Update test context to avoid discovery of external Pythons (#4222)

By setting the test search path to an empty path, we avoid accidentally
pulling interpreters from the system during a test case.

Cherry-picked from https://github.com/astral-sh/uv/pull/4214

crates/uv/tests/common/mod.rs

@@ -45,6 +45,12 @@ pub const INSTA_FILTERS: &[(&str, &str)] = &[
     ),
 ];
 
+/// Create a context for tests which simplfiies shared behavior across tests.
+///
+/// * Set the current directory to a temporary directory (`temp_dir`).
+/// * Set the cache dir to a different temporary directory (`cache_dir`).
+/// * Set a cutoff for versions used in the resolution so the snapshots don't change after a new release.
+/// * Set the venv to a fresh `.venv` in `temp_dir`.
 #[derive(Debug)]
 pub struct TestContext {
     pub temp_dir: assert_fs::TempDir,
@@ -153,11 +159,7 @@ impl TestContext {
         }
     }
 
-    /// Set shared defaults between tests:
+    /// Create a `pip compile` command for testing.
-    /// * Set the current directory to a temporary directory (`temp_dir`).
-    /// * Set the cache dir to a different temporary directory (`cache_dir`).
-    /// * Set a cutoff for versions used in the resolution so the snapshots don't change after a new release.
-    /// * Set the venv to a fresh `.venv` in `temp_dir`.
     pub fn compile(&self) -> std::process::Command {
         let mut command = self.compile_without_exclude_newer();
         command.arg("--exclude-newer").arg(EXCLUDE_NEWER);
@@ -178,6 +180,7 @@ impl TestContext {
             .arg(self.cache_dir.path())
             .env("VIRTUAL_ENV", self.venv.as_os_str())
             .env("UV_NO_WRAP", "1")
+            .env("UV_TEST_PYTHON_PATH", "/dev/null")
             .current_dir(self.temp_dir.path());
 
         if cfg!(all(windows, debug_assertions)) {
@@ -211,6 +214,7 @@ impl TestContext {
             .arg(self.cache_dir.path())
             .env("VIRTUAL_ENV", self.venv.as_os_str())
             .env("UV_NO_WRAP", "1")
+            .env("UV_TEST_PYTHON_PATH", "/dev/null")
             .current_dir(&self.temp_dir);
 
         if cfg!(all(windows, debug_assertions)) {
@@ -231,6 +235,7 @@ impl TestContext {
             .arg(self.cache_dir.path())
             .env("VIRTUAL_ENV", self.venv.as_os_str())
             .env("UV_NO_WRAP", "1")
+            .env("UV_TEST_PYTHON_PATH", "/dev/null")
             .current_dir(&self.temp_dir);
 
         if cfg!(all(windows, debug_assertions)) {
@@ -263,6 +268,7 @@ impl TestContext {
             .arg(self.cache_dir.path())
             .env("VIRTUAL_ENV", self.venv.as_os_str())
             .env("UV_NO_WRAP", "1")
+            .env("UV_TEST_PYTHON_PATH", "/dev/null")
             .current_dir(&self.temp_dir);
 
         if cfg!(all(windows, debug_assertions)) {
@@ -295,6 +301,7 @@ impl TestContext {
             .arg(self.cache_dir.path())
             .env("VIRTUAL_ENV", self.venv.as_os_str())
             .env("UV_NO_WRAP", "1")
+            .env("UV_TEST_PYTHON_PATH", "/dev/null")
             .current_dir(&self.temp_dir);
 
         if cfg!(all(windows, debug_assertions)) {

crates/uv/tests/run.rs

@@ -4,8 +4,7 @@ use anyhow::Result;
 use assert_fs::prelude::*;
 use indoc::indoc;
 
-use crate::common::get_toolchain;
+use common::{python_path_with_versions, uv_snapshot, TestContext};
-use common::{uv_snapshot, TestContext};
 
 mod common;
 
@@ -13,6 +12,8 @@ mod common;
 #[test]
 fn run_with_python_version() -> Result<()> {
     let context = TestContext::new("3.12");
+    let python_path = python_path_with_versions(&context.temp_dir, &["3.11", "3.12"])
+        .expect("Failed to create Python test path");
 
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(indoc! { r#"
@@ -43,7 +44,8 @@ fn run_with_python_version() -> Result<()> {
         .arg("--preview")
         .arg("python")
         .arg("-B")
-        .arg("main.py");
+        .arg("main.py")
+        .env("UV_TEST_PYTHON_PATH", &python_path);
     uv_snapshot!(context.filters(), command_with_args, @r###"
     success: true
     exit_code: 0
@@ -66,10 +68,11 @@ fn run_with_python_version() -> Result<()> {
     let command_with_args = command
         .arg("--preview")
         .arg("-p")
-        .arg(get_toolchain("3.12"))
+        .arg("3.12")
         .arg("python")
         .arg("-B")
-        .arg("main.py");
+        .arg("main.py")
+        .env("UV_TEST_PYTHON_PATH", &python_path);
     uv_snapshot!(context.filters(), command_with_args, @r###"
     success: true
     exit_code: 0
@@ -87,16 +90,20 @@ fn run_with_python_version() -> Result<()> {
     let command_with_args = command
         .arg("--preview")
         .arg("-p")
-        .arg(get_toolchain("3.11"))
+        .arg("3.11")
         .arg("python")
         .arg("-B")
-        .arg("main.py");
+        .arg("main.py")
+        .env("UV_TEST_PYTHON_PATH", &python_path)
+        .env_remove("VIRTUAL_ENV");
+
     let mut filters = context.filters();
     filters.push((
         r"Using Python 3.11.\d+ interpreter at: .*",
         "Using Python 3.11.[X] interpreter at: [PYTHON]",
     ));
     filters.push((r"3.11.\d+", "3.11.[X]"));
+
     uv_snapshot!(filters, command_with_args, @r###"
     success: true
     exit_code: 0