Preface: the migration to pytest

Last year, after 17 years since the inception of the project, I decided to start adopting pytest into psutil (see psutil/#2446). The advantages over unittest are numerous, but the two I cared about most are:

Being able to use base assert statements instead of unittest's self.assert*() APIs.
The excellent pytest-xdist extension, that lets you run tests in parallel, basically for free.

Beyond that, I don't rely on any pytest-specific features in the code, like fixtures or conftest.py. I still organize tests in classes, with each one inheriting from unittest.TestCase. Why?

I like unittest's self.addCleanup too much to give it up (see some usages). I find it superior to fixtures. Less magical and more explicit.
I want users to be able to test their psutil installation in production environments where pytest might not be installed. To accommodate this, I created a minimal "fake" pytest class that emulates essential features like pytest.raises, @pytest.skip etc. (see PR-2456).

But that's a separate topic. What I want to focus on here is one of pytest's most frustrating aspects: slow startup times.

pytest invocation is slow

To measure pytest's startup time, let's run a very simple test where execution time won't significantly affect the results:

$ time python3 -m pytest --no-header psutil/tests/test_misc.py::TestMisc::test_version
============================= test session starts =============================
collected 1 item
psutil/tests/test_misc.py::TestMisc::test_version PASSED
============================== 1 passed in 0.05s ==============================

real    0m0,427s
user    0m0,375s
sys     0m0,051s

0,427s. Almost half of a second. That's excessive for something I frequently execute during development. For comparison, running the same test with unittest:

$ time python3 -m unittest psutil.tests.test_misc.TestMisc.test_version
----------------------------------------------------------------------
Ran 1 test in 0.000s
OK

real    0m0,204s
user    0m0,169s
sys     0m0,035s

0,204 secs. Meaning unittest is roughly twice as fast as pytest. But why?

Where is time being spent?

A significant portion of pytest's overhead comes from import time:

$ time python3 -c "import pytest"
real    0m0,151s
user    0m0,135s
sys     0m0,016s

$ time python3 -c "import unittest"
real    0m0,065s
user    0m0,055s
sys     0m0,010s

There's nothing I can do about that. For the record, psutil import timing is:

$ time python3 -c "import psutil"
real    0m0,056s
user    0m0,050s
sys     0m0,006s

Disable plugin auto loading

After some research, I discovered that pytest automatically loads all plugins installed on the system, even if they aren't used. Here's how to list them (output is cut):

$ pytest --trace-config --collect-only
...
active plugins:
    ...
    setupplan           : ~/.local/lib/python3.12/site-packages/_pytest/setupplan.py
    stepwise            : ~/.local/lib/python3.12/site-packages/_pytest/stepwise.py
    warnings            : ~/.local/lib/python3.12/site-packages/_pytest/warnings.py
    logging             : ~/.local/lib/python3.12/site-packages/_pytest/logging.py
    reports             : ~/.local/lib/python3.12/site-packages/_pytest/reports.py
    python_path         : ~/.local/lib/python3.12/site-packages/_pytest/python_path.py
    unraisableexception : ~/.local/lib/python3.12/site-packages/_pytest/unraisableexception.py
    threadexception     : ~/.local/lib/python3.12/site-packages/_pytest/threadexception.py
    faulthandler        : ~/.local/lib/python3.12/site-packages/_pytest/faulthandler.py
    instafail           : ~/.local/lib/python3.12/site-packages/pytest_instafail.py
    anyio               : ~/.local/lib/python3.12/site-packages/anyio/pytest_plugin.py
    pytest_cov          : ~/.local/lib/python3.12/site-packages/pytest_cov/plugin.py
    subtests            : ~/.local/lib/python3.12/site-packages/pytest_subtests/plugin.py
    xdist               : ~/.local/lib/python3.12/site-packages/xdist/plugin.py
    xdist.looponfail    : ~/.local/lib/python3.12/site-packages/xdist/looponfail.py
    ...

