pytest-hot-reloading 0.1.0a7

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A PyTest Hot Reloading Plugin

A hot reloading pytest daemon, implemented as a plugin.

This uses the jurigged library to watch files.

If it takes less than 5 seconds to do all of the imports necessary to run a unit test, then you probably don't need this. If you use Django, you'll probably love this.

The minimum Python version is 3.10

Demo

With hot reloading

Without hot reloading

Installation

Do not install in production code. This is exclusively for the developer environment.

pip: Add pytest-hot-reloading to your dev-requirements.txt file and pip install -r dev-requirements.txt

poetry: poetry add --group=dev pytest-hot-reloading

Usage

Add the plugin to the pytest arguments. Example using pyproject.toml:

[tool.pytest.ini_options]
addopts = "-p pytest_hot_reloading.plugin"

When running pytest, the plugin will detect whether the daemon is running, and start it if is not. Note that a pid file is created to track the pid.

Imports are not reran on subsequent runs, which can be a huge time saver.

Currently, if you want to debug, you will want to run the daemon manually with debugging. This can easily be done in VS Code with the following launch profile:

        {
            "name": "Pytest Daemon",
            "type": "python",
            "request": "launch",
            "module": "pytest",
            "justMyCode": false,
            "args": [
                "--daemon",
                //
                // everything below this is optional
                //
                "--daemon-port",
                "4852", // the default value
                "--daemon-watch-globs",
                "./*.py" // the default value
                // "./my-project/*.py:./some-thing-else/*.py",  // example of colon separated globs
                "--daemon-ignore-watch-globs",
                "./.venv/*" // this is the default value, also colon separated globs
            ]
        },

The only reason you would need to limit the watched files is because the jurigged library opens every file it watches, so it can exhaust the open file limit if you have a lot of files.

If the daemon is already running and you run pytest with --daemon, then the old one will be stopped and a new one will be started. Note that pytest --daemon is NOT how you run tests. It is only used to start the daemon.

The daemon can be stopped with pytest --stop-daemon. This can be used if it gets into a bad state.

Arguments and Env Variables

• PYTEST_DAEMON_PORT
  • The port the daemon listens on.
  • Default: 4852.
  • Command line: --daemon-port
• PYTEST_DAEMON_PYTEST_NAME
  • The name of the pytest executable.
  • Default: pytest.
  • Command line: --pytest-name
• PYTEST_DAEMON_WATCH_GLOBS
  • The colon separated globs to watch.
  • Default: ./**/*.py.
  • Command line: --daemon-watch-globs
• PYTEST_DAEMON_IGNORE_WATCH_GLOBS
  • The colon separated globs to ignore.
  • Default: ./.venv/*.
  • Command line: --daemon-ignore-watch-globs

Workarounds

Libraries that use mutated globals may need a workaround to work with this plugin. The preferred route is to have the library update its code to not mutate globals in a test environment, or to restore them after a test suite has ran. In some cases, that isn't possible, usually because the person with the problem doesn't own the library and can't wait around for a fix.

To register a workaround, create a function that is decorated by the pytest_hot_reloading.workaround.register_workaround decorator. It may optionally yield. If it does, then code after the yield is executed after the test suite has ran.

Example:

from pytest_hot_reloading.workaround import register_workaround

@register_workaround("my_library")
def my_library_workaround():
    import my_library

    yield

    my_library.some_global = BackToOriginalValue()

If you are a library author, you can disable any workarounds for your library by creating an empty module _clear_hot_reload_workarounds.py. If this is successfully imported, then workarounds for the given module will not be executed.

Known Issues

• This is early alpha
• The jurigged library is not perfect and sometimes it gets in a bad state
• Some libraries were not written with hot reloading in mind, and will not work without some changes. There is going to be logic to work around other issues with other libraries, such as pytest-django's mutation of the settings module that runs every session, but it hasn't been implemented yet.

Notes

• pytest-xdist will have its logic disabled, even if args are passed in to enable it
• pytest-django will not create test database suffixes for multiworker runs such as tox.