Add support for embedded Python on Windows (#3161)

## Summary

References:
-
cad550030a/src/virtualenv/create/via_global_ref/builtin/cpython/cpython3.py (L58-L68)
- https://github.com/pypa/virtualenv/pull/2353
- https://github.com/pypa/virtualenv/issues/2368

Closes https://github.com/astral-sh/uv/issues/1656.

.github/workflows/ci.yml

@@ -814,3 +814,37 @@ jobs:
 
       - name: "Validate global Python install"
         run: python3 scripts/check_system_python.py --uv ./uv
+
+  system-test-windows-embedded-python-310:
+    needs: build-binary-windows
+    name: "check system | embedded python3.10 on windows"
+    runs-on: windows-latest
+    env:
+      # Avoid debug build stack overflows.
+      UV_STACK_SIZE: 2000000 # 2 megabyte, double the default on windows
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: "Download binary"
+        uses: actions/download-artifact@v4
+        with:
+          name: uv-windows-${{ github.sha }}
+
+      # Download embedded Python.
+      - name: "Download embedded Python"
+        run: curl -LsSf https://www.python.org/ftp/python/3.11.8/python-3.11.8-embed-amd64.zip -o python-3.11.8-embed-amd64.zip
+
+      - name: "Unzip embedded Python"
+        run: 7z x python-3.11.8-embed-amd64.zip -oembedded-python
+
+      - name: "Show embedded Python contents"
+        run: ls embedded-python
+
+      - name: "Set PATH"
+        run: echo "${{ github.workspace }}\embedded-python" >> $env:GITHUB_PATH
+
+      - name: "Print Python path"
+        run: echo $(which python)
+
+      - name: "Validate embedded Python install"
+        run: python ./scripts/check_embedded_python.py --uv ./uv.exe

crates/uv-virtualenv/src/bare.rs

@@ -139,6 +139,14 @@ pub fn create_bare_venv(
     // Create a `.gitignore` file to ignore all files in the venv.
     fs::write(location.join(".gitignore"), "*")?;
 
+    // Per PEP 405, the Python `home` is the parent directory of the interpreter.
+    let python_home = base_python.parent().ok_or_else(|| {
+        io::Error::new(
+            io::ErrorKind::NotFound,
+            "The Python interpreter needs to have a parent directory",
+        )
+    })?;
+
     // Different names for the python interpreter
     fs::create_dir(&scripts)?;
     let executable = scripts.join(format!("python{EXE_SUFFIX}"));
@@ -163,55 +171,23 @@ pub fn create_bare_venv(
     }
 
     // No symlinking on Windows, at least not on a regular non-dev non-admin Windows install.
-    #[cfg(windows)]
+    if cfg!(windows) {
-    {
+        copy_launcher_windows(
-        // https://github.com/python/cpython/blob/d457345bbc6414db0443819290b04a9a4333313d/Lib/venv/__init__.py#L261-L267
+            WindowsExecutable::Python,
-        // https://github.com/pypa/virtualenv/blob/d9fdf48d69f0d0ca56140cf0381edbb5d6fe09f5/src/virtualenv/create/via_global_ref/builtin/cpython/cpython3.py#L78-L83
+            interpreter,
-        // There's two kinds of applications on windows: Those that allocate a console (python.exe) and those that
+            &base_python,
-        // don't because they use window(s) (pythonw.exe).
+            &scripts,
-        for python_exe in ["python.exe", "pythonw.exe"] {
+            python_home,
-            let shim = interpreter
+        )?;
-                .stdlib()
+        copy_launcher_windows(
-                .join("venv")
+            WindowsExecutable::Pythonw,
-                .join("scripts")
+            interpreter,
-                .join("nt")
+            &base_python,
-                .join(python_exe);
+            &scripts,
-            match fs_err::copy(shim, scripts.join(python_exe)) {
+            python_home,
-                Ok(_) => {}
+        )?;
-                Err(err) if err.kind() == io::ErrorKind::NotFound => {
-                    let launcher = match python_exe {
-                        "python.exe" => "venvlauncher.exe",
-                        "pythonw.exe" => "venvwlauncher.exe",
-                        _ => unreachable!(),
-                    };
-
-                    // If `python.exe` doesn't exist, try the `venvlauncher.exe` shim.
-                    let shim = interpreter
-                        .stdlib()
-                        .join("venv")
-                        .join("scripts")
-                        .join("nt")
-                        .join(launcher);
-
-                    // If the `venvlauncher.exe` shim doesn't exist, then on Conda at least, we
-                    // can look for it next to the Python executable itself.
-                    match fs_err::copy(shim, scripts.join(python_exe)) {
-                        Ok(_) => {}
-                        Err(err) if err.kind() == io::ErrorKind::NotFound => {
-                            let shim = base_python.with_file_name(launcher);
-                            fs_err::copy(shim, scripts.join(python_exe))?;
-                        }
-                        Err(err) => {
-                            return Err(err.into());
-                        }
-                    }
-                }
-                Err(err) => {
-                    return Err(err.into());
-                }
-            }
-        }
     }
+
     #[cfg(not(any(unix, windows)))]
     {
         compile_error!("Only Windows and Unix are supported")
@@ -242,18 +218,6 @@ pub fn create_bare_venv(
         fs::write(scripts.join(name), activator)?;
     }
 
