Only query interpreter for system_site_packages = true virtualenv

crates/uv-python/python/get_interpreter_info.py

@@ -543,6 +543,11 @@ def get_operating_system_and_architecture():
 
 
 def main() -> None:
+    sys_path_before_site = [os.path.abspath(p) for p in sys.path]
+    import site
+    site.main()
+    sys_path = sys.path[:]
+
     markers = {
         "implementation_name": implementation_name,
         "implementation_version": implementation_version,
@@ -579,7 +584,8 @@ def main() -> None:
         "sys_prefix": sys.prefix,
         "sys_base_executable": getattr(sys, "_base_executable", None),
         "sys_executable": sys.executable,
-        "sys_path": sys.path,
+        "sys_path_before_site": sys_path_before_site,
+        "sys_path": sys_path,
         "stdlib": sysconfig.get_path("stdlib"),
         # Prior to the introduction of `sysconfig` patching, python-build-standalone installations would always use
         # "/install" as the prefix. With `sysconfig` patching, we rewrite the prefix to match the actual installation

crates/uv-python/src/interpreter.rs

@@ -44,6 +44,7 @@ pub struct Interpreter {
     sys_base_prefix: PathBuf,
     sys_base_executable: Option<PathBuf>,
     sys_executable: PathBuf,
+    sys_path_before_site: Vec<PathBuf>,
     sys_path: Vec<PathBuf>,
     stdlib: PathBuf,
     standalone: bool,
@@ -78,6 +79,7 @@ impl Interpreter {
             sys_base_prefix: info.sys_base_prefix,
             sys_base_executable: info.sys_base_executable,
             sys_executable: info.sys_executable,
+            sys_path_before_site: info.sys_path_before_site,
             sys_path: info.sys_path,
             stdlib: info.stdlib,
             standalone: info.standalone,
@@ -90,10 +92,17 @@ impl Interpreter {
     /// Return a new [`Interpreter`] with the given virtual environment root.
     #[must_use]
     pub fn with_virtualenv(self, virtualenv: VirtualEnvironment) -> Self {
+        let sys_path: Vec<PathBuf> = self
+            .sys_path_before_site
+            .iter()
+            .cloned()
+            .chain(self.site_packages().map(Cow::into_owned))
+            .collect();
         Self {
             scheme: virtualenv.scheme,
             sys_executable: virtualenv.executable,
             sys_prefix: virtualenv.root,
+            sys_path,
             target: None,
             prefix: None,
             ..self
@@ -618,6 +627,7 @@ struct InterpreterInfo {
     sys_base_prefix: PathBuf,
     sys_base_executable: Option<PathBuf>,
     sys_executable: PathBuf,
+    sys_path_before_site: Vec<PathBuf>,
     sys_path: Vec<PathBuf>,
     stdlib: PathBuf,
     standalone: bool,
@@ -640,6 +650,7 @@ impl InterpreterInfo {
         );
         let output = Command::new(interpreter)
             .arg("-I") // Isolated mode.
+            .arg("-S") // Don't import site for initialization.
             .arg("-B") // Don't write bytecode.
             .arg("-c")
             .arg(script)

crates/uv-virtualenv/src/lib.rs

@@ -72,6 +72,10 @@ pub fn create_venv(
     )?;
 
     // Create the corresponding `PythonEnvironment`.
-    let interpreter = Interpreter::query(virtualenv.executable, cache)?;
+    let interpreter = if system_site_packages {
+        Interpreter::query(virtualenv.executable, cache)?
+    } else {
+        interpreter.with_virtualenv(virtualenv)
+    };
     Ok(PythonEnvironment::from_interpreter(interpreter))
 }