refactor: streamline test Python cache helpers

Make `with_python_download_cache()` private and remove redundant explicit calls across test files.

Fixes #16721

Author: nooscraft

crates/uv/tests/it/common/mod.rs

@@ -486,7 +486,7 @@ impl TestContext {
 
     /// Use a shared global cache for Python downloads.
     #[must_use]
-    pub fn with_python_download_cache(mut self) -> Self {
+    fn with_python_download_cache(mut self) -> Self {
         let key: OsString = EnvVars::UV_PYTHON_CACHE_DIR.into();
         self.extra_env.retain(|(existing, _)| existing != &key);
         let value = env::var_os(EnvVars::UV_PYTHON_CACHE_DIR).unwrap_or_else(|| {
@@ -504,9 +504,6 @@ impl TestContext {
     pub fn without_python_download_cache(mut self) -> Self {
         let key: OsString = EnvVars::UV_PYTHON_CACHE_DIR.into();
         self.extra_env.retain(|(existing, _)| existing != &key);
-        let empty_cache = self.temp_dir.child("python-cache-empty");
-        self.extra_env
-            .push((key, empty_cache.as_os_str().to_owned()));
         self
     }
 

crates/uv/tests/it/pip_install.rs

@@ -13121,7 +13121,6 @@ fn pip_install_no_sources_editable_to_registry_switch() -> Result<()> {
 #[test]
 fn install_with_system_interpreter() {
     let context = TestContext::new_with_versions(&[])
-        .with_python_download_cache()
         .with_managed_python_dirs()
         .with_filtered_python_keys();
 
@@ -13145,3 +13144,81 @@ fn install_with_system_interpreter() {
     "
     );
 }
+
+/// Test that a missing Python version is not installed when not using `--target` or `--prefix`.
+#[cfg(feature = "python-managed")]
+#[test]
+fn install_missing_python_no_target() {
+    // Create a context that only has Python 3.11 available.
+    let context = TestContext::new("3.11").with_managed_python_dirs();
+
+    // Request Python 3.12; which should fail
+    uv_snapshot!(context.filters(), context.pip_install()
+        .arg("--python").arg("3.12")
+        .arg("anyio"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: No virtual environment found for Python 3.12; run `uv venv` to create an environment, or pass `--system` to install into a non-virtual environment
+    "###
+    );
+}
+
+// If there are no python interpreters available, `uv pip install` into a target should install one.
+#[cfg(feature = "python-managed")]
+#[test]
+fn install_missing_python_with_target() {
+    // Create a context with no installed python interpreters.
+    let context = TestContext::new_with_versions(&[]).with_managed_python_dirs();
+
+    let target_dir = context.temp_dir.child("target-dir");
+
+    uv_snapshot!(context.filters(), context.pip_install()
+        .arg("anyio")
+        .arg("--target").arg(target_dir.path()), @r###"
+        success: true
+        exit_code: 0
+        ----- stdout -----
+
+        ----- stderr -----
+        Using CPython 3.14.0
+        Resolved 3 packages in [TIME]
+        Prepared 3 packages in [TIME]
+        Installed 3 packages in [TIME]
+         + anyio==4.3.0
+         + idna==3.6
+         + sniffio==1.3.1
+        "###
+    );
+}
+
+#[cfg(feature = "python-managed")]
+#[test]
+fn install_missing_python_version_with_target() {
+    // Create a context that only has Python 3.11 available.
+    let context = TestContext::new("3.11").with_managed_python_dirs();
+
+    let target_dir = context.temp_dir.child("target-dir");
+
+    // Request Python 3.12 which is not installed in this context.
+    uv_snapshot!(context.filters(), context.pip_install()
+        .arg("anyio")
+        .arg("--python").arg("3.12")
+        .arg("--target").arg(target_dir.path()), @r###"
+        success: true
+        exit_code: 0
+        ----- stdout -----
+
+        ----- stderr -----
+        Using CPython 3.12.12
+        Resolved 3 packages in [TIME]
+        Prepared 3 packages in [TIME]
+        Installed 3 packages in [TIME]
+         + anyio==4.3.0
+         + idna==3.6
+         + sniffio==1.3.1
+        "###
+    );
+}

crates/uv/tests/it/python_find.rs

@@ -1264,7 +1264,6 @@ fn python_find_freethreaded_313() {
         .with_filtered_python_keys()
         .with_filtered_python_sources()
         .with_managed_python_dirs()
-        .with_python_download_cache()
         .with_filtered_python_install_bin()
         .with_filtered_python_names()
         .with_filtered_exe_suffix();
@@ -1303,7 +1302,6 @@ fn python_find_freethreaded_314() {
         .with_filtered_python_keys()
         .with_filtered_python_sources()
         .with_managed_python_dirs()
-        .with_python_download_cache()
         .with_filtered_python_install_bin()
         .with_filtered_python_names()
         .with_filtered_exe_suffix();
@@ -1380,7 +1378,6 @@ fn python_find_prerelease_version_specifiers() {
         .with_filtered_python_keys()
         .with_filtered_python_sources()
         .with_managed_python_dirs()
-        .with_python_download_cache()
         .with_filtered_python_install_bin()
         .with_filtered_python_names()
         .with_filtered_exe_suffix();
@@ -1479,7 +1476,6 @@ fn python_find_prerelease_with_patch_request() {
         .with_filtered_python_keys()
         .with_filtered_python_sources()
         .with_managed_python_dirs()
-        .with_python_download_cache()
         .with_filtered_python_install_bin()
         .with_filtered_python_names()
         .with_filtered_exe_suffix();