Running tests across multiple CPUs

To send tests to multiple CPUs, use the -n (or --numprocesses) option:

pytest -n auto

This can lead to considerable speed ups, especially if your test suite takes a noticeable amount of time.

With -n auto, pytest-xdist will use as many processes as your computer has CPU cores.

Use -n logical to use the number of logical CPU cores rather than physical ones. This currently requires the psutil package to be installed; if it is not, pytest-xdist will fall back to -n auto behavior.

Pass a number, e.g. -n 8, to specify the number of processes explicitly.

To specify a different meaning for -n auto and -n logical for your tests, you can:

  • Set the environment variable PYTEST_XDIST_AUTO_NUM_WORKERS to the desired number of processes.
  • Implement the pytest_xdist_auto_num_workers pytest hook (a pytest_xdist_auto_num_workers(config) function in e.g. conftest.py) that returns the number of processes to use. The hook can use config.option.numprocesses to determine if the user asked for "auto" or "logical", and it can return None to fall back to the default.

If both the hook and environment variable are specified, the hook takes priority.

Parallelization can be configured further with these options:

  • --maxprocesses=maxprocesses: limit the maximum number of workers to process the tests.
  • --max-worker-restart: maximum number of workers that can be restarted when crashed (set to zero to disable this feature).

The test distribution algorithm is configured with the --dist command-line option:

  • --dist load (default): Sends pending tests to any worker that is available, without any guaranteed order.

  • --dist loadscope: Tests are grouped by module for test functions and by class for test methods. Groups are distributed to available workers as whole units. This guarantees that all tests in a group run in the same process. This can be useful if you have expensive module-level or class-level fixtures. Grouping by class takes priority over grouping by module.

  • --dist loadfile: Tests are grouped by their containing file. Groups are distributed to available workers as whole units. This guarantees that all tests in a file run in the same worker.

  • --dist loadgroup: Tests are grouped by the xdist_group mark. Groups are distributed to available workers as whole units. This guarantees that all tests with same xdist_group name run in the same worker.

    @pytest.mark.xdist_group(name="group1")
    def test1():
        pass
    
    
    class TestA:
        @pytest.mark.xdist_group("group1")
        def test2():
            pass
    

    This will make sure test1 and TestA::test2 will run in the same worker. Tests without the xdist_group mark are distributed normally as in the --dist=load mode.

  • --dist worksteal: Initially, tests are distributed evenly among all available workers. When a worker completes most of its assigned tests and doesn’t have enough tests to continue (currently, every worker needs at least two tests in its queue), an attempt is made to reassign (“steal”) a portion of tests from some other worker’s queue. The results should be similar to the load method, but worksteal should handle tests with significantly differing duration better, and, at the same time, it should provide similar or better reuse of fixtures.

  • --dist no: The normal pytest execution mode, runs one test at a time (no distribution at all).