Build and Setup Python

This action wraps around actions/setup-python allowing for deprecated CPython versions to be built from source instead of leading to a failure while appearing transparent in all other cases.

Table of contents

• Build and Setup Python
  • Table of contents
  • Basic usage
  • Motivation
    • But why? It's deprecated: just use a more recent version
    • But why a dedicated action? Just use ubuntu-20.04 for that 1 job
  • Guarantees
  • Known limits
  • Performance
  • Security guarantees
  • Inputs
    • allow-build input
  • Outputs
  • FAQ
  • Contributing

Basic usage

For a more complete view on the actions see action.yml or look in the Inputs and Outputs sections. In general you could replace the original action with this one and it should already work (all its supported inputs are also supported in this one):

- uses: MatteoH2O1999/setup-python@v3
  with:
    python-version: '3.6'

But if you wish for a more optimized experience you could use inputs exclusive to this action:

- uses: MatteoH2O1999/setup-python@v3
  with:
    python-version: '3.6'
    allow-build: info
    cache-build: true
    cache: pip

Motivation

Since the replacement of ubuntu-20.04 with ubuntu-22.04 runner images actions/setup-python no longer supports Python 3.6 for the ubuntu-latest label. This broke a large number of pipelines as Python 3.6 is still widely used.

This made apparent that a way to support deprecated versions was needed.

But why? It's deprecated: just use a more recent version

Yes, in a perfect world that would be the go to solution. Unfortunately, we do not live in that world, so we can't always choose our dependencies. In fact, I would be willing to bet that most deprecated dependencies are there despite the developers, not because of them. That is why this action came about: to offer an easier way to continue supporting deprecated builds even if github does not anymore.

But why a dedicated action? Just use ubuntu-20.04 for that 1 job

Once again, that is possible, but that is

1. not good practice as editing the main job matrix will not edit the "special" one;
2. a pain to maintain as each new deprecated version needs another job to be created.

Guarantees

The objective of this action is to guarantee that for every major Python version starting from 2.7 at least one specific version can be successfully installed on the ...-latest images using the default architecture.

TLDR: If you use the major version specification (3.6 instead of 3.6.5) without specifying the architecture as shown in Basic usage this action is guaranteed to work (hopefully...😉) on all the ...-latest labels.

Exceptions

Python versions < 3.5 are too difficult to build from source on Windows. For these versions the action tries to use the official installer from python.org. If this fails, the action fails with an appropriate error message. In this case you should request a version for which python.org offers a binary installer, and not just the source code. On the positive side, every version of Python 2.7 has a binary installer so it shouldn't be a problem.

Known limits

This action at the moment does not support:

• installing multiple Python versions
• building PyPy from source
• building from source in UNIX systems for a different architecture

This actions tries to but does not guarantee to work on any arbitrary pair (python-version, architecture).

Performance

The action's performance depends on the runner OS and on whether Python needs to be built, installed, restored from cache or is already installed:

1. Supported by actions/setup-python
   1. Already installed on runner
      1. ubuntu-latest → ~10s
      2. windows-latest → ~20s
      3. macos-latest → ~10s
   2. Available from actions/python-versions
      1. ubuntu-latest → ~20s
      2. windows-latest → ~1m
      3. macos-latest → ~30s
2. Not supported by actions/setup-python
   1. Already cached
      1. ubuntu-latest → ~20s
      2. windows-latest → ~30s
      3. macos-latest → ~30s
   2. Built from source
      1. ubuntu-latest → ~3m30s
      2. windows-latest → ~25m
      3. macos-latest → ~13m

Security guarantees

This action is composite and uses the original actions/setup-python and a custom action MatteoH2O1999/build-and-install-python. Both are referenced by commit hash in order to provide better security: if you use this action using its commit hash it will be safe from future malicious edits to the tags.

Inputs

This action supports the following inputs (in bold are the names of the exclusive inputs for this action):

Input name	Description	Accepted values	Default value
python-version	Version range or exact version of Python or PyPy to use, using SemVer's version range syntax. Reads from .python-version if both this and python-version-file are unset.	pypy3.7, pypy-3.9, 3.5, 2.7.6, ...	None
python-version-file	File containing the Python version to use. Reads from .python-version if both this and python-version are unset.	.python-version, path/to/python/version, ...	None
cache	Used to specify a package manager for caching in the default directory.	pip, pipenv and poetry	None
architecture	The target architecture of the Python or PyPy interpreter.	x64, x86, arm64, ...	None
check-latest	Set this option if you want the action to check for the latest available version that satisfies the version spec.	false, true	false
token	The token used to authenticate when fetching Python distributions from actions/python-versions. When running this action on github.com, the default value is sufficient. When running on GHES, you can pass a personal access token for github.com if you are experiencing rate limiting.	example: TokenString	github.token
cache-dependency-path	Used to specify the path to dependency files. Supports wildcards or a list of file names for caching multiple dependencies.	example: path/to/dependency/files	''
update-environment	Set this option if you want the action to update environment variables.	true, false	true
cache-build	Whether to cache the built Python distribution to speed up successive runs.	true, false	false
allow-build	Set the behavior of the action when actions/setup-python fails and has to be built from source.	allow, info, warn, error, force	warn

allow-build input

This action can apply for different behaviors when it tries to install a CPython version not supported by actions/setup-python:

• error this emulates actions/setup-python and throws an error (thus failing the job) if the Python version cannot be downloaded from actions/python-versions;
• warn(default) this will proceed to build the specified version of CPython but will still throw a warning. This was chosen as the default behavior as the user may not know a deprecated version has been requested;
• info this will build from source and only print to the logs the fact that a specific Python version will be built from source;
• allow same as info but does not even print to logs the fact that CPython will be built from source;
• force this will force Python to be built from source regardless of whether actions/setup-python supports it or not.

Outputs

This action will emit the following outputs:

Output name	Description
python-version	The installed Python or PyPy version. Useful when given a version range as input.
cache-hit	A boolean value to indicate a cache entry was found (for pip, pipenv and poetry).
python-path	The absolute path to the Python or PyPy executable.

FAQ

No file in (...) matched to [**/requirements.txt or **/pyproject.toml], make sure you have checked out the target repository

This is a byproduct of actions/setup-python. If you wish to cache your pip dependencies, you need to have anywhere in your repository a requirements.txt or a pyproject.toml file. The solution in this case is either creating a blank requirements.txt file or stop caching pip (in order to do so simply remove cache: pip from your .yml file).

Contributing

This action is pretty much only a wrapper for a javascript action. Any issue you might encounter with this action is probably caused by MatteoH2O1999/build-and-install-python. If you wish to contribute, you should do so there.