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Pytest plugin for testing applications that use Pyodide

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Pytest plugin for testing applications that use Pyodide


pytest-pyodide requires Python 3.10+ and can be installed with

pip install pytest-pyodide

You would also need one at least one of the following runtimes:

  • Chrome and chromedriver
  • Firefox and geckodriver
  • Safari and safaridriver
  • node v18+

Github Reusable workflow

pytest-pyodide also supports testing on github actions by means of a reusable workflow in /.github/workflows/main.yml This allows you to test on a range of browser/OS combinations without having to install all the testing stuff, and integrate it easily into your CI process.

In your github actions workflow, call it with as a aseparate job. To pass in your build wheel use an upload-artifact step in your build step.

This will run your tests on the given browser/pyodide version/OS configuration. It runs pytest in the root of your repo, which should catch any test_*.py files in subfolders.

  # Build for pyodide 0.24.1
    runs-on: ubuntu-latest
    - uses: actions/checkout@v3
    - uses: actions/setup-python@v4
        python-version: 3.11
    - uses: mymindstorm/setup-emsdk@v11
        version: 3.1.45
    - run: pip install pyodide-build==0.24.1
    - run: pyodide build
    - uses: actions/upload-artifact@v3
        name: pyodide wheel
        path: dist
  # this is the job which you add to run pyodide-test
    needs: build
    uses: pyodide/pytest-pyodide/.github/workflows/main.yaml@main
      build-artifact-name: pyodide wheel
      build-artifact-path: dist
      browser: firefox
      runner: selenium
      pyodide-version: 0.24.1

If you want to run on multiple browsers / pyodide versions etc., you can either use a matrix strategy and run main.yaml as above, or you can use testall.yaml. This by default tests on all browsers (and node) with multiple configurations. If you want to reduce the configurations you can filter with lists of browsers, runners, pyodide-versions as shown below.

    needs: build
    uses: pyodide/pytest-pyodide/.github/workflows/testall.yaml@main
      build-artifact-name: pyodide wheel
      build-artifact-path: dist
      pyodide-versions: "0.23.4, 0.24.1"
      runners: "selenium, playwright"
      browsers: "firefox, chrome, node"
      os: "ubuntu-latest, macos-latest"


  1. First you need a compatible version of Pyodide. You can download the Pyodide build artifacts from releases with,

    tar xjf pyodide-build-0.24.1.tar.bz2
    mv pyodide dist/
  2. You can then use the provided fixtures (selenium, selenium_standalone) in tests,

    def test_a(selenium):"assert 1+1 == 2")   # run Python with Pyodide

    which you can run with

    pytest --dist-dir=./dist/


Some tests simply involve running a chunk of code in Pyodide and ensuring it doesn't error. In this case, one can use the run_in_pyodide decorate from pytest_pyodide, e.g.

from pytest_pyodide import run_in_pyodide
def test_add(selenium):
    assert 1 + 1 == 2

In this case, the body of the function will automatically be run in Pyodide. The decorator can also be called with a packages argument to load packages before running the test. For example:

from pytest_pyodide import run_in_pyodide
@run_in_pyodide(packages = ["regex"])
def test_regex(selenium_standalone):
    import regex
    assert"o", "foo").end() == 2

You can also use @run_in_pyodide with pytest.mark.parametrize, with hypothesis, etc. @run_in_pyodide MUST be the innermost decorator. Any decorators inside of @run_in_pyodide will be have no effect on the behavior of the test.

from pytest_pyodide import run_in_pyodide
@pytest.mark.parametrize("x", [1, 2, 3])
@run_in_pyodide(packages = ["regex"])
def test_type_of_int(selenium, x):
    assert type(x) is int

The first argument to a @run_in_pyodide function must be a browser runner, generally a selenium fixture. The remaining arguments and the return value of the @run_in_pyodide function must be picklable. The arguments will be pickled in the host Python and unpickled in the Pyodide Python. The reverse will happen to the return value. The first selenium argument will be None inside the body of the function (it is used internally by the fixture). Note that a consequence of this is that the received arguments are copies. Changes made to an argument will not be reflected in the host Python:

def mutate_dict(selenium, x):
    x["a"] = -1
    return x

def test_mutate_dict():
    d = {"a" : 9, "b" : 7}
    assert mutate_dict(d) == { "a" : -1, "b" : 7 }
    # d is unchanged because it was implicitly copied into the Pyodide runtime!
    assert d == {"a" : 9, "b" : 7}

You can also use fixtures as long as the return values of the fixtures are picklable (most commonly, if they are None). As a special case, the function will see the selenium fixture as None inside the test.

