Adjust base Python lookup logic for Windows (#2121)

## Summary

When I install via the Windows Store, `interpreter.base_prefix` contains
a bunch of resolved information that leads to a broken environment.

Instead, we now use `sys._base_executable` on Windows by default,
falling back to `sys.base_prefix` if it doesn't exist. (There are some
issues with `sys.base_executable` that lead to complexity in
`virtualenv`, but they only affect POSIX.) Admittedly, I don't know when
`sys._base_executable` wouldn't exist. It exists in all the environments
I've tested.

Additionally, we use the system interpreter directly if we're outside of
a virtualenv.

crates/uv-interpreter/src/get_interpreter_info.py

@@ -88,6 +88,7 @@ interpreter_info = {
     "base_prefix": sys.base_prefix,
     "base_exec_prefix": sys.base_exec_prefix,
     "prefix": sys.prefix,
+    "base_executable": getattr(sys, "_base_executable", None),
     "sys_executable": sys.executable,
     "sysconfig_paths": sysconfig.get_paths(),
 }

crates/uv-interpreter/src/interpreter.rs

@@ -32,6 +32,7 @@ pub struct Interpreter {
     prefix: PathBuf,
     base_exec_prefix: PathBuf,
     base_prefix: PathBuf,
+    base_executable: Option<PathBuf>,
     sys_executable: PathBuf,
     tags: OnceCell<Tags>,
 }
@@ -54,6 +55,7 @@ impl Interpreter {
             prefix: info.prefix,
             base_exec_prefix: info.base_exec_prefix,
             base_prefix: info.base_prefix,
+            base_executable: info.base_executable,
             sys_executable: info.sys_executable,
             tags: OnceCell::new(),
         })
@@ -77,6 +79,7 @@ impl Interpreter {
             prefix: PathBuf::from("/dev/null"),
             base_exec_prefix: PathBuf::from("/dev/null"),
             base_prefix: PathBuf::from("/dev/null"),
+            base_executable: None,
             sys_executable: PathBuf::from("/dev/null"),
             tags: OnceCell::new(),
         }
@@ -355,6 +358,12 @@ impl Interpreter {
         &self.prefix
     }
 
+    /// Return the `sys._base_executable` path for this Python interpreter. Some platforms do not
+    /// have this attribute, so it may be `None`.
+    pub fn base_executable(&self) -> Option<&Path> {
+        self.base_executable.as_deref()
+    }
+
     /// Return the `sys.executable` path for this Python interpreter.
     pub fn sys_executable(&self) -> &Path {
         &self.sys_executable
@@ -455,6 +464,7 @@ struct InterpreterInfo {
     prefix: PathBuf,
     base_exec_prefix: PathBuf,
     base_prefix: PathBuf,
+    base_executable: Option<PathBuf>,
     sys_executable: PathBuf,
 }
 

crates/uv-virtualenv/src/bare.rs

@@ -48,9 +48,36 @@ pub fn create_bare_venv(
     system_site_packages: bool,
     extra_cfg: Vec<(String, String)>,
 ) -> Result<Virtualenv, Error> {
-    // We have to canonicalize the interpreter path, otherwise the home is set to the venv dir instead of the real root.
-    // This would make python-build-standalone fail with the encodings module not being found because its home is wrong.
-    let base_python = fs_err::canonicalize(interpreter.sys_executable())?;
+    // Determine the base Python executable; that is, the Python executable that should be
+    // considered the "base" for the virtual environment. This is typically the Python executable
+    // from the [`Interpreter`]; however, if the interpreter is a virtual environment itself, then
+    // the base Python executable is the Python executable of the interpreter's base interpreter.
+    let base_python = if cfg!(unix) {
+        // On Unix, follow symlinks to resolve the base interpreter, since the Python executable in
+        // a virtual environment is a symlink to the base interpreter.
+        fs_err::canonicalize(interpreter.sys_executable())?
+    } else if cfg!(windows) {
+        // On Windows, follow `virtualenv`. If we're in a virtual environment, use
+        // `sys._base_executable` if it exists; if not, use `sys.base_prefix`. For example, with
+        // Python installed from the Windows Store, `sys.base_prefix` is slightly "incorrect".
+        //
+        // If we're _not_ in a virtual environment, use the interpreter's executable, since it's
+        // already a "system Python". We canonicalize the path to ensure that it's real and
+        // consistent, though we don't expect any symlinks on Windows.
+        if interpreter.is_virtualenv() {
+            if let Some(base_executable) = interpreter.base_executable() {
+                base_executable.to_path_buf()
+            } else {
+                // Assume `python.exe`, though the exact executable name is never used (below) on
+                // Windows, only its parent directory.
+                interpreter.base_prefix().join("python.exe")
+            }
+        } else {
+            fs_err::canonicalize(interpreter.sys_executable())?
+        }
+    } else {
+        unimplemented!("Only Windows and Unix are supported")
+    };
 
