Cache Python downloads by default

i.e., without requiring opt-in vi `UV_PYTHON_CACHE_DIR`

crates/uv-python/src/downloads.rs

@@ -21,6 +21,7 @@ use tokio_util::either::Either;
 use tracing::{debug, instrument};
 use url::Url;
 
+use uv_cache::{Cache, CacheBucket};
 use uv_client::{BaseClient, WrappedReqwestError, is_transient_network_error};
 use uv_distribution_filename::{ExtensionError, SourceDistExtension};
 use uv_extract::hash::Hasher;
@@ -1099,13 +1100,14 @@ impl ManagedPythonDownload {
     }
 
     /// Download and extract a Python distribution, retrying on failure.
-    #[instrument(skip(client, installation_dir, scratch_dir, reporter), fields(download = % self.key()))]
+    #[instrument(skip(client, installation_dir, scratch_dir, cache, reporter), fields(download = % self.key()))]
     pub async fn fetch_with_retry(
         &self,
         client: &BaseClient,
         retry_policy: &ExponentialBackoff,
         installation_dir: &Path,
         scratch_dir: &Path,
+        cache: &Cache,
         reinstall: bool,
         python_install_mirror: Option<&str>,
         pypy_install_mirror: Option<&str>,
@@ -1120,6 +1122,7 @@ impl ManagedPythonDownload {
                     client,
                     installation_dir,
                     scratch_dir,
+                    cache,
                     reinstall,
                     python_install_mirror,
                     pypy_install_mirror,
@@ -1167,12 +1170,13 @@ impl ManagedPythonDownload {
     }
 
     /// Download and extract a Python distribution.
-    #[instrument(skip(client, installation_dir, scratch_dir, reporter), fields(download = % self.key()))]
+    #[instrument(skip(client, installation_dir, scratch_dir, cache, reporter), fields(download = % self.key()))]
     pub async fn fetch(
         &self,
         client: &BaseClient,
         installation_dir: &Path,
         scratch_dir: &Path,
+        cache: &Cache,
         reinstall: bool,
         python_install_mirror: Option<&str>,
         pypy_install_mirror: Option<&str>,
@@ -1205,10 +1209,13 @@ impl ManagedPythonDownload {
 
         let temp_dir = tempfile::tempdir_in(scratch_dir).map_err(Error::DownloadDirError)?;
 
-        if let Some(python_builds_dir) =
+        // Use the cache directory from the environment variable if set, otherwise use the default
-            env::var_os(EnvVars::UV_PYTHON_CACHE_DIR).filter(|s| !s.is_empty())
+        // cache bucket.
-        {
+        let python_builds_dir = env::var_os(EnvVars::UV_PYTHON_CACHE_DIR)
-            let python_builds_dir = PathBuf::from(python_builds_dir);
+            .filter(|s| !s.is_empty())
+            .map(PathBuf::from)
+            .unwrap_or_else(|| cache.bucket(CacheBucket::Python));
+
         fs_err::create_dir_all(&python_builds_dir)?;
         let hash_prefix = match self.sha256.as_deref() {
             Some(sha) => {
@@ -1272,26 +1279,6 @@ impl ManagedPythonDownload {
             Direction::Extract,
         )
         .await?;
-        } else {
-            // Avoid overlong log lines
-            debug!("Downloading {url}");
-            debug!(
-                "Extracting {filename} to temporary location: {}",
-                temp_dir.path().simplified_display()
-            );
-
-            let (reader, size) = read_url(&url, client).await?;
-            self.extract_reader(
-                reader,
-                temp_dir.path(),
-                &filename,
-                ext,
-                size,
-                reporter,
-                Direction::Download,
-            )
-            .await?;
-        }
 
         // Extract the top-level directory.
         let mut extracted = match uv_extract::strip_component(temp_dir.path()) {

crates/uv-python/src/installation.rs

@@ -261,6 +261,7 @@ impl PythonInstallation {
                 retry_policy,
                 installations_dir,
                 &scratch_dir,
+                cache,
                 false,
                 python_install_mirror,
                 pypy_install_mirror,

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

@@ -14,6 +14,7 @@ use owo_colors::{AnsiColors, OwoColorize};
 use rustc_hash::{FxHashMap, FxHashSet};
 use tracing::{debug, trace};
 
+use uv_cache::Cache;
 use uv_client::BaseClientBuilder;
 use uv_fs::Simplified;
 use uv_platform::{Arch, Libc};
@@ -188,6 +189,7 @@ pub(crate) async fn install(
     pypy_install_mirror: Option<String>,
     python_downloads_json_url: Option<String>,
     client_builder: BaseClientBuilder<'_>,
+    cache: &Cache,
     default: bool,
     python_downloads: PythonDownloads,
     no_config: bool,
@@ -470,6 +472,7 @@ pub(crate) async fn install(
                         &retry_policy,
                         installations_dir,
                         &scratch_dir,
+                        cache,
                         reinstall,
                         python_install_mirror.as_deref(),
                         pypy_install_mirror.as_deref(),

crates/uv/src/lib.rs

@@ -1601,6 +1601,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
             let args = settings::PythonInstallSettings::resolve(args, filesystem, environment);
             show_settings!(args);
 
+            let cache = cache.init().await?;
             commands::python_install(
                 &project_dir,
                 args.install_dir,
@@ -1614,6 +1615,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 args.pypy_install_mirror,
                 args.python_downloads_json_url,
                 client_builder.subcommand(vec!["python".to_owned(), "install".to_owned()]),
+                &cache,
                 args.default,
                 globals.python_downloads,
                 cli.top_level.no_config,
@@ -1630,6 +1632,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
             show_settings!(args);
             let upgrade = commands::PythonUpgrade::Enabled(commands::PythonUpgradeSource::Upgrade);
 
+            let cache = cache.init().await?;
             commands::python_install(
                 &project_dir,
                 args.install_dir,
@@ -1643,6 +1646,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 args.pypy_install_mirror,
                 args.python_downloads_json_url,
                 client_builder.subcommand(vec!["python".to_owned(), "upgrade".to_owned()]),
+                &cache,
                 args.default,
                 globals.python_downloads,
                 cli.top_level.no_config,

crates/uv/tests/it/python_install.rs

@@ -2712,10 +2712,10 @@ fn python_install_no_cache() {
      - cpython-3.14.2-[PLATFORM] (python3.14)
     ");
 
-    // 3.12 isn't cached, so it can't be installed
+    // 3.12 isn't cached, so it can't be installed offline
     let mut filters = context.filters();
     filters.push((
-        "cpython-3.12.*.tar.gz",
+        r"cpython-3\.12\.\d+(%2B|\+)\d+-[a-z0-9_-]+\.tar\.gz",
         "cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz",
     ));
     filters.push((r"releases/download/\d{8}/", "releases/download/[DATE]/"));
@@ -2729,8 +2729,7 @@ fn python_install_no_cache() {
 
     ----- stderr -----
     error: Failed to install cpython-3.12.12-[PLATFORM]
-      Caused by: Failed to download https://github.com/astral-sh/python-build-standalone/releases/download/[DATE]/cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz
+      Caused by: An offline Python installation was requested, but cpython-3.12.12-[PLATFORM] (from https://github.com/astral-sh/python-build-standalone/releases/download/[DATE]/cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz) is missing in [CACHE_DIR]/python-v0
-      Caused by: Network connectivity is disabled, but the requested data wasn't found in the cache for: `https://github.com/astral-sh/python-build-standalone/releases/download/[DATE]/cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz`
     ");
 }