It turns out PYTEST_DISABLE_PLUGIN_AUTOLOAD environment variable can be used to disable them. By running PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest --trace-config --collect-only again I can see that the following plugins disappeared:

anyio
pytest_cov
pytest_instafail
pytest_subtests
xdist
xdist.looponfail

Now let's run the test again by using PYTEST_DISABLE_PLUGIN_AUTOLOAD:

$ time PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python3 -m pytest --no-header psutil/tests/test_misc.py::TestMisc::test_version
============================= test session starts =============================
collected 1 item
psutil/tests/test_misc.py::TestMisc::test_version PASSED
============================== 1 passed in 0.05s ==============================

real    0m0,285s
user    0m0,267s
sys     0m0,040s

We went from 0,427 secs to 0,285 secs, a ~40% improvement. Not bad. We now need to selectively enable only the plugins we actually use, via -p CLI option. Plugins used by psutil are pytest-instafail and pytest-subtests (we'll think about pytest-xdist later):

$ time PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python3 -m pytest -p instafail -p subtests --no-header psutil/tests/test_misc.py::TestMisc::test_version
========================================================= test session starts =========================================================
collected 1 item
psutil/tests/test_misc.py::TestMisc::test_version PASSED
========================================================== 1 passed in 0.05s ==========================================================
real    0m0,320s
user    0m0,283s
sys     0m0,037s

Time went up again, from 0,285 secs to 0,320s. Quite a slowdown, but still better than the initial 0,427s. Now, let's add pytest-xdist to the mix:

$ time PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python3 -m pytest -p instafail -p subtests -p xdist --no-header psutil/tests/test_misc.py::TestMisc::test_version
========================================================= test session starts =========================================================
collected 1 item
psutil/tests/test_misc.py::TestMisc::test_version PASSED
========================================================== 1 passed in 0.05s ==========================================================

real    0m0,369s
user    0m0,286s
sys     0m0,049s

We now went from 0,320s to 0,369s. Not too much, but still it's a pity to pay the price when NOT running tests in parallel.

Handling pytest-xdist

If we disable pytest-xdist psutil tests still run, but we get a warning:

psutil/tests/test_testutils.py:367
  ~/svn/psutil/psutil/tests/test_testutils.py:367: PytestUnknownMarkWarning: Unknown pytest.mark.xdist_group - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    @pytest.mark.xdist_group(name="serial")

This warning appears for methods that are intended to run serially, those decorated with @pytest.mark.xdist_group(name="serial"). However, since pytest-xdist is now disabled, the decorator no longer exists. To address this, I implemented the following solution in psutil/tests/__init__.py:

import pytest, functools

PYTEST_PARALLEL = "PYTEST_XDIST_WORKER" in os.environ  # True if running parallel tests

if not PYTEST_PARALLEL:
    def fake_xdist_group(*_args, **_kwargs):
        """Mimics `@pytest.mark.xdist_group` decorator. No-op: it just
        calls the test method or return the decorated class."""
        def wrapper(obj):
            @functools.wraps(obj)
            def inner(*args, **kwargs):
                return obj(*args, **kwargs)

            return obj if isinstance(obj, type) else inner

        return wrapper

    pytest.mark.xdist_group = fake_xdist_group  # monkey patch

With this in place the warning disappears when running tests serially. To run tests in parallel, we'll manually enable xdist:

$ python3 -m pytest -p xdist -n auto --dist loadgroup

Disable some default plugins

pytests also loads quite a bunch of plugins by default (see output of pytest --trace-config --collect-only). I tried to disable some of them with:

pytest -p no:junitxml -p no:doctest -p no:nose -p no:pastebin

...but that didn't make much of a difference.

Optimizing test collection time

By default, pytest searches the entire directory for tests, adding unnecessary overhead. In pyproject.toml you can tell pytest where test files are located, and only to consider test_*.py files:

[tool.pytest.ini_options]
testpaths = ["psutil/tests/"]
python_files = ["test_*.py"]

With this I saved another 0.03 seconds. Before:

$ python3 -m pytest --collect-only
...
======================== 685 tests collected in 0.20s =========================

After:

$ python3 -m pytest --collect-only
...
======================== 685 tests collected in 0.17s =========================

Putting it all together

With these small optimizations, I managed to reduce pytest startup time by ~0.12 seconds, bringing it down from 0.42 seconds. While this improvement is insignificant for full test runs, it somewhat makes a noticeable difference (~28% faster) when repeatedly running individual tests from the command line, which is something I do frequently during development. Final result is visible in PR-2538.

Giampaolo Rodola Python enthusiast, core developer, psutil author

Blog posts for tags/pytest

Speedup pytest startup

Preface: the migration to pytest

pytest invocation is slow

Where is time being spent?

Disable plugin auto loading

Handling pytest-xdist

Disable some default plugins

Optimizing test collection time

Putting it all together

Other links which may be useful

Feeds

Blog posts for tags/pytest

Preface: the migration to pytest

pytest invocation is slow

Where is time being spent?

Disable plugin auto loading

Handling pytest-xdist

Disable some default plugins

Optimizing test collection time

Putting it all together

Other links which may be useful

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