Anže's Blog

Python, Django, and the Web

06 Oct 2023

Writing a Pytest plugin

I’ve been working on a pytest plugin, and I’ve learned how some of this black magic works, so I thought I’d share.


When you install a pytest plugin, it will be automatically loaded when you run pytest.

❯ pytest
=================== test session starts ====================
platform darwin -- Python 3.11.4, pytest-7.4.2, pluggy-1.3.0
rootdir: /Users/anze/coding/pytest-plugin-example
plugins: exampleplugin-2023.10.6
collected 3 items

We can see that my exampleplugin-2023.10.6 plugin was loaded when we ran the tests.

pytest has many different ways to load a plugin at startup, but the simplest way for the users of the plugin is to use the pytest11 entrypoint. This entrypoint is a convention that allows pytest to automatically load a plugin for any installed package. You can define the entrypoint in your pyproject.toml file:

exampleplugin = "exampleplugin.hook"

The exampleplugin.hook module contains the plugin’s hook implementations. The module’s name is unimportant, but it’s good to name it after the plugin.

Hook implementations

A pytest plugin is a Python module that implements one or more hooks. Hooks are plain functions with specific names that pytest calls at particular points during its execution. For example, the pytest_addoption hook is called when pytest parses the command line arguments. The pytest_runtest_setup hook is called before each test is run.

You can find the whole list of hooks in the pytest documentation, but some of the most important ones are listed below:

 +- PyTestPluginManager()
 +- Config()
 +- import+register default built-in plugins
 |   +- pytest_plugin_registerd()
 +- pytest_namespace()
 +- pytest_addoption()
 +- pytest_cmdline_parse() 1:1
 +- pytest_cmdline_main() 1:1
     +- Session()
     +- pytest_configure()
     +- pytest_session_start()
     +- pytest_collection() 1:1
     |   +- pytest_collectreport() per item
     |   +- pytest_collection_modifyitems()
     |   +- pytest_collection_finish()
     +- pytest_runtestloop()
     |   +- pytest_runtest_protocol() per item
     |       +- pytest_runtest_logstart()
     |       +- pytest_runtest_setup()
     |       +- pytest_runtest_call()
     |       +- pytest_runtest_teardown()
     +- pytest_sessionfinish()
     +- pytest_unconfigure()


Publishing the plugin

The easiest way for users to use your pytest plugin is to publish it on PyPI. There are many ways to publish your plugin, but I like using Hatch because it has excellent defaults and it’s easy to automate with GitHub Actions. Hatch’s documentation does a good job of explaining how to create a new project and publish it.

Wrapping it up

That’s it! You now know how to write a simple pytest plugin and publish it. If you need examples, try searching for the pytest classifier on PyPI. There are a lot of fantastic plugins out there that you can learn from!