- 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.
More details on PyPy syntax and examples of using preview / nightly versions of PyPy can be found in the [Available versions of PyPy](#available-versions-of-pypy) section.
>The environment variable `pythonLocation` also becomes available after Python or PyPy installation. It contains the absolute path to the folder where the desired version of Python or PyPy is installed.
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 [Supported software](https://docs.github.com/en/actions/reference/specifications-for-github-hosted-runners#supported-software).
- 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.
- Use `-dev` instead of a patch number (e.g., `3.11-dev`) to install the latest patch version release for a given minor version, *alpha and beta releases included*.
**Note:** Python versions used in this action are generated in the [python-versions](https://github.com/actions/python-versions) repository. For macOS and Ubuntu images python versions are built from the source code. For Windows the python-versions repository uses installation executable. For more information please refer to the [python-versions](https://github.com/actions/python-versions) repository.
- For detailed information regarding the available versions of PyPy that are installed, see [Supported software](https://docs.github.com/en/actions/reference/specifications-for-github-hosted-runners#supported-software).
- Cache is updated with a 1-2 week delay. If you specify the PyPy version as `pypy3.7` or `pypy-3.7`, the cached version will be used although a newer version is available. If you need to start using the recently released version right after release, you should specify the exact PyPy version using `pypy3.7-v7.3.3` or `pypy-3.7-v7.3.3`.
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.
The `check-latest` flag defaults to `false`. Use the default or set `check-latest` to `false` if you prefer stability and if you want to ensure a specific `Python/PyPy` version is always used.
If `check-latest` is set to `true`, the action first checks if the cached version is the latest one. If the locally cached version is not the most up-to-date, a `Python/PyPy` version will then be downloaded. Set `check-latest` to `true` if you want the most up-to-date `Python/PyPy` version to always be used.
> Setting `check-latest` to `true` has performance implications as downloading `Python/PyPy` versions is slower than using cached versions.
The action has built-in functionality for caching and restoring dependencies. It uses [actions/cache](https://github.com/actions/toolkit/tree/main/packages/cache) under the hood for caching dependencies but requires less configuration settings. Supported package managers are `pip`, `pipenv` and `poetry`. The `cache` input is optional, and caching is turned off by default.
The action defaults to searching for a dependency file (`requirements.txt` for pip, `Pipfile.lock` for pipenv or `poetry.lock` for poetry) in the repository, and uses its hash as a part of the cache key. Use `cache-dependency-path` for cases where multiple dependency files are used, they are located in different subdirectories or different files for the hash want to be used.
**Please Note:** Restored cache will not be used if the requirements.txt file is not updated for a long time and a newer version of the dependency is available that can lead to an increase in total build time.
The requirements file format allows to specify dependency versions using logical operators (for example chardet>=3.0.4) or specify dependencies without any versions. In this case the pip install -r requirements.txt command will always try to install the latest available package version. To be sure that the cache will be used, please stick to a specific dependency version and update it manually if necessary.
With this setting, the action will add/update environment variables (e.g. `PATH`, `PKG_CONFIG_PATH`, `pythonLocation`) for `python` to just work out of the box.
If `update-environment` is set to `false`, the action will not add/update environment variables.
This can prove useful if you want the only side-effect to be to ensure python is installed and rely on the `python-path` output to run python.
Such a requirement on side-effect could be because you don't want your composite action messing with your user's workflows.
Python distributions are only available for the same [environments](https://github.com/actions/virtual-environments#available-environments) that GitHub Actions hosted environments are available for. If you are using an unsupported version of Ubuntu such as `19.04` or another Linux distribution such as Fedora, `setup-python` will not work. If you have a supported self-hosted runner and you would like to use `setup-python`, 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.
If you are experiencing problems while configuring Python on your self-hosted runner, turn on [step debugging](https://github.com/actions/toolkit/blob/main/docs/action-debugging.md#step-debug-logs) to see addition logs.
- 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 with the [--enable-shared](https://github.com/actions/python-versions/blob/main/builders/ubuntu-python-builder.psm1#L37) flag.
- By default runner downloads and install the tools to the `RUNNER_TOOL_CACHE` directory, however `AGENT_TOOLSDIRECTORY` can be set to override this location.
`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`.
`setup-python` comes pre-installed on the appliance with GHES if Actions is enabled. When dynamically downloading Python distributions, `setup-python` downloads distributions from [`actions/python-versions`](https://github.com/actions/python-versions) on github.com (outside of the appliance). These calls to `actions/python-versions` are made via unauthenticated requests, which are limited to [60 requests per hour per IP](https://docs.github.com/en/rest/overview/resources-in-the-rest-api#rate-limiting). If more requests are made within the time frame, then you will start to see rate-limit errors during download that read `##[error]API rate limit exceeded for...`.
To avoid hitting rate-limit problems, we recommend [setting up your own runner tool cache](https://docs.github.com/en/enterprise-server@2.22/admin/github-actions/managing-access-to-actions-from-githubcom/setting-up-the-tool-cache-on-self-hosted-runners-without-internet-access#about-the-included-setup-actions-and-the-runner-tool-cache).
