Auto activate venv when python-version is set

.github/workflows/test.yml

@@ -155,5 +155,11 @@ jobs:
           if [ "$UV_PYTHON" != "${{ matrix.python-version }}" ]; then
             exit 1
           fi
-      - run: uv sync
-        working-directory: __tests__/fixtures/uv-project
+      - name: Verify packages can be installed
+        run: uv pip install --python=${{ matrix.python-version }} pip
+      - name: Verify python version is correct
+        run: |
+          python --version
+          if [ "$(python --version)" != "${{ matrix.python-version }}" ]; then
+            exit 1
+          fi

README.md

@@ -73,14 +73,16 @@ to install the latest version that satisfies the range.
 ### Python version
 
 You can use the input `python-version` to set the environment variable `UV_PYTHON` for the rest
-of your workflow.
+of your workflow and automatically create a new virtual environment with the specified python version.
+
 This will override any python version specifications in `pyproject.toml` and `.python-version`
 
 ```yaml
-- name: Install the latest version of uv and set the python version to 3.12
+- name: Install the latest version of uv and set the python version to ${{ matrix.python-version }}
   uses: astral-sh/setup-uv@v4
   with:
-    python-version: "3.12"
+    python-version: ${{ matrix.python-version }}
+- run: uv pip install --python=${{ matrix.python-version }} pip
 ```
 
 You can combine this with a matrix to test multiple python versions:

action.yml

@@ -46,6 +46,9 @@ inputs:
   tool-bin-dir:
     description: "Custom path to set UV_TOOL_BIN_DIR to."
     required: false
+  python-install-dir:
+    description: "Custom path where uv will install python interpreters."
+    required: false
 outputs:
   uv-version:
     description: "The installed uv version. Useful when using latest."

src/setup-uv.ts

@@ -18,11 +18,13 @@ import {
   checkSum,
   enableCache,
   githubToken,
+  pythonInstallDir,
   pythonVersion,
   toolBinDir,
   toolDir,
   version,
 } from "./utils/inputs";
+import * as exec from "@actions/exec";
 
 async function run(): Promise<void> {
   const platform = getPlatform();
@@ -46,7 +48,8 @@ async function run(): Promise<void> {
     addUvToPath(setupResult.uvDir);
     addToolBinToPath();
     setToolDir();
-    setupPython();
+    addUvPythonInstallDirToPath();
+    await setupPython();
     addMatchers();
     setCacheDir(cacheLocalPath);
 
@@ -98,6 +101,13 @@ function addUvToPath(cachedPath: string): void {
   core.info(`Added ${cachedPath} to the path`);
 }
 
+function addUvPythonInstallDirToPath(): void {
+  core.exportVariable("UV_PYTHON_INSTALL_DIR", toolBinDir);
+  core.info(`Set UV_PYTHON_INSTALL_DIR to ${toolBinDir}`);
+  core.addPath(pythonInstallDir);
+  core.info(`Added ${pythonInstallDir} to the path`);
+}
+
 function addToolBinToPath(): void {
   if (toolBinDir !== undefined) {
     core.exportVariable("UV_TOOL_BIN_DIR", toolBinDir);
@@ -125,10 +135,17 @@ function setToolDir(): void {
   }
 }
 
-function setupPython(): void {
+async function setupPython(): Promise<void> {
   if (pythonVersion !== "") {
     core.exportVariable("UV_PYTHON", pythonVersion);
     core.info(`Set UV_PYTHON to ${pythonVersion}`);
+    const options: exec.ExecOptions = {
+      silent: !core.isDebug(),
+    };
+    const execArgs = ["venv", "--python", pythonVersion];
+
+    core.info("Activating python venv...");
+    await exec.exec("uv", execArgs, options);
   }
 }
 

src/utils/inputs.ts

@@ -13,6 +13,7 @@ export const ignoreNothingToCache =
   core.getInput("ignore-nothing-to-cache") === "true";
 export const toolBinDir = getToolBinDir();
 export const toolDir = getToolDir();
+export const pythonInstallDir = getPythonInstallDir();
 export const githubToken = core.getInput("github-token");
 
 function getEnableCache(): boolean {
@@ -55,6 +56,19 @@ function getToolDir(): string | undefined {
   return undefined;
 }
 
+function getPythonInstallDir(): string {
+  const pythonInstallDirInput = core.getInput("python-install-dir");
+  if (pythonInstallDirInput !== "") {
+    return expandTilde(pythonInstallDirInput);
+  }
+  if (process.env.RUNNER_TEMP !== undefined) {
+    return `${process.env.RUNNER_TEMP}${path.sep}uv-python-install-dir`;
+  }
+  throw Error(
+    "Could not determine UV_PYTHON_INSTALL_DIR. Please make sure RUNNER_TEMP is set or provide the python-install-dir input",
+  );
+}
+
 function getCacheLocalPath(): string {
   const cacheLocalPathInput = core.getInput("cache-local-path");
   if (cacheLocalPathInput !== "") {