@@ -505,3 +505,30 @@ jobs:
           }
           $pythonVersion
         shell: pwsh
+
+  setup-python-version-required:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          [
+            macos-latest,
+            windows-latest,
+            ubuntu-22.04,
+            ubuntu-22.04-arm,
+            ubuntu-24.04-arm,
+            ubuntu-latest,
+            macos-13
+          ]
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup Python
+        id: setup-python
+        uses: ./
+        env:
+          PYTHON_VERSION_REQUIRED: true
+        continue-on-error: true
+      - name: Validate failure
+        if: steps.setup-python.outcome != 'failure'
+        run: exit 1
@@ -97729,7 +97729,14 @@ function run() {
                 }
             }
             else {
-                core.warning('The `python-version` input is not set.  The version of Python currently in `PATH` will be used.');
+                const trueValue = ['true', 'True', 'TRUE'];
+                const pythonVersionRequired = process.env['PYTHON_VERSION_REQUIRED'] || '';
+                if (!trueValue.includes(pythonVersionRequired)) {
+                    core.warning('The `python-version` input is not set.  The version of Python currently in `PATH` will be used.');
+                }
+                else {
+                    throw new Error('The `python-version` input is required.');
+                }
             }
             const matchersPath = path.join(__dirname, '../..', '.github');
             core.info(`##[add-matcher]${path.join(matchersPath, 'python.json')}`);

@@ -138,6 +138,16 @@ steps:
 ```
 Please refer to the [Advanced range syntax section](https://github.com/npm/node-semver#advanced-range-syntax) of the [semver](https://github.com/npm/node-semver) to check other available range syntaxes.
 
+If you want to make an error when unspecified, set `PYTHON_VERSION_REQUIRED: true` to env.
+```yaml
+steps:
+- uses: actions/checkout@v4
+- uses: actions/setup-python@v5
+  env:
+    PYTHON_VERSION_REQUIRED: true
+  # the action fail with error.
+```
+
 ### Specifying a PyPy version
 The version of PyPy should be specified in the format `pypy<python_version>[-v<pypy_version>]` or `pypy-<python_version>[-v<pypy_version>]`.
 The `-v<pypy_version>` parameter is optional and can be skipped. The latest PyPy version will be used in this case.

@@ -146,9 +146,17 @@ async function run() {
         await cacheDependencies(cache, pythonVersion);
       }
     } else {
-      core.warning(
-        'The `python-version` input is not set.  The version of Python currently in `PATH` will be used.'
-      );
+      const trueValue = ['true', 'True', 'TRUE'];
+      const pythonVersionRequired = process.env['PYTHON_VERSION_REQUIRED'] || '';
+      if(!trueValue.includes(pythonVersionRequired)) {
+        core.warning(
+          'The `python-version` input is not set.  The version of Python currently in `PATH` will be used.'
+        );
+      } else {
+        throw new Error(
+          'The `python-version` input is required.'
+        );
+      }
     }
     const matchersPath = path.join(__dirname, '../..', '.github');
     core.info(`##[add-matcher]${path.join(matchersPath, 'python.json')}`);