setup-python not caching pip dependencies

Question

Why is GitHub Action's setup-python action's cache still making pip install all the dependencies?

I have this simple workflow

name: Python Testing

on:
  push:
    branches: [ master ]
  pull_request:
    branches: [ master ]

permissions:
  contents: read

jobs:
  coverage_check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-python@v4
        with:
          python-version: "3.6"
          cache: 'pip' # caching pip dependencies
      - run: pip install -r requirements.txt --use-deprecated=legacy-resolver

From the logs, it's a cache hit:

Successfully set up CPython (3.6.15)
/opt/hostedtoolcache/Python/3.6.15/x64/bin/pip cache dir
/home/runner/.cache/pip
Received 123461741 of 123461741 (100.0%), 111.2 MBs/sec
Cache Size: ~118 MB (123461741 B)
/usr/bin/tar --use-compress-program unzstd -xf /home/runner/work/_temp/a5d8f986-ce6b-424f-8de9-c082916cdb2f/cache.tzst -P -C /home/runner/work/backend/backend
Cache restored successfully
Cache restored from key: setup-python-Linux-20.04-Ubuntu-python-3.6.15-pip-d0b220da2d89ebae7a0978e97e9d1ce87061e5e2e0f5d3d242c2770b7f983de6

Yet the Python dependencies are installed

Collecting Flask-Cors==3.0.9
  Using cached Flask_Cors-3.0.9-py2.py3-none-any.whl (14 kB)
Collecting Flask-GraphQL==2.0.1
....

I expected dependency to be already present and return something like this

Requirement already satisfied: Flask-Cors==3.0.9 in 
....

Answer 1

I use this to cache the dependencies, before this step you checkout the repo, load the cache deps, and then install or carry onwards with your actions.

      - name: Load Cached Virtualenv
        id: cached-pip-wheels
        uses: actions/cache@v3
        with:
          path: ~/.cache
          key: venv-${{ runner.os }}-${{ hashFiles('**/poetry.lock') }}

Keep in mind that this is using poetry.lock, you would need to replace the file with your requirements.txt