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:

[tool.pytest.ini_options]
testpaths = ["psutil/tests/"]

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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