kurtmckee/detect-pythons

A GitHub action to detect installed Pythons. Excellent for cache-busting.

Detect Python interpreters

Robust cache-busting based on Python implementations, versions, and architectures.

---

Purpose

If you're caching Python virtual environments to speed up your CI runs, you've likely encountered broken symlinks or missing libraries when a new Python patch version was released. This is particularly likely to happen when using tox to test multiple Python versions simultaneously.

detect-pythons provides that much-needed cache busting.

It detects all Python executables in every directory on the $PATH and identifies:

• The Python interpreter (like CPython, PyPy, or GraalPy)
• The Python version (like "3.12.1") or the Python ABI version (like "3.12")
• The target architecture (like "x64")

In most cases, the path to the executable already contains this information so it's not necessary to run the Python interpreter to extract any info. That makes detect-pythons fast.

There are some exceptions, however; these are covered in the "Implementation" section below.

Usage

The following example demonstrates how detect-pythons can be used when caching a Python virtual environment stored in .venv/ and tox test environments stored in .tox/.

- uses: "actions/setup-python@v5"
  with:
    python-version: |
      graalpy-23.1
      pypy-3.10
      3.12

- uses: "kurtmckee/detect-pythons@v1"

- uses: "actions/cache@v4"
  id: "restore-cache"
  with:
    # You may need to augment the list of files to hash.
    # For example, you might add 'requirements/*.txt' or 'pyproject.toml'.
    key: "${{ hashFiles('.python-identifiers') }}"
    path: |
      .tox/
      .venv/

- name: "Identify .venv path"
  shell: "bash"
  run: "echo 'venv-path=.venv/${{ runner.os == 'Windows' && 'Scripts' || 'bin' }}' >> $GITHUB_ENV"

- name: "Create a virtual environment"
  if: "steps.restore-cache.outputs.cache-hit == false"
  run: |
    python -m venv .venv
    ${{ env.venv-path }}/python -m pip install --upgrade pip setuptools wheel

    # You may need to customize what gets installed next.
    # However, tox is able to run test suites against multiple Pythons,
    # so it's a helpful tool for efficient testing.
    ${{ env.venv-path }}/pip install tox

- name: "Run the test suite against all installed Pythons"
  run: "${{ env.venv-path }}/tox"

Inputs

By default, the action writes to a file named .python-identifiers, which can then be passed to GitHub's hashFiles function for convenient cache-busting.

You can customize the path and filename by modifying the input variable identifiers-filename:

- uses: "kurtmckee/detect-pythons@v1"
  with:
    identifiers-filename: "favored_filename.txt"

To prevent writing a file at all, set identifiers-filename to an empty string:

- uses: "kurtmckee/detect-pythons@v1"
  with:
    identifiers-filename: ""

Outputs

In addition to writing to a file, the action creates an output named python-identifiers. This may be useful in other contexts.

Implementation

detect-pythons finds all Python interpreters available on the $PATH and ensures that critical information about each interpreter is included in its output:

• Implementation
• Version
• Architecture

Cached Python interpreters

GitHub runners have common CPython and PyPy versions pre-installed. These are installed under $RUNNER_TOOL_CACHE in informative directory paths, so the paths are used without executing the interpreters.

Platform	Sample path under $RUNNER_TOOL_CACHE
Linux	/opt/hostedtoolcache/Python/3.12.1/x64/bin
macOS	/Users/runner/hostedtoolcache/PyPy/3.10.13/x64/bin
Windows	C:\hostedtoolcache\windows\Python\3.12.1\x64

System CPython interpreters

GitHub's Linux and macOS runners have system CPython interpreters installed. These are available at paths like /usr/bin/python, which contains no useful information.

For these interpreters, the interpreter is executed and the value of sysconfig.get_config_var("EXT_SUFFIX") is extracted. This results in a value like the following:

Platform	Sample EXT_SUFFIX value
Linux	.cpython-310-x86_64-linux-gnu.so
macOS	.cpython-311-darwin.so

macOS runner variability

At the time of writing, GitHub's macOS runners sometimes have CPython 2.7 pre-installed. CPython 2.7 doesn't have an EXT_SUFFIX config value, so detect-pythons constructs one.

Platform	Constructed EXT_SUFFIX equivalent
macOS 12.6	.cpython-27-darwin-x86_64

Note that CPython 2.7 is only installed sometimes; sometimes it isn't. This is because macos-latest is sometimes macOS 12.6, and sometimes it's macOS 12.7. See actions/runner-images#8642 for more information.

When using detect-pythons to help with cache-busting, this variability means that you may see two caches appear in regular use for your macOS-based workflows; one that is valid when CPython 2.7 is pre-installed, and another that is valid when it isn't installed.