How do I cache an installed tool between runs in GitHub Actions?

Question

I need to run poetry version to get the pyproject.toml version on every push to master touching pyproject.toml. However, since Poetry is not installed on GitHub Actions runner virtual environments, I also need to install it before I can run any Poetry commands. I want to cache this tool installation so I don't have to install it on every run and use up Actions minutes. The only Poetry command I need to run is poetry version, so I'm not worried about the tool being outdated - it only needs to parse the pyproject.toml and get the project version number. I'm also not sure what to use as my key for the caching action - I assume it can be static

The desired order of operations would be something like:

1. Check out repo.
2. Check cache for Poetry. If it's not installed, install it.
3. Run poetry version.

Answer 1

The key input of actions/cache@v2 can be a string - provide it something arbitrary. The path input is the location of the tool.

Potential pitfalls:

• Note that the path argument does NOT resolve environment variables like $HOME, but the tilde (~) can be used to signify the home directory.
• Poetry has to be prepended to the PATH on every run, as default environment variables are not preserved between runs.
• Poetry may complain that it's dropping support for Python2 soon - to ensure it's running with Python 3, make sure to setup the run with any of the Python 3 versions.

on:
  push:
    branches:
      - master
    paths:
      - 'pyproject.toml'

jobs:
  pyproject-version:
    runs-on: 'ubuntu-latest'
    steps:
      - name: Checkout code
        uses: actions/checkout@v2
      - name: Setup Python
        uses: actions/setup-python@v2
        with:
          python-version: '3.7'
      # Perma-cache Poetry since we only need it for checking pyproject version
      - name: Cache Poetry
        id: cache-poetry
        uses: actions/cache@v2
        with:
          path: ~/.poetry
          key: poetry
      # Only runs when key from caching step changes
      - name: Install latest version of Poetry
        if: steps.cache-poetry.outputs.cache-hit != 'true'
        run: |
          curl -sSL https://install.python-poetry.org | python -
      # Poetry still needs to be re-prepended to the PATH on each run, since
      # PATH does not persist between runs.
      - name: Add Poetry to $PATH
        run: |
          echo "$HOME/.poetry/bin" >> $GITHUB_PATH
      - name: Get pyproject version
        run: poetry version