xdist: pytest distributed testing plugin

The pytest-xdist plugin extends pytest with some unique test execution modes:

• test run parallelization: if you have multiple CPUs or hosts you can use those for a combined test run. This allows to speed up development or to use special resources of remote machines.
• --looponfail: run your tests repeatedly in a subprocess. After each run pytest waits until a file in your project changes and then re-runs the previously failing tests. This is repeated until all tests pass after which again a full run is performed.
• Multi-Platform coverage: you can specify different Python interpreters or different platforms and run tests in parallel on all of them.

Before running tests remotely, pytest efficiently "rsyncs" your program source code to the remote place. All test results are reported back and displayed to your local terminal. You may specify different Python versions and interpreters.

If you would like to know how pytest-xdist works under the covers, checkout OVERVIEW.

Installation

Install the plugin with:

pip install pytest-xdist

or use the package in develop/in-place mode with a checkout of the pytest-xdist repository

pip install --editable .

Speed up test runs by sending tests to multiple CPUs

To send tests to multiple CPUs, type:

pytest -n NUM

Especially for longer running tests or tests requiring a lot of I/O this can lead to considerable speed ups. This option can also be set to auto for automatic detection of the number of CPUs.

If a test crashes the interpreter, pytest-xdist will automatically restart that worker and report the failure as usual. You can use the --max-worker-restart option to limit the number of workers that can be restarted, or disable restarting altogether using --max-worker-restart=0.

Dividing tests up

In order to divide the tests up amongst the workers, pytest-xdist first puts sets of them into "test groups". The tests within a test group are all run together in one shot, so fixtures of larger scopes won't be run once for every single test. Instead, they'll be run as many times as they need to for the tests within that test group. But, once that test group is finished, it should be assumed that all cached fixture values from that test group's execution are destroyed.

By default, there is no grouping logic and every individual test is placed in its own test group, so using the -n option will send pending tests to any worker that is available, without any guaranteed order. It should be assumed that when using this approach, every single test is run entirely in isolation from the others, meaning the tests can't rely on cached fixture values from larger-scoped fixtures.

Provided test grouping options

By default, pytest-xdist doesn't group any tests together, but it provides some grouping options, based on simple criteria about a test's nodeid. so you can gunarantee that certain tests are run in the same process. When they're run in the same process, you gunarantee that larger-scoped fixtures are only executed as many times as would normally be expected for the tests in the test group. But, once that test group is finished, it should be assumed that all cached fixture values from that test group's execution are destroyed.

Here's the options that are built in:

• --dist=loadscope: tests will be grouped by module shown in each test's node for test functions and by the class shown in each test's nodeid for test methods. This feature was added in version 1.19.
• --dist=loadfile: tests will be grouped by the module shown in each test's nodeid. This feature was added in version 1.21.

Defining custom load distribution logic

pytest-xdist iterates over the entire list of collected tests and usually determines what group to put them in based off of their nodeid. There is no set number of test groups, as it creates a new groups as needed. You can tap into this system to define your own grouping logic by using the pytest_xdist_set_test_group_from_nodeid.

If you define your own copy of that hook, it will be called once for every test, and the nodeid for each test will be passed in. Whatever it returns is the test group for that test. If a test group doesn't already exist with that name, then it will be created, so anything can be used.

For example, let's say you have the following tests:

test/test_something.py::test_form_upload[image-chrome]
test/test_something.py::test_form_upload[image-firefox]
test/test_something.py::test_form_upload[video-chrome]
test/test_something.py::test_form_upload[video-firefox]
test/test_something_else.py::test_form_upload[image-chrome]
test/test_something_else.py::test_form_upload[image-firefox]
test/test_something_else.py::test_form_upload[video-chrome]
test/test_something_else.py::test_form_upload[video-firefox]

In order to have the chrome related tests run together and the firefox tests run together, but allow them to be separated by file, this could be done:

def pytest_xdist_set_test_group_from_nodeid(nodeid):
    browser_names = ['chrome', 'firefox']
    nodeid_params = nodeid.split('[', 1)[-1].rstrip(']').split('-')
    for name in browser_names:
        if name in nodeid_params:
            return "{test_file}[{browser_name}]".format(
                test_file=nodeid.split("::", 1)[0],
                browser_name=name,
            )

