pytest-random-order 1.2.0

Randomise the order in which pytest tests are run with some control over the randomness

pytest-random-order is a pytest plugin that randomises the order of tests. This can be useful to detect a test that passes just because it happens to run after an unrelated test that leaves the system in a favourable state.

The plugin allows user to control the level of randomness they want to introduce and to disable reordering on subsets of tests. Tests can be rerun in a specific order by passing a seed value reported in a previous test run.

Quick Start

Installation:

$ pip install pytest-random-order

From v1.0.0 onwards, this plugin no longer randomises tests by default. To enable randomisation, you have to run pytest in one of the following ways:

pytest --random-order
pytest --random-order-bucket=<bucket_type>
pytest --random-order-seed=<seed>

If you want to always randomise the order of tests, configure pytest. There are many ways to do it, my favourite one is to add addopts = --random-order in your project-specific configuration file under the pytest options (usually [pytest] or [tool:pytest] section).

Alternatively, you can set environment variable PYTEST_ADDOPTS:

export PYTEST_ADDOPTS="--random-order"

To randomise the order of tests within modules and shuffle the order of test modules (which is the default behaviour of the plugin), run pytest as follows:

$ pytest --random-order

To change the scope of re-ordering, run pytest with --random-order-bucket=<bucket-type> option where <bucket-type> can be class, module, package, global:

$ pytest -v --random-order-bucket=package

To disable reordering of tests in a module or class, use pytest marker notation:

pytestmark = pytest.mark.random_order(disabled=True)

To rerun tests in a particular order:

$ pytest -v --random-order-seed=<seed>

All runs in which the randomisation is enabled report seed so if you encounter a specific ordering of tests that causes problems you can look up the value in the test report and repeat the run with the above command.

platform darwin -- Python 3.5.6, pytest-3.9.1, py-1.7.0, pluggy-0.8.0
Using --random-order-bucket=module
Using --random-order-seed=383013

Design

The plugin groups tests in buckets, shuffles them within buckets and then shuffles the buckets.

Given the test suite above, here are two of a few possible generated orders of tests:

You can choose from a few types of buckets:

class

Tests will be shuffled within a class and classes will be shuffled, but tests from one class will never have tests from other classes or modules run in-between them.

module

Same as above at module level. This is the setting applied if you run pytest with just --random-order flag or --random-order-seed=<seed>.

package

Same as above at package level. Note that modules (and hence tests inside those modules) that belong to package x.y.z do not belong to package x.y, so they will fall in different buckets when randomising with package bucket type.

parent

If you are using custom test items which don’t belong to any module, you can use this to limit reordering of test items to within the parent to which they belong. For normal test functions the parent is the module in which they are declared.

grandparent

Similar to parent above, but use the parent of the parent of the test item as the bucket key instead.

global

All tests fall in the same bucket, full randomness, tests probably take longer to run.

none

Disable shuffling. This plugin no longer shuffles tests by default so there is nothing to disable, however, there are scenarios where this is useful due to the way test configs are specified, see #40.

If you have three buckets of tests A, B, and C with three tests 1 and 2, and 3 in each of them, then one of many potential orderings that non-global randomisation can produce could be:

c2, c1, c3, a3, a1, a2, b3, b2, b1

As you can see, all C tests are executed “next” to each other and so are tests in buckets A and B. Tests from any bucket X are guaranteed to not be interspersed with tests from another bucket Y. For example, if you choose bucket type module then bucket X contains all tests that are in this module.

By default, when randomisation is enabled, your tests will be randomised at module level which means that tests within a single module X will be executed in no particular order, but tests from other modules will not be mixed in between tests of module X.

The randomised reordering can be disabled per module or per class irrespective of the chosen bucket type.

Usage and Tips

Bucket Type Choice

It is best to start with smallest bucket type (class or module depending on whether you have class-based tests), and switch to a larger bucket type when you are sure your tests handle that.

If your tests rely on fixtures that are module or session-scoped, more randomised order of tests will mean slower tests. You probably don’t want to randomise at global or package level while you are coding and need a quick confirmation that nothing big is broken.

Disable Shuffling in Module or Class

You can disable shuffling of tests within a single module or class by marking the module or class with random_order marker and passing disabled=True to it:

pytestmark = pytest.mark.random_order(disabled=True)

def test_number_one():
    assert True

def test_number_two():
    assert True

class MyTest(TestCase):
    pytestmark = pytest.mark.random_order(disabled=True)

    def test_number_one(self):
        self.assertTrue(True)

No matter what will be the bucket type for the test run, test_number_one will always run before test_number_two.

Rerun Tests in the Same Order (Same Seed)

If you discover a failing test because you reordered tests, you will probably want to be able to rerun the tests in the same failing order. To allow reproducing test order, the plugin reports the seed value it used with pseudo random number generator:

============================= test session starts ==============================
..
Using --random-order-bucket=module
Using --random-order-seed=24775
...

You can now use the --random-order-seed=... bit as an argument to the next run to produce the same order:

$ pytest -v --random-order-seed=24775

Run Last Failed Tests First

Since v0.8.0 pytest cache plugin’s --failed-first flag is supported – tests that failed in the last run will be run before tests that passed irrespective of shuffling bucket type.

Disable the Plugin

If the plugin misbehaves or you just want to assure yourself that it is not the plugin making your tests fail or pass undeservedly, you can disable it:

$ pytest -p no:random_order

Note that randomisation is disabled by default. By passing -p no:random_order you are stopping the plugin from being registered so its hooks won’t be registered and its command line options won’t appear in --help.

Credits

• The shuffle icon in the diagram is by artist Daniele De Santis and it was found on iconarchive.

• The diagram is drawn with sketchboard.io