Download versions in `uv python pin` if not found (#13946)

As another follow-up in the vein of
https://github.com/astral-sh/uv/pull/13944, I noticed `uv python pin`
doesn't download Python versions, which is a bit weird because we'll
warn it's not found.

crates/uv/src/commands/python/pin.rs

@@ -7,17 +7,22 @@ use owo_colors::OwoColorize;
 use tracing::debug;
 
 use uv_cache::Cache;
+use uv_client::BaseClientBuilder;
 use uv_dirs::user_uv_config_dir;
 use uv_fs::Simplified;
 use uv_python::{
-    EnvironmentPreference, PYTHON_VERSION_FILENAME, PythonInstallation, PythonPreference,
-    PythonRequest, PythonVersionFile, VersionFileDiscoveryOptions,
+    EnvironmentPreference, PYTHON_VERSION_FILENAME, PythonDownloads, PythonInstallation,
+    PythonPreference, PythonRequest, PythonVersionFile, VersionFileDiscoveryOptions,
 };
+use uv_settings::PythonInstallMirrors;
 use uv_warnings::warn_user_once;
 use uv_workspace::{DiscoveryOptions, VirtualProject, WorkspaceCache};
 
-use crate::commands::{ExitStatus, project::find_requires_python};
+use crate::commands::{
+    ExitStatus, project::find_requires_python, reporters::PythonDownloadReporter,
+};
 use crate::printer::Printer;
+use crate::settings::NetworkSettings;
 
 /// Pin to a specific Python version.
 #[allow(clippy::fn_params_excessive_bools)]
@@ -26,9 +31,12 @@ pub(crate) async fn pin(
     request: Option<String>,
     resolved: bool,
     python_preference: PythonPreference,
+    python_downloads: PythonDownloads,
     no_project: bool,
     global: bool,
     rm: bool,
+    install_mirrors: PythonInstallMirrors,
+    network_settings: NetworkSettings,
     cache: &Cache,
     printer: Printer,
 ) -> Result<ExitStatus> {
@@ -98,12 +106,26 @@ pub(crate) async fn pin(
         bail!("Requests for arbitrary names (e.g., `{name}`) are not supported in version files");
     }
 
-    let python = match PythonInstallation::find(
-        &request,
+    let client_builder = BaseClientBuilder::new()
+        .connectivity(network_settings.connectivity)
+        .native_tls(network_settings.native_tls)
+        .allow_insecure_host(network_settings.allow_insecure_host.clone());
+    let reporter = PythonDownloadReporter::single(printer);
+
+    let python = match PythonInstallation::find_or_download(
+        Some(&request),
         EnvironmentPreference::OnlySystem,
         python_preference,
+        python_downloads,
+        &client_builder,
         cache,
-    ) {
+        Some(&reporter),
+        install_mirrors.python_install_mirror.as_deref(),
+        install_mirrors.pypy_install_mirror.as_deref(),
+        install_mirrors.python_downloads_json_url.as_deref(),
+    )
+    .await
+    {
         Ok(python) => Some(python),
         // If no matching Python version is found, don't fail unless `resolved` was requested
         Err(uv_python::Error::MissingPython(err)) if !resolved => {

crates/uv/src/lib.rs

@@ -1464,9 +1464,12 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 args.request,
                 args.resolved,
                 globals.python_preference,
+                globals.python_downloads,
                 args.no_project,
                 args.global,
                 args.rm,
+                args.install_mirrors,
+                globals.network_settings,
                 &cache,
                 printer,
             )

crates/uv/src/settings.rs

@@ -1057,12 +1057,13 @@ pub(crate) struct PythonPinSettings {
     pub(crate) no_project: bool,
     pub(crate) global: bool,
     pub(crate) rm: bool,
+    pub(crate) install_mirrors: PythonInstallMirrors,
 }
 
 impl PythonPinSettings {
     /// Resolve the [`PythonPinSettings`] from the CLI and workspace configuration.
     #[allow(clippy::needless_pass_by_value)]
-    pub(crate) fn resolve(args: PythonPinArgs, _filesystem: Option<FilesystemOptions>) -> Self {
+    pub(crate) fn resolve(args: PythonPinArgs, filesystem: Option<FilesystemOptions>) -> Self {
         let PythonPinArgs {
             request,
             no_resolved,
@@ -1072,12 +1073,17 @@ impl PythonPinSettings {
             rm,
         } = args;
 
+        let install_mirrors = filesystem
+            .map(|fs| fs.install_mirrors.clone())
+            .unwrap_or_default();
+
         Self {
             request,
             resolved: flag(resolved, no_resolved).unwrap_or(false),
             no_project,
             global,
             rm,
+            install_mirrors,
         }
     }
 }

crates/uv/tests/it/python_pin.rs

@@ -816,6 +816,32 @@ fn python_pin_with_comments() -> Result<()> {
     Ok(())
 }
 
+#[test]
+#[cfg(feature = "python-managed")]
+fn python_pin_install() {
+    let context: TestContext = TestContext::new_with_versions(&[]).with_filtered_python_sources();
+
+    // Should not install 3.12 when downloads are not automatic
+    uv_snapshot!(context.filters(), context.python_pin().arg("3.12"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Pinned `.python-version` to `3.12`
+
+    ----- stderr -----
+    warning: No interpreter found for Python 3.12 in [PYTHON SOURCES]
+    ");
+
+    uv_snapshot!(context.filters(), context.python_pin().arg("3.12").env("UV_PYTHON_DOWNLOADS", "auto"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Pinned `.python-version` to `3.12`
+
+    ----- stderr -----
+    ");
+}
+
 #[test]
 fn python_pin_rm() {
     let context: TestContext = TestContext::new_with_versions(&["3.12"]);