WIP

.github/workflows/test.yaml

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+name: "Test"
+
+on:
+  push:
+      branches:
+        - "main"
+
+jobs:
+  test-linux:
+    name: "Test on ${{ matrix.config.os-name }}"
+    runs-on: "${{ matrix.config.runner }}"
+    strategy:
+      matrix:
+        config:
+          - os-name: "Linux"
+            runner: "ubuntu-latest"
+            test-label: "ci-test-linux"
+          - os-name: "macOS"
+            runner: "macos-latest"
+            test-label: "ci-test-macos"
+      fail-fast: false
+
+    steps:
+      - name: "Checkout"
+        uses: "actions/checkout@v4"
+
+      - name: "Linux / macOS"
+        id: linux
+        if: "runner.os != 'windows'"
+        shell: "bash"
+        run: |
+          env | sort
+          bash ./finder.sh false
+          bash ./finder.sh true
+
+  test-windows:
+    name: "Test on ${{ matrix.config.os-name }}"
+    runs-on: "${{ matrix.config.runner }}"
+    strategy:
+      matrix:
+        config:
+          - os-name: "Windows"
+            runner: "windows-latest"
+            test-label: "ci-test-windows"
+
+    steps:
+      - name: "Checkout"
+        uses: "actions/checkout@v4"
+
+      - name: "Windows"
+        shell: "powershell"
+        run: |
+          dir env:
+          & .\finder.ps1 false
+          & .\finder.ps1 true

action.yml

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+name: "Identify installed Python interpreters"
+description: |
+  Find installed Python interpreters and output them as "python-paths".
+  This can be useful for cache busting of tox, virtual environments,
+  and other items that are sensitive to changes in Python minor versions.
+
+  Only Python interpreters installed in $RUNNER_TOOL_CACHE will be found.
+
+inputs:
+  search-runner-tool-cache:
+    description: |
+      By default, only paths in the PATH environment variable will be searched
+      because the "actions/setup-python" action typically modifies the PATH
+      after installing various Python versions.
+      
+      Because it is possible to prevent "actions/setup-python" from modifying the PATH,
+      and because some GitHub runner images have multiple Python versions pre-installed,
+      it may be desirable to find every Python installed in $RUNNER_TOOL_CACHE,
+      even if their paths don't appear in the PATH environment variable.
+
+      If this input is specified, its value must be the string value "true".
+      Use quotation marks to ensure the correct value is passed.
+    required: false
+    default: "false"
+
+outputs:
+  python-paths:
+    description: |
+      A string of sorted Python executable paths.
+
+      The string will be separated by OS-specific PATH separators;
+      ":" is used on Linux and macOS, and ";" is used on Windows.
+    value: ${{ steps.final-step.outputs.python-path }}
+
+runs:
+  using: "composite"
+  steps:
+    - name: "Find Pythons on Linux / macOS"
+      id: "linux"
+      if: "runner.os != 'windows'"
+      shell: "bash"
+      run: 'bash ./finder.sh "${{ inputs.search-runner-tool-cache }}" > "$GITHUB_OUTPUT"'
+
+    - name: "Find Pythons on Windows"
+      id: "windows"
+      if: "runner.os == 'windows'"
+      shell: "powershell"
+      run: '& .\finder.ps1 "${{ inputs.search-runner-tool-cache }}" > "$env:GITHUB_OUTPUT"'
+
+    - name: "Output"
+      id: "final-step"
+      shell: "bash"
+      run: |
+        echo "${{ steps.linux.outputs.python-paths }}${{ steps.windows.outputs.python-paths }}" > $GITHUB_OUTPUT

finder.ps1

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+$search_runner_tool_cache = $args[0]
+
+if ($search_runner_tool_cache -eq "true") {
+    # Search paths in $RUNNER_TOOL_CACHE for Python interpreters.
+    $all_paths = `
+        Get-ChildItem `
+            -Path `
+                "$env:RUNNER_TOOL_CACHE\Python\*\*", `
+                "$env:RUNNER_TOOL_CACHE\PyPy\*\*" `
+            -Filter "x??" `
+            -Directory `
+        | Select-Object -ExpandProperty "FullName"
+} else {
+    # Search paths in $PATH for Python interpreters.
+    $all_paths = $env:PATH -split ";"
+}
+
+# Find Python interpreters.
+$paths = @()
+foreach ($path in $all_paths) {
+    # Only consider interpreters in $RUNNER_TOOL_CACHE.
+    if ($path.StartsWith($env:RUNNER_TOOL_CACHE)) {
+        if (Test-Path "$path\python.exe") {
+            $paths += $path
+        }
+    }
+}
+
+# Sort the paths, ensure each path is unique,
+# and create the output result.
+$result = (
+    $paths `
+    | Sort-Object `
+    | Get-Unique
+) -join ";"
+
+# Output path information.
+Write-Output "python-paths=$result"

finder.sh

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+search_runner_tool_cache="$1"
+
+if [ "$search_runner_tool_cache" = "true" ]; then
+    # Search paths in $RUNNER_TOOL_CACHE for Python interpreters.
+    IFS=: read -r -a all_paths <<< "$(
+        find \
+            "$RUNNER_TOOL_CACHE/Python" \
+            "$RUNNER_TOOL_CACHE/PyPy" \
+            -mindepth 2 \
+            -maxdepth 2 \
+            -name 'x86' \
+            -o \
+            -name 'x64' \
+        | tr '\n' ':'
+    )"
+else
+    # Search paths in $PATH for Python interpreters.
+    IFS=: read -r -a all_paths <<< "$PATH"
+fi
+
+# Find Python interpreters.
+paths=()
+for path in "${all_paths[@]}"; do
+    # Only consider interpreters in $RUNNER_TOOL_CACHE.
+    if [[ "${path/#${RUNNER_TOOL_CACHE}/}" != "${path}" ]]; then
+        # Check for bin/python first;
+        # this results in duplicate paths which are later removed.
+        if [[ -x "${path}/bin/python" ]]; then
+            paths+=("${path}/bin")
+        elif [[ -x "${path}/python" ]]; then
+            paths+=("${path}")
+        fi
+    fi
+done
+
+# If no paths were found, look for a "python" symlink.
+if [[ "${#paths[@]}" -eq 0 ]]; then
+    if command -v python &> /dev/null; then
+        path="$(realpath "$(command -v python)")"
+        echo "${path}" 1>&2
+        if [[ "${path/#${RUNNER_TOOL_CACHE}}" != "${path}" ]]; then
+            paths+=("${path%/python}")
+        fi
+    fi
+fi
+
+# Sort the paths, ensure each path is unique
+# and create the output result.
+result="$(
+    echo "${paths[*]}" \
+    | tr ' ' '\n' \
+    | sort \
+    | uniq \
+    | tr '\n' ':'
+)"
+
+# Trim trailing colons.
+result="${result%:}"
+
+# Output path information.
+echo "python-paths=${result}"