- optionally installing and adding to PATH a version of Python that is already installed in the tools cache
- downloading, installing and adding to PATH an available version of Python from GitHub Releases ([actions/python-versions](https://github.com/actions/python-versions/releases)) if a specific version is not available in the tools cache
- failing if a specific version of Python is not preinstalled or available for download
Check out our detailed guide on using [Python with GitHub Actions](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/using-python-with-github-actions).
- For detailed information regarding the available versions of Python that are installed see [Software installed on GitHub-hosted runners](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/software-installed-on-github-hosted-runners).
- For every minor version of Python, expect only the latest patch to be preinstalled. See [Semantic Versioning](https://semver.org/) for more information
- If `3.8.1` is installed for example, and `3.8.2` is released, expect `3.8.1` to be removed and replaced by `3.8.2` in the tools cache
- If the exact patch version doesn't matter to you, specifying just the major and minor version will get you the latest preinstalled patch version. In this case, for example, the version spec 3.8 will use the 3.8.2 found in the cache
GitHub hosted runners have a tools cache that comes with a few versions of Python + PyPy already installed. This tools cache helps speed up runs and tool setup by not requiring any new downloads. There is an environment variable called `RUNNER_TOOL_CACHE` on each runner that describes the location of this tools cache and there is where you will find Python and PyPy installed. `setup-python` works by taking a specific version of Python or PyPy in this tools cache and adding it to PATH.
GitHub virtual environments are setup in [actions/virtual-environments](https://github.com/actions/virtual-environments). During the setup, the available versions of Python and PyPy are automatically downloaded, setup and documented.
If you would like to use `setup-python` and a self-hosted runner, there isn't much that you need to do. When `setup-python` is run for the first time with a version of Python that it doesn't have, it will download the appropriate version, and setup the tools cache on your machine. Any subsequent runs will use the python versions that were previously downloaded.
A few things to look out for when `setup-python` is first setting up the tools cache
- If using Windows, your runner needs to be running as an administrator so that the appropriate directories and files can be setup. On Linux and Mac, you also need to be running with elevated permissions
- On Windows, you need `7zip` installed and added to your `PATH` so that files can be extracted properly during setup
- MSI installers are used when setting up Python on Windows. A word of caution as MSI installers update registry settings
- The 3.8 MSI installer for Windows will not let you install another 3.8 version of Python. If `setup-python` fails for a 3.8 version of Python, make sure any previously installed versions are removed by going to "Apps and Features" in the Windows settings and uninstalling any
- Create an global environment variable called `AGENT_TOOLSDIRECTORY` that will point to the root directory of where you want the tools installed. The env variable is preferably global as it must be set in the shell that will install the tools cache, along with the shell that the runner will be using.
- This env variable is used internally by the runner to set the `RUNNER_TOOL_CACHE` env variable
- Example for Administrator Powershell: `[System.Environment]::SetEnvironmentVariable("AGENT_TOOLSDIRECTORY", "C:\hostedtoolcache\windows", [System.EnvironmentVariableTarget]::Machine)` (restart the shell afterwards)
- Download the appropriate NPM packages from the [GitHub Actions NPM registry](https://github.com/orgs/actions/packages)
- Make sure to have `npm` installed, and then [configure npm for use with GitHub packages](https://help.github.com/en/packages/using-github-packages-with-your-projects-ecosystem/configuring-npm-for-use-with-github-packages#authenticating-to-github-package-registry)
- Create an empty npm project for easier installation (`npm init`) in the tools cache directory. You can delete `package.json`, `package.lock.json` and `node_modules` after all tools get installed
- Before downloading a specific package, create an empty folder for the version of Python/PyPY that is being installed. If downloading Python 3.6.8 for example, create `C:\hostedtoolcache\windows\Python\3.6.8`
- Once configured, download a specific package by calling `npm install`. Note (if downloading a PyPy package on Windows, you will need 7zip installed along with `7z.exe` added to your PATH)
- Each NPM package has multiple versions that determine the version of Python or PyPy that should be installed.
-`npm install @actions/toolcache-python-windows-x64@3.7.61579791175` for example installs Python 3.7.6 while `npm install @actions/toolcache-python-windows-x64@3.6.81579791177` installs Python 3.6.8
- You can browse and find all available versions of a package by searching the GitHub Actions NPM registry
`setup-python` helps keep your dependencies explicit and ensures consistent behavior between different runners. If you use `python` in a shell on a GitHub hosted runner without `setup-python` it will default to whatever is in PATH. The default version of Python in PATH vary between runners and can change unexpectedly so we recommend you always use `setup-python`.
Python versions available for `setup-python` can be found in the [actions/python-versions](https://github.com/actions/python-versions) repository. You can see the build scripts, configurations, and everything that is used. You should open an issue in the `python-versions` repository if:
If you suspect something might be wrong with the tools cache or how Python gets installed on GitHub hosted runners, please open an issue in [actions/virtual-environments](https://github.com/actions/virtual-environments).
