Issue caching python dependencies in GitHub Actions

Question

I have the following steps in a github action:

steps:
      - name: Check out repository code
        uses: actions/checkout@v2

      - name: Cache dependencies
        id: pip-cache
        uses: actions/cache@v2
        with:
          path: ~.cache/pip
          key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
          restore-keys: |
            ${{ runner.os }}-pip-

      - name: Install dependencies
        if: steps.pip-cache.outputs.cache-hit != 'true'
        run: pip install -r requirements.txt
     
      - name: run mypy
        run: mypy .

The caching works fine, but when a cache hit occurs, and I try to run mypy, it fails with:

Run mypy .
/home/runner/work/_temp/9887df5b-d5cc-46d7-90e1-b884d8c49272.sh: line 1: mypy: command not found
Error: Process completed with exit code 127.

The whole point of caching dependencies is so I don't have to install them every time I run the workflow. How do I use the cached dependencies?

Answer 1

You're only caching source tarballs and binary wheels downloaded by pip . You're not caching:

• Installed Python packages (ie, the site-packages/ subdirectory of the active Python interpreter).
• Installed entry points (ie, executable commands residing in the current ${PATH} ).

That isn't necessarily a bad thing. Merely downloading assets tends to consume a disproportionate share of scarce GitHub Actions (GA) minutes; caching assets trivially alleviates that issue.

In other words, remove the if: steps.pip-cache.outputs.cache-hit != 'true' line to restore your GitHub Actions (GA) workflow to sanity.

But... I Want to Cache Installed Packages!

Challenge accepted. This is feasible – albeit more fragile. I'd advise just caching pip downloads unless you've profiled the pip install command to be a significant installation bottleneck.

Let's assume that you still want to do this. In this case, something resembling the following snippet should get you where you want to go:

  - uses: actions/cache@v2
    id: cache
    with:
      # CAUTION: Replace this hardcoded "python3.7" dirname with
      # the dirname providing your desired Python interpreter.
      path: ${{ env.pythonLocation}}/lib/python3.7/site-packages/*
      key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
      restore-keys: |
        ${{ runner.os }}-pip-
        ${{ runner.os }}-

As noted, you'll want to manually replace the hardcoded python3.7 substring above with something specific to your use case. In theory, adding the above directly under your existing uses: actions/cache@v2 list item should suffice.