If you need to return a persistent reference to a Pyodide Python object, you can use the special PyodideHandle class:

def get_pyodide_handle(selenium):
    from pytest_pyodide.decorator import PyodideHandle
    d = { "a" : 2 }
    return PyodideHandle(d)

def set_value(selenium, h, key, value):
    h[key] = value

def get_value(selenium, h, key):
    return h[key]

def test_pyodide_handle(selenium):
    h = get_pyodide_handle(selenium)
    assert get_value(selenium, h, "a") == 2
    set_value(selenium, h, "a", 3)
    assert get_value(selenium, h, "a") == 3

This can be used to create fixtures for use with @run_in_pyodide.

It is possible to use run_in_pyodide as an inner function:

def test_inner_function(selenium):
    def inner_function(selenium, x):
        assert x == 6
        return 7
    assert inner_function(selenium_mock, 6) == 7

However, the function will not see closure variables at all:

def test_inner_function_closure(selenium):
    x = 6
    def inner_function(selenium):
        assert x == 6
        return 7
    # Raises `NameError: 'x' is not defined`
    assert inner_function(selenium_mock) == 7

Thus, the only value of inner @run_in_pyodide functions is to limit the scope of the function definition. If you need a closure, you will have to wrap it in a second function call.

Copying files to Pyodide

You can copy files to the pyodide filesystem using the copy_files_to_pyodide decorator. This takes two arguments - a list of (src,destination) pairs. These can be any of: 1) A filename, 2) A folder name, which is copied to the destination path (along with all subdirectories if recurse_directories is True), 3) A glob pattern, which will fetch all files matching the pattern and copy them to a destination directory, whilst preserving the folder structure.

If you set install_wheels to True, any .whl files will be installed on pyodide. This is useful for installing your package.

from pytest_pyodide.decorator import copy_files_to_pyodide


Running non-pyodide tests in Pyodide

This plugin also supports running standard pytest tests on pyodide in a browser. So if you have an existing codebase and you want to check if your pyodide build works, just run it like this:

# Make the emscripten/wasm32  wheel in the dist folder
pyodide build
# the following code
# a) copies the test_path directory and subfolders to a Pyodide instance, and
# b) installs any wheels in the dist subfolder so that this package is available on the Pyodide VM
pytest --run-in-pyodide test_path --runtime <runtime> --dist-dir=<pyodide/dist>

Specifying a browser

You can specify a browser runtime using --runtime (--rt) commandline option.

Possible options for --runtime are:

  • node (default)
  • firefox
  • chrome
  • safari
  • host (do not run browser-based tests)
pytest --runtime firefox
pytest --runtime firefox --runtime chrome

# Adding -no-host suffix will disable running host tests
pytest --runtime chrome-no-host

Running tests with Playwright (optional)

By default, the tests will be run with Selenium. It is possible to run tests with playwright instead as follows.

First install playwright browsers

python -m playwright install --with-deps

Then use the --runner argument to specify to run tests with playwright.

pytest --runner playwright

Custom test marks

Custom test marks supported by pytest-pyodide:

pytest.mark.driver_timeout(timeout): Set script timeout in WebDriver. If the test is known to take a long time, you can extend the deadline with this marker.

pytest.mark.xfail_browsers(chrome="why chrome fails"): xfail a test in specific browsers.


See examples.

Compatible Pyodide versions

See compatibility table.


pytest-pyodide uses the Mozilla Public License Version 2.0.

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