Respect .python-version in uv venv --preview (#4360)

Adds support for reading Python version files (introduced in #4335) to
`uv venv`. If present, we'll use the file version as the default.

.python-versions

@@ -1,6 +1,6 @@
-3.8.12
-3.8.18
-3.9.18
-3.10.13
-3.11.7
 3.12.1
+3.11.7
+3.10.13
+3.9.18
+3.8.18
+3.8.12

crates/uv-toolchain/src/lib.rs

@@ -13,7 +13,7 @@ pub use crate::prefix::Prefix;
 pub use crate::python_version::PythonVersion;
 pub use crate::target::Target;
 pub use crate::toolchain::Toolchain;
-pub use crate::version_files::{request_from_version_files, requests_from_version_files};
+pub use crate::version_files::{request_from_version_file, requests_from_version_file};
 pub use crate::virtualenv::{Error as VirtualEnvError, PyVenvConfiguration, VirtualEnvironment};
 
 mod discovery;

crates/uv-toolchain/src/version_files.rs

@@ -8,7 +8,7 @@ use crate::ToolchainRequest;
 ///
 /// Prefers `.python-versions` then `.python-version`.
 /// If only one Python version is desired, use [`request_from_version_files`] which prefers the `.python-version` file.
-pub async fn requests_from_version_files() -> Result<Option<Vec<ToolchainRequest>>, io::Error> {
+pub async fn requests_from_version_file() -> Result<Option<Vec<ToolchainRequest>>, io::Error> {
     if let Some(versions) = read_versions_file().await? {
         Ok(Some(
             versions
@@ -27,7 +27,7 @@ pub async fn requests_from_version_files() -> Result<Option<Vec<ToolchainRequest
 ///
 /// Prefers `.python-version` then the first entry of `.python-versions`.
 /// If multiple Python versions are desired, use [`requests_from_version_files`] instead.
-pub async fn request_from_version_files() -> Result<Option<ToolchainRequest>, io::Error> {
+pub async fn request_from_version_file() -> Result<Option<ToolchainRequest>, io::Error> {
     if let Some(version) = read_version_file().await? {
         Ok(Some(ToolchainRequest::parse(&version)))
     } else if let Some(versions) = read_versions_file().await? {

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

@@ -8,7 +8,7 @@ use uv_configuration::PreviewMode;
 use uv_fs::Simplified;
 use uv_toolchain::downloads::{self, DownloadResult, PythonDownload, PythonDownloadRequest};
 use uv_toolchain::managed::{InstalledToolchain, InstalledToolchains};
-use uv_toolchain::{requests_from_version_files, ToolchainRequest};
+use uv_toolchain::{requests_from_version_file, ToolchainRequest};
 use uv_warnings::warn_user;
 
 use crate::commands::ExitStatus;
@@ -35,7 +35,7 @@ pub(crate) async fn install(
     let toolchain_dir = toolchains.root();
 
     let requests: Vec<_> = if targets.is_empty() {
-        if let Some(requests) = requests_from_version_files().await? {
+        if let Some(requests) = requests_from_version_file().await? {
             requests
         } else {
             vec![ToolchainRequest::Any]

crates/uv/src/commands/venv.rs

@@ -23,7 +23,7 @@ use uv_dispatch::BuildDispatch;
 use uv_fs::Simplified;
 use uv_git::GitResolver;
 use uv_resolver::{ExcludeNewer, FlatIndex, InMemoryIndex, OptionsBuilder};
-use uv_toolchain::{SystemPython, Toolchain, ToolchainRequest};
+use uv_toolchain::{request_from_version_file, SystemPython, Toolchain, ToolchainRequest};
 use uv_types::{BuildContext, BuildIsolation, HashStrategy, InFlight};
 
 use crate::commands::{pip, ExitStatus};
@@ -125,9 +125,14 @@ async fn venv_impl(
         .connectivity(connectivity)
         .native_tls(native_tls);
 
+    let mut interpreter_request = python_request.map(ToolchainRequest::parse);
+    if preview.is_enabled() && interpreter_request.is_none() {
+        interpreter_request = request_from_version_file().await.into_diagnostic()?;
+    }
+
     // Locate the Python interpreter to use in the environment
     let interpreter = Toolchain::find_or_fetch(
-        python_request.map(ToolchainRequest::parse),
+        interpreter_request,
         SystemPython::Required,
         preview,
         client_builder,

crates/uv/tests/venv.rs

@@ -88,6 +88,114 @@ fn create_venv_ignores_virtual_env_variable() {
     );
 }
 
+#[test]
+fn create_venv_reads_request_from_python_version_file() {
+    let context = TestContext::new_with_versions(&["3.11", "3.12"]);
+
+    // Without the file, we should use the first on the PATH
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "###
+    );
+
+    // With a version file, we should prefer that version
+    context
+        .temp_dir
+        .child(".python-version")
+        .write_str("3.12")
+        .unwrap();
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.12.[X] interpreter at: [PYTHON-3.12]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "###
+    );
+
+    context.venv.assert(predicates::path::is_dir());
+}
+
+#[test]
+fn create_venv_reads_request_from_python_versions_file() {
+    let context = TestContext::new_with_versions(&["3.11", "3.12"]);
+
+    // Without the file, we should use the first on the PATH
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "###
+    );
+
+    // With a versions file, we should prefer the first listed version
+    context
+        .temp_dir
+        .child(".python-versions")
+        .write_str("3.12\n3.11")
+        .unwrap();
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.12.[X] interpreter at: [PYTHON-3.12]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "###
+    );
+
+    context.venv.assert(predicates::path::is_dir());
+}
+
+#[test]
+fn create_venv_explicit_request_takes_priority_over_python_version_file() {
+    let context = TestContext::new_with_versions(&["3.11", "3.12"]);
+
+    context
+        .temp_dir
+        .child(".python-version")
+        .write_str("3.12")
+        .unwrap();
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview").arg("--python").arg("3.11"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "###
+    );
+
+    context.venv.assert(predicates::path::is_dir());
+}
+
 #[test]
 fn seed() {
     let context = TestContext::new_with_versions(&["3.12"]);