     // Validate the existing location.
     match location.metadata() {
@@ -188,28 +215,17 @@ pub fn create_bare_venv(
         fs::write(scripts.join(name), activator)?;
     }
 
-    // pyvenv.cfg
-    let python_home = if cfg!(unix) {
-        // On Linux and Mac, Python is symlinked so the base home is the parent of the resolved-by-canonicalize path.
-        base_python
+    // 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",
+                "The Python interpreter needs to have a parent directory",
             )
         })?
         .simplified_display()
-            .to_string()
-    } else if cfg!(windows) {
-        // `virtualenv` seems to rely on the undocumented, private `sys._base_executable`. When I tried,
-        // `sys.base_prefix` was the same as the parent of `sys._base_executable`, but a much simpler logic and
-        // documented.
-        // https://github.com/pypa/virtualenv/blob/d9fdf48d69f0d0ca56140cf0381edbb5d6fe09f5/src/virtualenv/discovery/py_info.py#L136-L156
-        interpreter.base_prefix().simplified_display().to_string()
-    } else {
-        unimplemented!("Only Windows and Unix are supported")
-    };
+        .to_string();
 
     // Validate extra_cfg
     let reserved_keys = [

crates/uv/tests/venv.rs

@@ -3,6 +3,7 @@
 use std::process::Command;
 
 use anyhow::Result;
+use assert_cmd::prelude::*;
 use assert_fs::prelude::*;
 
 use uv_fs::Simplified;
@@ -663,3 +664,70 @@ fn verify_pyvenv_cfg() {
     let search_string = format!("uv = {version}");
     pyvenv_cfg.assert(predicates::str::contains(search_string));
 }
+
+/// Ensure that a nested virtual environment uses the same `home` directory as the parent.
+#[test]
+fn verify_nested_pyvenv_cfg() -> Result<()> {
+    let temp_dir = assert_fs::TempDir::new()?;
+    let cache_dir = assert_fs::TempDir::new()?;
+    let bin = create_bin_with_executables(&temp_dir, &["3.12"]).expect("Failed to create bin dir");
+    let venv = temp_dir.child(".venv");
+
+    // Create a virtual environment at `.venv`.
+    Command::new(get_bin())
+        .arg("venv")
+        .arg(venv.as_os_str())
+        .arg("--python")
+        .arg("3.12")
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .arg("--exclude-newer")
+        .arg(EXCLUDE_NEWER)
+        .env("UV_TEST_PYTHON_PATH", bin.clone())
+        .current_dir(&temp_dir)
+        .assert()
+        .success();
+
+    let pyvenv_cfg = venv.child("pyvenv.cfg");
+
+    // Check pyvenv.cfg exists
+    pyvenv_cfg.assert(predicates::path::is_file());
+
+    // Extract the "home" line from the pyvenv.cfg file.
+    let contents = fs_err::read_to_string(pyvenv_cfg.path())?;
+    let venv_home = contents
+        .lines()
+        .find(|line| line.starts_with("home"))
+        .expect("home line not found");
+
+    // Now, create a virtual environment from within the virtual environment.
+    let subvenv = temp_dir.child(".subvenv");
+    Command::new(get_bin())
+        .arg("venv")
+        .arg(subvenv.as_os_str())
+        .arg("--python")
+        .arg("3.12")
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .arg("--exclude-newer")
+        .arg(EXCLUDE_NEWER)
+        .env("VIRTUAL_ENV", venv.as_os_str())
+        .env("UV_TEST_PYTHON_PATH", bin.clone())
+        .current_dir(&temp_dir)
+        .assert()
+        .success();
+
+    let sub_pyvenv_cfg = subvenv.child("pyvenv.cfg");
+
+    // Extract the "home" line from the pyvenv.cfg file.
+    let contents = fs_err::read_to_string(sub_pyvenv_cfg.path())?;
+    let sub_venv_home = contents
+        .lines()
+        .find(|line| line.starts_with("home"))
+        .expect("home line not found");
+
+    // Check that both directories point to the same home.
+    assert_eq!(sub_venv_home, venv_home);
+
+    Ok(())
+}