Enable the Python download cache in CI

.github/workflows/ci.yml

@@ -214,7 +214,18 @@ jobs:
         run: rustup show
 
       - uses: astral-sh/setup-uv@e92bafb6253dcd438e0484186d7669ea7a8ca1cc # v6.4.3
+
+      - name: "Cache Python downloads"
+        uses: actions/cache@v4
+        with:
+          path: /tmp/uv/python
+          key: uv-${{ runner.os }}-${{ hashFiles('.python-versions') }}
+          restore-keys: |
+            uv-${{ runner.os }}-${{ hashFiles('.python-versions') }}
+            uv-${{ runner.os }}
       - name: "Install required Python versions"
+        env:
+          UV_PYTHON_CACHE_DIR: /tmp/uv/python
         run: uv python install
 
       - name: "Install cargo nextest"
@@ -226,6 +237,7 @@ jobs:
         env:
           # Retry more than default to reduce flakes in CI
           UV_HTTP_RETRIES: 5
+          UV_PYTHON_CACHE_DIR: /tmp/uv/python
         run: |
           cargo nextest run \
             --features python-patch \
@@ -250,7 +262,18 @@ jobs:
         run: rustup show
 
       - uses: astral-sh/setup-uv@e92bafb6253dcd438e0484186d7669ea7a8ca1cc # v6.4.3
+
+      - name: "Cache Python downloads"
+        uses: actions/cache@v4
+        with:
+          path: /tmp/uv/python
+          key: uv-${{ runner.os }}-${{ hashFiles('.python-versions') }}
+          restore-keys: |
+            uv-${{ runner.os }}-${{ hashFiles('.python-versions') }}
+            uv-${{ runner.os }}
       - name: "Install required Python versions"
+        env:
+          UV_PYTHON_CACHE_DIR: /tmp/uv/python
         run: uv python install
 
       - name: "Install cargo nextest"
@@ -262,6 +285,7 @@ jobs:
         env:
           # Retry more than default to reduce flakes in CI
           UV_HTTP_RETRIES: 5
+          UV_PYTHON_CACHE_DIR: /tmp/uv/python
         run: |
           cargo nextest run \
             --no-default-features \
@@ -287,7 +311,19 @@ jobs:
           Copy-Item -Path "${{ github.workspace }}" -Destination "${{ env.UV_WORKSPACE }}" -Recurse
 
       - uses: astral-sh/setup-uv@e92bafb6253dcd438e0484186d7669ea7a8ca1cc # v6.4.3
+
+      - name: "Cache Python downloads"
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.UV_WORKSPACE }}/python
+          key: uv-${{ runner.os }}-${{ hashFiles('.python-versions') }}
+          restore-keys: |
+            uv-${{ runner.os }}-${{ hashFiles('.python-versions') }}
+            uv-${{ runner.os }}
+
       - name: "Install required Python versions"
+        env:
+          UV_PYTHON_CACHE_DIR: ${{ env.UV_WORKSPACE }}/python
         run: uv python install
 
       - uses: Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0 # v2.8.0
@@ -311,6 +347,7 @@ jobs:
           # Avoid permission errors during concurrent tests
           # See https://github.com/astral-sh/uv/issues/6940
           UV_LINK_MODE: copy
+          UV_PYTHON_CACHE_DIR: ${{ env.UV_WORKSPACE }}/python
         shell: bash
         run: |
           cargo nextest run \

crates/uv/tests/it/python_install.rs

@@ -2261,7 +2261,8 @@ fn python_install_no_cache() {
         .with_managed_python_dirs();
 
     // Install the latest version
-    uv_snapshot!(context.filters(), context.python_install(), @r"
+    uv_snapshot!(context.filters(), context.python_install()
+        .env_remove(EnvVars::UV_PYTHON_CACHE_DIR), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -2279,7 +2280,8 @@ fn python_install_no_cache() {
     bin_python.assert(predicate::path::exists());
 
     // Should be a no-op when already installed
-    uv_snapshot!(context.filters(), context.python_install(), @r###"
+    uv_snapshot!(context.filters(), context.python_install()
+        .env_remove(EnvVars::UV_PYTHON_CACHE_DIR), @r###"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -2289,7 +2291,8 @@ fn python_install_no_cache() {
     "###);
 
     // Similarly, when a requested version is already installed
-    uv_snapshot!(context.filters(), context.python_install().arg("3.13"), @r###"
+    uv_snapshot!(context.filters(), context.python_install().arg("3.13")
+        .env_remove(EnvVars::UV_PYTHON_CACHE_DIR), @r###"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -2298,7 +2301,8 @@ fn python_install_no_cache() {
     "###);
 
     // You can opt-in to a reinstall
-    uv_snapshot!(context.filters(), context.python_install().arg("3.13").arg("--reinstall"), @r"
+    uv_snapshot!(context.filters(), context.python_install().arg("3.13").arg("--reinstall")
+        .env_remove(EnvVars::UV_PYTHON_CACHE_DIR), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -2309,7 +2313,8 @@ fn python_install_no_cache() {
     ");
 
     // Uninstallation requires an argument
-    uv_snapshot!(context.filters(), context.python_uninstall(), @r###"
+    uv_snapshot!(context.filters(), context.python_uninstall()
+        .env_remove(EnvVars::UV_PYTHON_CACHE_DIR), @r###"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -2323,7 +2328,8 @@ fn python_install_no_cache() {
     For more information, try '--help'.
     "###);
 
-    uv_snapshot!(context.filters(), context.python_uninstall().arg("3.13"), @r"
+    uv_snapshot!(context.filters(), context.python_uninstall().arg("3.13")
+        .env_remove(EnvVars::UV_PYTHON_CACHE_DIR), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -2343,7 +2349,8 @@ fn python_install_no_cache() {
     uv_snapshot!(filters, context
         .python_install()
         .arg("3.12")
-        .arg("--offline"), @r"
+        .arg("--offline")
+        .env_remove(EnvVars::UV_PYTHON_CACHE_DIR), @r"
     success: false
     exit_code: 1
     ----- stdout -----