This chapter describes how to run various tests locally in a development environment, guidelines for writing tests, and information regarding the continuous testing infrastructure.

Running Tests Locally#

Before attempting to run Panel tests, make sure you have successfully run through all of the instructions in the developer setup guide.


Set up pre-commit hooks to ensure that your code is linted correctly whenever you commit a change.

To run the pre-commit hooks explicitly:

pre-commit run -a

Test Selection#

Currently Panel uses linting and three types of tests:

  • Unit tests: usually small tests executed with pytest, they can be found in panel/tests/.

  • UI tests: Panel provides web components that users can interact with through their browser, UI tests allow to check that these components get displayed as expected, and that the backend <-> front-end bi-communication (e.g. updating a widget value in the front end should update its value in Python) works correctly. UI tests are possible thanks to Playwright, they can be found in the panel/tests/ui/ folder.

  • Notebooks smoke tests: Panel’s documentation consists mostly of Jupyter Notebooks, these smoke tests execute all the notebooks and fail if an error is raised during their execution. Notebook smoke tests are possible thanks to nbval and can be found in the examples/ folder.

To run flake checking explicitly run:

doit test_flakes

To run unit tests use:

doit test_unit

To run UI tests use:

doit test_ui

To run example smoke tests use:

doit test_examples

Writing Tests#

In order to help keep Panel maintainable, all Pull Requests that touch code should normally be accompanied by relevant tests. While exceptions may be made for specific circumstances, the default assumption should be that a Pull Request without tests may not be merged.

Python Unit Tests#

Python unit tests maintain the basic functionality of the Python portion of the Panel library. A few general guidelines will help you write Python unit tests:

  • absolute imports

    In order to ensure that Panel’s unit tests as relocatable and unambiguous as possible, always prefer absolute imports in test files. When convenient, import and use the entire module under test:

    • GOOD: import panel.widgets

    • GOOD: from panel.layout import Column

    • BAD: from ..models.widgets import Player

  • pytest

    All new tests should use and assume pytest for test running, fixtures, parameterized testing, etc. New tests should not use the unittest module of the Python standard library.

UI tests#

It may be difficult to write UI tests robust enough to pass all the time. For instance some tests may need to download some CSS resources online, if the domain that hosts these resources do not serve them in a limited time (defined by a Playwright timeout) then the test will fail. To cover these cases you can mark your test as flaky with @pytest.mark.flaky, refer to flaky’s documentation for more information.

Continuous Integration#

Every push to the main branch or any Pull Request branch on GitHub automatically triggers a full test build on the GitHub Actions continuous integration service. This is most often useful for running the full Panel test suite continuously, but also triggers automated scripts for publishing releases when a tagged branch is pushed.

You can see the list of all current and previous builds at this URL: https://github.com/holoviz/panel/actions


There are a number of files that affect the build configuration:

  • .github/worksflows/test.yaml

    Defines the build matrix and global configurations for the stages described below.

  • conda.recipe/meta.yaml

    Instructions for building a conda noarch package for Panel.

  • setup.py

    Used to build sdist packages and “dev” installs. This file is the single source of truth of all dependencies.

  • tox.ini

    Contains the configuration for the doit commands .


GitHub Actions provides free build workers to Open Source projects. A few considerations will help you be considerate of others needing these limited resources:

  • Run the tests locally before opening a Pull Request, or before committing to an opened Pull Request.

  • Group commits into meaningful chunks of work before pushing to GitHub (i.e. don’t push on every commit).

  • If you must make multiple commits in succession, navigate to GitHub Actions and cancel all but the last build, in order to free up build workers.