-    // Per PEP 405, the Python `home` is the parent directory of the interpreter.
-    let python_home = base_python
-        .parent()
-        .ok_or_else(|| {
-            io::Error::new(
-                io::ErrorKind::NotFound,
-                "The Python interpreter needs to have a parent directory",
-            )
-        })?
-        .simplified_display()
-        .to_string();
-
     // Validate extra_cfg
     let reserved_keys = [
         "home",
@@ -272,7 +236,10 @@ pub fn create_bare_venv(
     }
 
     let mut pyvenv_cfg_data: Vec<(String, String)> = vec![
-        ("home".to_string(), python_home),
+        (
+            "home".to_string(),
+            python_home.simplified_display().to_string(),
+        ),
         (
             "implementation".to_string(),
             interpreter.markers().platform_python_implementation.clone(),
@@ -322,3 +289,153 @@ pub fn create_bare_venv(
         executable,
     })
 }
+
+#[derive(Debug, Copy, Clone)]
+enum WindowsExecutable {
+    /// The `python.exe` executable (or `venvlauncher.exe` launcher shim).
+    Python,
+    /// The `pythonw.exe` executable (or `venvwlauncher.exe` launcher shim).
+    Pythonw,
+}
+
+impl WindowsExecutable {
+    /// The name of the Python executable.
+    fn exe(self) -> &'static str {
+        match self {
+            WindowsExecutable::Python => "python.exe",
+            WindowsExecutable::Pythonw => "pythonw.exe",
+        }
+    }
+
+    /// The name of the launcher shim.
+    fn launcher(self) -> &'static str {
+        match self {
+            WindowsExecutable::Python => "venvlauncher.exe",
+            WindowsExecutable::Pythonw => "venvwlauncher.exe",
+        }
+    }
+}
+
+/// <https://github.com/python/cpython/blob/d457345bbc6414db0443819290b04a9a4333313d/Lib/venv/__init__.py#L261-L267>
+/// <https://github.com/pypa/virtualenv/blob/d9fdf48d69f0d0ca56140cf0381edbb5d6fe09f5/src/virtualenv/create/via_global_ref/builtin/cpython/cpython3.py#L78-L83>
+///
+/// There's two kinds of applications on windows: Those that allocate a console (python.exe)
+/// and those that don't because they use window(s) (pythonw.exe).
+fn copy_launcher_windows(
+    executable: WindowsExecutable,
+    interpreter: &Interpreter,
+    base_python: &Path,
+    scripts: &Path,
+    python_home: &Path,
+) -> Result<(), Error> {
+    // First priority: the `python.exe` and `pythonw.exe` shims.
+    let shim = interpreter
+        .stdlib()
+        .join("venv")
+        .join("scripts")
+        .join("nt")
+        .join(executable.exe());
+    match fs_err::copy(shim, scripts.join(executable.exe())) {
+        Ok(_) => return Ok(()),
+        Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+        Err(err) => {
+            return Err(err.into());
+        }
+    }
+
+    // Second priority: the `venvlauncher.exe` and `venvwlauncher.exe` shims.
+    // These are equivalent to the `python.exe` and `pythonw.exe` shims, which were
+    // renamed in Python 3.13.
+    let shim = interpreter
+        .stdlib()
+        .join("venv")
+        .join("scripts")
+        .join("nt")
+        .join(executable.launcher());
+    match fs_err::copy(shim, scripts.join(executable.exe())) {
+        Ok(_) => return Ok(()),
+        Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+        Err(err) => {
+            return Err(err.into());
+        }
+    }
+
+    // Third priority: on Conda at least, we can look for the launcher shim next to
+    // the Python executable itself.
+    let shim = base_python.with_file_name(executable.launcher());
+    match fs_err::copy(shim, scripts.join(executable.exe())) {
+        Ok(_) => return Ok(()),
+        Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+        Err(err) => {
+            return Err(err.into());
+        }
+    }
+
+    // Fourth priority: if the launcher shim doesn't exist, assume this is
+    // an embedded Python. Copy the Python executable itself, along with
+    // the DLLs, `.pyd` files, and `.zip` files in the same directory.
+    match fs_err::copy(
+        base_python.with_file_name(executable.exe()),
+        scripts.join(executable.exe()),
+    ) {
+        Ok(_) => {
+            // Copy `.dll` and `.pyd` files from the top-level, and from the
+            // `DLLs` subdirectory (if it exists).
+            for directory in [
+                python_home,
+                interpreter.base_prefix().join("DLLs").as_path(),
+            ] {
+                let entries = match fs_err::read_dir(directory) {
+                    Ok(read_dir) => read_dir,
+                    Err(err) if err.kind() == io::ErrorKind::NotFound => {
+                        continue;
+                    }
+                    Err(err) => {
+                        return Err(err.into());
+                    }
+                };
+                for entry in entries {
+                    let entry = entry?;
+                    let path = entry.path();
+                    if path.extension().is_some_and(|ext| {
+                        ext.eq_ignore_ascii_case("dll") || ext.eq_ignore_ascii_case("pyd")
+                    }) {
+                        if let Some(file_name) = path.file_name() {
+                            fs_err::copy(&path, scripts.join(file_name))?;
+                        }
+                    }
+                }
+            }
+
+            // Copy `.zip` files from the top-level.
+            match fs_err::read_dir(python_home) {
+                Ok(entries) => {
+                    for entry in entries {
+                        let entry = entry?;
+                        let path = entry.path();
+                        if path
+                            .extension()
+                            .is_some_and(|ext| ext.eq_ignore_ascii_case("zip"))
+                        {
+                            if let Some(file_name) = path.file_name() {
+                                fs_err::copy(&path, scripts.join(file_name))?;
+                            }
+                        }
+                    }
+                }
+                Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+                Err(err) => {
+                    return Err(err.into());
+                }
+            };
+
+            return Ok(());
+        }
+        Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+        Err(err) => {
+            return Err(err.into());
+        }
+    }
+
+    Err(Error::NotFound(base_python.user_display().to_string()))
+}

