How-tos

This section show cases how to accomplish some specialized tasks with pytest-xdist.

Identifying the worker process during a test

New in version 1.15.

If you need to determine the identity of a worker process in a test or fixture, you may use the worker_id fixture to do so:

@pytest.fixture()
def user_account(worker_id):
    """ use a different account in each xdist worker """
    return "account_%s" % worker_id

When xdist is disabled (running with -n0 for example), then worker_id will return "master".

Worker processes also have the following environment variables defined:

PYTEST_XDIST_WORKER

The name of the worker, e.g., "gw2".

PYTEST_XDIST_WORKER_COUNT

The total number of workers in this session, e.g., "4" when -n 4 is given in the command-line.

The information about the worker_id in a test is stored in the TestReport as well, under the worker_id attribute.

Since version 2.0, the following functions are also available in the xdist module:

Identifying workers from the system environment

New in version 2.4

If the setproctitle package is installed, pytest-xdist will use it to update the process title (command line) on its workers to show their current state. The titles used are [pytest-xdist running] file.py/node::id and [pytest-xdist idle], visible in standard tools like ps and top on Linux, Mac OS X and BSD systems. For Windows, please follow setproctitle’s pointer regarding the Process Explorer tool.

This is intended purely as an UX enhancement, e.g. to track down issues with long-running or CPU intensive tests. Errors in changing the title are ignored silently. Please try not to rely on the title format or title changes in external scripts.

Uniquely identifying the current test run

New in version 1.32.

If you need to globally distinguish one test run from others in your workers, you can use the testrun_uid fixture. For instance, let’s say you wanted to create a separate database for each test run:

import pytest
from posix_ipc import Semaphore, O_CREAT


@pytest.fixture(scope="session", autouse=True)
def create_unique_database(testrun_uid):
    """ create a unique database for this particular test run """
    database_url = f"psql://myapp-{testrun_uid}"

    with Semaphore(f"/{testrun_uid}-lock", flags=O_CREAT, initial_value=1):
        if not database_exists(database_url):
            create_database(database_url)


@pytest.fixture()
def db(testrun_uid):
    """ retrieve unique database """
    database_url = f"psql://myapp-{testrun_uid}"
    return database_get_instance(database_url)

Additionally, during a test run, the following environment variable is defined:

PYTEST_XDIST_TESTRUNUID

The unique id of the test run.

Accessing sys.argv from the controller node in workers

To access the sys.argv passed to the command-line of the controller node, use request.config.workerinput["mainargv"].

Specifying test exec environments in an ini file

You can use pytest’s ini file configuration to avoid typing common options. You can for example make running with three subprocesses your default like this:

[pytest]
addopts = -n3

You can also add default environments like this:

[pytest]
addopts = --tx ssh=myhost//python=python3.9 --tx ssh=myhost//python=python3.6

and then just type:

pytest --dist=each

to run tests in each of the environments.

Specifying “rsync” dirs in an ini-file

In a tox.ini or setup.cfg file in your root project directory you may specify directories to include or to exclude in synchronisation:

[pytest]
rsyncdirs = . mypkg helperpkg
rsyncignore = .hg

These directory specifications are relative to the directory where the configuration file was found.

Making session-scoped fixtures execute only once

pytest-xdist is designed so that each worker process will perform its own collection and execute a subset of all tests. This means that tests in different processes requesting a high-level scoped fixture (for example session) will execute the fixture code more than once, which breaks expectations and might be undesired in certain situations.

While pytest-xdist does not have a builtin support for ensuring a session-scoped fixture is executed exactly once, this can be achieved by using a lock file for inter-process communication.

The example below needs to execute the fixture session_data only once (because it is resource intensive, or needs to execute only once to define configuration options, etc), so it makes use of a FileLock to produce the fixture data only once when the first process requests the fixture, while the other processes will then read the data from a file.

Here is the code:

import json

import pytest
from filelock import FileLock


@pytest.fixture(scope="session")
def session_data(tmp_path_factory, worker_id):
    if worker_id == "master":
        # not executing in with multiple workers, just produce the data and let
        # pytest's fixture caching do its job
        return produce_expensive_data()

    # get the temp directory shared by all workers
    root_tmp_dir = tmp_path_factory.getbasetemp().parent

    fn = root_tmp_dir / "data.json"
    with FileLock(str(fn) + ".lock"):
        if fn.is_file():
            data = json.loads(fn.read_text())
        else:
            data = produce_expensive_data()
            fn.write_text(json.dumps(data))
    return data

The example above can also be use in cases a fixture needs to execute exactly once per test session, like initializing a database service and populating initial tables.

This technique might not work for every case, but should be a starting point for many situations where executing a high-scope fixture exactly once is important.

Creating one log file for each worker

To create one log file for each worker with pytest-xdist, you can leverage PYTEST_XDIST_WORKER to generate a unique filename for each worker.

Example:

# content of conftest.py
def pytest_configure(config):
    worker_id = os.environ.get("PYTEST_XDIST_WORKER")
    if worker_id is not None:
        log_file = config.getini("worker_log_file")
        logging.basicConfig(
            format=config.getini("log_file_format"),
            filename=f"tests_{worker_id}.log",
            level=config.getini("log_file_level"),
        )

When running the tests with -n3, for example, three files will be created in the current directory: tests_gw0.log, tests_gw1.log and tests_gw2.log.