- 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))
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 are a few extra things you need to make sure are set up so that new versions of Python can be downloaded and configured on your runner.
- Your runner needs to be running with administrator privileges so that the appropriate directories and files can be set up when downloading and installing a new version of Python for the first time.
- If your runner is configured as a service, make sure the account that is running the service has the appropriate write permissions so that Python can get installed. The default `NT AUTHORITY\NETWORK SERVICE` should be sufficient.
- You need `7zip` installed and added to your `PATH` so that the downloaded versions of Python files can be extracted properly during first-time 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.
- The Python packages that are downloaded from `actions/python-versions` are originally compiled from source in `/opt/hostedtoolcache/` with the [--enable-shared](https://github.com/actions/python-versions/blob/94f04ae6806c6633c82db94c6406a16e17decd5c/builders/ubuntu-python-builder.psm1#L35) flag, which makes them non-relocatable.
- Create an environment variable called `AGENT_TOOLSDIRECTORY` and set it to `/opt/hostedtoolcache`. This controls where the runner downloads and installs tools.
- In the same shell that your runner is using, type `export AGENT_TOOLSDIRECTORY=/opt/hostedtoolcache`
- A more permanent way of setting the environment variable is to create a `.env` file in the same directory as your runner and to add `AGENT_TOOLSDIRECTORY=/opt/hostedtoolcache`. This ensures the variable is always set if your runner is configured as a service.
- Create a directory called `hostedtoolcache` inside `/opt`.
- The user starting the runner must have write permission to the `/opt/hostedtoolcache` directory. It is not possible to start the Linux runner with `sudo` and the `/opt` directory usually requires root privileges to write to. Check the current user and group that the runner belongs to by typing `ls -l` inside the runners root directory.
- The runner can be granted write access to the `/opt/hostedtoolcache` directory using a few techniques:
- The user starting the runner is the owner, and the owner has write permission
- The user starting the runner is in the owning group, and the owning group has write permission
- If your runner is configured as a service and you run into problems, make sure the user that the service is running as is correct. For more information, you can [check the status of your self-hosted runner](https://help.github.com/en/actions/hosting-your-own-runners/configuring-the-self-hosted-runner-application-as-a-service#checking-the-status-of-the-service).
### Mac
- The same setup that applies to `Linux` also applies to `Mac`, just with a different tools cache directory.
- Create a directory called `/Users/runner/hostedtoolcache`
- Set the `AGENT_TOOLSDIRECTORY` environment variable to `/Users/runner/hostedtoolcache`.
- Change the permissions of `/Users/runner/hostedtoolcache` so that the runner has write access.
`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`.
