pytest-random-order

pytest-random-order is a plugin for pytest that randomises the order in which tests are run to reveal unwanted coupling between tests. 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

$ pip install pytest-random-order

The plugin is enabled by default. To randomise the order of tests within modules and shuffle the order of test modules (which is the default behaviour of the plugin), just run pytest as always:

$ pytest -v

To change the level of randomness allowed, run pytest with --random-order-bucket=<bucket-type> option where <bucket-type> can be class, module, package, global, or none:

$ 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=<value-reported-in-previous-run>

Design

pytest-random-order plugin groups tests in buckets, shuffles them within buckets and then shuffles the buckets.

You can choose from four 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 default setting.

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.

global

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

none

Disable shuffling.

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, 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 Randomisation or 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 -v

To disable just the shuffling, but let the plugin exist:

$ pytest --random-order-bucket=none