first at the level of modules, then at the level of test classes (if you have them), then at the order of functions. This also works with things like doctests.
  • Resets random.seed() at the start of every test case and test to a fixed number - this defaults to time.time() from the start of your test run, but you can pass in --randomly-seed to repeat a randomness-induced failure.
  • If factory boy is installed, its random state is reset at the start of every test. This allows for repeatable use of its random 'fuzzy' features.
  • If faker is installed, its random state is reset at the start of every test. This is also for repeatable fuzzy data in tests - factory boy uses faker for lots of data.
  • If numpy is installed, its random state is reset at the start of every test.

About

Randomness in testing can be quite powerful to discover hidden flaws in the tests themselves, as well as giving a little more coverage to your system.

By randomly ordering the tests, the risk of surprising inter-test dependencies is reduced - a technique used in many places, for example Google's C++ test runner googletest.

By resetting the random seed to a repeatable number for each test, tests can create data based on random numbers and yet remain repeatable, for example factory boy's fuzzy values. This is good for ensuring that tests specify the data they need and that the tested system is not affected by any data that is filled in randomly due to not being specified.

This plugin is a Pytest port of my plugin for nose, nose-randomly.

Usage

Install from pip with:

pip install pytest-randomly

Pytest will automatically find the plugin and use it when you run pytest. The output will start with an extra line that tells you the random seed that is being used:

$ pytest
platform darwin -- Python 2.7.11, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
Using --randomly-seed=1460130750
...

If the tests fail due to ordering or randomly created data, you can restart them with that seed using the flag as suggested:

pytest --randomly-seed=1234

Since the ordering is by module, then by class, you can debug inter-test pollution failures by narrowing down which tests are being run to find the bad interaction by rerunning just the module/class:

pytest --randomly-seed=1234 tests/module_that_failed/

You can disable behaviours you don't like with the following flags:

  • --randomly-dont-reset-seed - turn off the reset of random.seed() at the start of every test
  • --randomly-dont-reorganize - turn off the shuffling of the order of tests

The plugin appears to Pytest with the name 'randomly'. To disable it altogether, you can use the -p argument, for example:

pytest -p no:randomly