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"
+          - os-name: "Windows"
+            runner: "windows-latest"
+            test-label: "ci-test-windows"
+      fail-fast: false
+
+    steps:
+      - name: "Use it!"
+        id: "finder"
+        uses: "kurtmckee/setup-python-version-detector@main"
+
+      - name: "Print it!"
+        shell: "bash"
+        run: |
+          echo '${{ steps.finder.outputs.python-identifiers }}'

README.rst

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+Identify Python interpreters
+############################
+
+*Robust cache-busting based on Python implementations, architectures, and versions.*
+
+----
+

action.yml

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+name: "Identify installed Python interpreters"
+description: |
+  Find installed Python interpreters and output them as "python-identifiers".
+  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 named "python" in the $PATH will be found.
+
+outputs:
+  python-identifiers:
+    description: |
+      A string of sorted Python identifiers.
+      
+      In most cases the identifiers will be paths,
+      but for system Pythons, 'sysconfig.get_config_var("EXT_SUFFIX")' will be included.
+
+      The string will be separated by OS-specific PATH separators;
+      ":" is used on Linux and macOS, and ";" is used on Windows.
+    value: "${{ steps.final-step.outputs.python-identifiers }}"
+
+runs:
+  using: "composite"
+  steps:
+    - name: "Find Pythons on Linux / macOS"
+      id: "linux"
+      if: "runner.os != 'windows'"
+      shell: "bash"
+      run: |
+        true > /dev/null  # Prevent YAML syntax errors when embedded in 'action.yml'.
+
+        code=$(
+        cat <<'END_CODE'
+        from __future__ import print_function
+        
+        import sysconfig
+        
+        ext_suffix = sysconfig.get_config_var("EXT_SUFFIX")
+        if ext_suffix is not None:
+            print(ext_suffix)
+        else:
+            # Python 2.7 on macOS
+            import platform
+            print(
+                "." + platform.python_implementation().lower(),
+                sysconfig.get_config_var("py_version_nodot"),
+                sysconfig.get_config_var("build"),
+                sep="-"
+            )
+        END_CODE
+        )
+        
+        # Search paths in $PATH for Python interpreters.
+        IFS=: read -r -a all_paths <<< "$PATH"
+        
+        # Find Python interpreters.
+        paths=()
+        for path in "${all_paths[@]}"; do
+            # Interpreters in RUNNER_TOOL_CACHE have directory names that include:
+            #
+            # * The implementation (like "Python" or "PyPy")
+            # * A version (like "3.10.12")
+            # * The architecture (like "x64")
+            #
+            # In such cases, the path can be used as the identifier.
+            if [[ "${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
+            else
+                # System Pythons (e.g. /usr/bin/python) have nothing unique in the path.
+                # In such cases, it's necessary to run the executable to get something unique.
+                if [[ -x "${path}/python" ]]; then
+                  paths+=("${path}" "$(echo "${code}" | "${path}/python" -)")
+                fi
+            fi
+        done
+        
+        # Sort the paths, ensure each path is unique, and create the output result.
+        result="$(
+            echo "${paths[*]}" \
+            | tr ' ' '\n' \
+            | sort \
+            | uniq \
+            | tr '\n' ':'
+        )"
+        
+        # Trim trailing colons.
+        result="${result%:}"
+        
+        # Output path information.
+        echo "python-identifiers=${result}" > "$GITHUB_OUTPUT"
+
+    - name: "Find Pythons on Windows"
+      id: "windows"
+      if: "runner.os == 'windows'"
+      shell: "powershell"
+      run: |
+        $true | Out-Null  # Prevent YAML syntax errors when embedded in 'action.yml'.
+
+        # Search paths in $PATH for Python interpreters.
+        $all_paths = $env:PATH -split ";"
+        
+        # Find Python interpreters.
+        $paths = @()
+        foreach ($path in $all_paths) {
+            # Only consider paths in RUNNER_TOOL_CACHE.
+            if ($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 = (
+            $paths `
+            | Sort-Object `
+            | Get-Unique
+        ) -join ";"
+        
+        # Output path information.
+        Write-Output "python-identifiers=$result" > "$env:GITHUB_OUTPUT"
+
+    - name: "Output"
+      id: "final-step"
+      shell: "bash"
+      run: |
+        echo 'python-identifiers=${{ steps.linux.outputs.python-identifiers }}${{ steps.windows.outputs.python-identifiers }}' > "$GITHUB_OUTPUT"

finder.ps1

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+$true | Out-Null  # Prevent YAML syntax errors when embedded in 'action.yml'.
+
+# Search paths in $PATH for Python interpreters.
+$all_paths = $env:PATH -split ";"
+
+# Find Python interpreters.
+$paths = @()
+foreach ($path in $all_paths) {
+    # Only consider paths in RUNNER_TOOL_CACHE.
+    if ($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 = (
+    $paths `
+    | Sort-Object `
+    | Get-Unique
+) -join ";"
+
+# Output path information.
+Write-Output "python-identifiers=$result"

finder.sh

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+true > /dev/null  # Prevent YAML syntax errors when embedded in 'action.yml'.
+
+code=$(
+cat <<'END_CODE'
+from __future__ import print_function
+
+import sysconfig
+
+ext_suffix = sysconfig.get_config_var("EXT_SUFFIX")
+if ext_suffix is not None:
+    print(ext_suffix)
+else:
+    # Python 2.7 on macOS
+    import platform
+    print(
+        "." + platform.python_implementation().lower(),
+        sysconfig.get_config_var("py_version_nodot"),
+        sysconfig.get_config_var("build"),
+        sep="-"
+    )
+END_CODE
+)
+
+# Search paths in $PATH for Python interpreters.
+IFS=: read -r -a all_paths <<< "$PATH"
+
+# Find Python interpreters.
+paths=()
+for path in "${all_paths[@]}"; do
+    # Interpreters in RUNNER_TOOL_CACHE have directory names that include:
+    #
+    # * The implementation (like "Python" or "PyPy")
+    # * A version (like "3.10.12")
+    # * The architecture (like "x64")
+    #
+    # In such cases, the path can be used as the identifier.
+    if [[ "${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
+    else
+        # System Pythons (e.g. /usr/bin/python) have nothing unique in the path.
+        # In such cases, it's necessary to run the executable to get something unique.
+        if [[ -x "${path}/python" ]]; then
+            paths+=("${path}" "$(echo "${code}" | "${path}/python" -)")
+        fi
+    fi
+done
+
+# Sort the paths, ensure each path is unique, and create the output result.
+result="$(
+    echo "${paths[*]}" \
+    | tr ' ' '\n' \
+    | sort \
+    | uniq \
+    | tr '\n' ':'
+)"
+
+# Trim trailing colons.
+result="${result%:}"
+
+# Output path information.
+echo "python-identifiers=${result}"

identify.py

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+from __future__ import print_function
+
+import platform
+import sys
+
+
+def main():
+    python_version = platform.python_version()
+    implementation = platform.python_implementation()
+    architecture = "/".join(platform.architecture())
+
+    # PyPy
+    if hasattr(sys, "pypy_version_info"):
+        implementation_version = ".".join(str(v) for v in sys.pypy_version_info[:3])
+
+    # CPython
+    else:
+        implementation_version, _, _ = sys.version.partition(" ")
+
+    # GraalPy
+    try:
+        implementation_version = __graalpython__.get_graalvm_version()
+    except NameError:
+        pass
+
+    output = "{0} [{1} {2}; {3}]".format(
+        python_version, implementation, implementation_version, architecture
+    )
+    print(output)
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())