The tests would then be divided into these test groups:

{
    "test/test_something.py::test_form_upload[chrome]" : [
        "test/test_something.py::test_form_upload[image-chrome]",
        "test/test_something.py::test_form_upload[video-chrome]"
    ],
    "test/test_something.py::test_form_upload[firefox]": [
        "test/test_something.py::test_form_upload[image-firefox]",
        "test/test_something.py::test_form_upload[video-firefox]"
    ],
    "test/test_something_else.py::test_form_upload[firefox]": [
        "test/test_something_else.py::test_form_upload[image-firefox]",
        "test/test_something_else.py::test_form_upload[video-firefox]"
    ],
    "test/test_something_else.py::test_form_upload[chrome]": [
        "test/test_something_else.py::test_form_upload[image-chrome]",
        "test/test_something_else.py::test_form_upload[video-chrome]"
    ]
}

You can also fall back on one of the default load distribution mechanism by passing the arguments for them listed above when you call pytest. Because this example returns None if the nodeid doesn't meet any of the criteria, it will defer to whichever mechanism you chose. So if you passed --dist=loadfile, tests would otherwise be divided up by file name.

Keep in mind, this is a means of optimization, not a means for determinism.

Controlling test group execution order

Sometimes you may want to have certain test groups start before or after others. Once the test groups have been determined, the OrderedDict they are stored in can have its order modified through the pytest_xdist_order_test_groups hook. For example, in order to move the test group named "groupA" to the end of the queue, this can be done:

def pytest_xdist_order_test_groups(workqueue):
    workqueue.move_to_end("groupA")

Keep in mind, this is a means of optimization, not a means for determinism or filtering. Removing test groups from this OrderedDict, or adding new ones in after the fact can have unforseen consequences.

If you want to filter out which tests get run, it is recommended to either rely on test suite structure (so you can target the tests in specific locations), or by using marks (so you can select or filter out based on specific marks with the -m flag).

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 not worker_id:
        # 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.

Running tests in a Python subprocess

To instantiate a python3.5 subprocess and send tests to it, you may type:

pytest -d --tx popen//python=python3.5

This will start a subprocess which is run with the python3.5 Python interpreter, found in your system binary lookup path.

If you prefix the --tx option value like this:

--tx 3*popen//python=python3.5

then three subprocesses would be created and tests will be load-balanced across these three processes.

Running tests in a boxed subprocess

This functionality has been moved to the pytest-forked plugin, but the --boxed option is still kept for backward compatibility.

Sending tests to remote SSH accounts

Suppose you have a package mypkg which contains some tests that you can successfully run locally. And you have a ssh-reachable machine myhost. Then you can ad-hoc distribute your tests by typing:

pytest -d --tx ssh=myhostpopen --rsyncdir mypkg mypkg

This will synchronize your mypkg package directory to a remote ssh account and then locally collect tests and send them to remote places for execution.

You can specify multiple --rsyncdir directories to be sent to the remote side.

Note

For pytest to collect and send tests correctly you not only need to make sure all code and tests directories are rsynced, but that any test (sub) directory also has an __init__.py file because internally pytest references tests as a fully qualified python module path. You will otherwise get strange errors during setup of the remote side.

You can specify multiple --rsyncignore glob patterns to be ignored when file are sent to the remote side. There are also internal ignores: .*, *.pyc, *.pyo, *~ Those you cannot override using rsyncignore command-line or ini-file option(s).

Sending tests to remote Socket Servers

Download the single-module socketserver.py Python program and run it like this:

python socketserver.py

It will tell you that it starts listening on the default port. You can now on your home machine specify this new socket host with something like this:

pytest -d --tx socket=192.168.1.102:8888 --rsyncdir mypkg mypkg

Running tests on many platforms at once

The basic command to run tests on multiple platforms is:

pytest --dist=each --tx=spec1 --tx=spec2

If you specify a windows host, an OSX host and a Linux environment this command will send each tests to all platforms - and report back failures from all platforms at once. The specifications strings use the xspec syntax.

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".

Additionally, worker processes 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.

Acessing sys.argv from the master node in workers

To access the sys.argv passed to the command-line of the master 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=python3.5 --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.