- 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).
`setup-python` is able to configure Python from two sources:
- Preinstalled versions of Python in the tools cache on GitHub-hosted runners
- 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.
- 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 the previous example, the version spec `3.8` will use the `3.8.2` Python version found in the cache.
- Downloadable Python versions from GitHub Releases ([actions/python-versions](https://github.com/actions/python-versions/releases))
- All available versions are listed in the [version-manifest.json](https://github.com/actions/python-versions/blob/master/versions-manifest.json) file.
- If there is a specific version of Python that is not available, you can open an issue in the `python-versions` repository.
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 there is a specific version of Python that you need and you don't want to worry about any potential breaking changes due to patch updates (going from `3.7.5` to `3.7.6` for example), you should specify the exact major, minor, and patch version (such as `3.7.5`)
- The only downside to this is that set up will take a little longer since the exact version will have to be downloaded if the exact version is not already installed on the runner due to more recent versions.
- MSI installers are used on Windows for this, so runs will take a little longer to set up vs Mac and Linux.
You should specify only a major and minor version if you are okay with the most recent patch version being used.
- There will be a single patch version already installed on each runner for every minor version of Python that is supported.
- The patch version that will be preinstalled, will generally be the latest and every time there is a new patch released, the older version that is preinstalled will be replaced.
- Using the most recent patch version will result in a very quick setup since no downloads will be required since a locally installed version Python on the runner will be used.
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 set up 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 & Features" in the Settings app.
`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:
- something might be compiled incorrectly
- certain modules might be missing
- there is a version of Python that you would like that is currently not available