crates/uv-virtualenv/src/lib.rs

@@ -1,9 +1,9 @@
 use std::io;
 use std::path::Path;
 
-use platform_tags::PlatformError;
 use thiserror::Error;
 
+use platform_tags::PlatformError;
 use uv_interpreter::{Interpreter, PythonEnvironment};
 
 pub use crate::bare::create_bare_venv;
@@ -20,6 +20,8 @@ pub enum Error {
     Platform(#[from] PlatformError),
     #[error("Reserved key used for pyvenv.cfg: {0}")]
     ReservedConfigKey(String),
+    #[error("Could not find a suitable Python executable for the virtual environment based on the interpreter: {0}")]
+    NotFound(String),
 }
 
 /// The value to use for the shell prompt when inside a virtual environment.

scripts/check_embedded_python.py

@@ -0,0 +1,97 @@
+#!/usr/bin/env python3
+
+"""Install `pylint` and `numpy` into an embedded Python."""
+
+import argparse
+import logging
+import os
+import subprocess
+import sys
+import tempfile
+
+
+if __name__ == "__main__":
+    logging.basicConfig(level=logging.INFO, format="%(levelname)s: %(message)s")
+
+    parser = argparse.ArgumentParser(
+        description="Check an embedded Python interpreter."
+    )
+    parser.add_argument("--uv", help="Path to a uv binary.")
+    args = parser.parse_args()
+
+    uv: str = os.path.abspath(args.uv) if args.uv else "uv"
+
+    # Create a temporary directory.
+    with tempfile.TemporaryDirectory() as temp_dir:
+        # Create a virtual environment with `uv`.
+        logging.info("Creating virtual environment with `uv`...")
+        subprocess.run(
+            [uv, "venv", ".venv", "--seed", "--python", sys.executable],
+            cwd=temp_dir,
+            check=True,
+        )
+
+        if os.name == "nt":
+            executable = os.path.join(temp_dir, ".venv", "Scripts", "python.exe")
+        else:
+            executable = os.path.join(temp_dir, ".venv", "bin", "python")
+
+        logging.info("Querying virtual environment...")
+        subprocess.run(
+            [executable, "--version"],
+            cwd=temp_dir,
+            check=True,
+        )
+
+        logging.info("Installing into `uv` virtual environment...")
+
+        # Disable the `CONDA_PREFIX` and `VIRTUAL_ENV` environment variables, so that
+        # we only rely on virtual environment discovery via the `.venv` directory.
+        # Our "system Python" here might itself be a Conda environment!
+        env = os.environ.copy()
+        env["CONDA_PREFIX"] = ""
+        env["VIRTUAL_ENV"] = ""
+
+        # Install, verify, and uninstall a few packages.
+        for package in ["pylint", "numpy"]:
+            # Install the package.
+            logging.info(
+                f"Installing the package `{package}` into the virtual environment..."
+            )
+            subprocess.run(
+                [uv, "pip", "install", package, "--verbose"],
+                cwd=temp_dir,
+                check=True,
+                env=env,
+            )
+
+            # Ensure that the package is installed in the virtual environment.
+            logging.info(f"Checking that `{package}` is installed.")
+            code = subprocess.run(
+                [executable, "-c", f"import {package}"],
+                cwd=temp_dir,
+            )
+            if code.returncode != 0:
+                raise Exception(
+                    f"The package `{package}` isn't installed in the virtual environment."
+                )
+
+            # Uninstall the package.
+            logging.info(f"Uninstalling the package `{package}`.")
+            subprocess.run(
+                [uv, "pip", "uninstall", package, "--verbose"],
+                cwd=temp_dir,
+                check=True,
+                env=env,
+            )
+
+            # Ensure that the package isn't installed in the virtual environment.
+            logging.info(f"Checking that `{package}` isn't installed.")
+            code = subprocess.run(
+                [executable, "-m", "pip", "show", package],
+                cwd=temp_dir,
+            )
+            if code.returncode == 0:
+                raise Exception(
+                    f"The package `{package}` is installed in the virtual environment (but shouldn't be)."
+                )