Support .python-version outside current directory

Preparing for supporting this in uv run, update the .python-version
reading functions so that they can read from more than just the current
directory.

Minor behaviour change(!). In some cases previously it could
end up with different python versions depending on from which directory
inside a wider workspace an operation was called from (if there were
different .python-version files). I think this is clear from
get_or_init_environment's use of discover.

crates/uv-python/src/version_files.rs

@@ -1,3 +1,5 @@
+use std::path::Path;
+
 use fs_err as fs;
 use tracing::debug;
 
@@ -13,15 +15,17 @@ pub static PYTHON_VERSIONS_FILENAME: &str = ".python-versions";
 ///
 /// 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_file() -> Result<Option<Vec<PythonRequest>>, std::io::Error> {
-    if let Some(versions) = read_versions_file().await? {
+pub async fn requests_from_version_file(
+    directory: Option<&Path>,
+) -> Result<Option<Vec<PythonRequest>>, std::io::Error> {
+    if let Some(versions) = read_versions_file(directory).await? {
         Ok(Some(
             versions
                 .into_iter()
                 .map(|version| PythonRequest::parse(&version))
                 .collect(),
         ))
-    } else if let Some(version) = read_version_file().await? {
+    } else if let Some(version) = read_version_file(directory).await? {
         Ok(Some(vec![PythonRequest::parse(&version)]))
     } else {
         Ok(None)
@@ -30,12 +34,17 @@ pub async fn requests_from_version_file() -> Result<Option<Vec<PythonRequest>>,
 
 /// Read a [`PythonRequest`] from a version file, if present.
 ///
+/// Find the version file inside directory, or the current directory
+/// if None.
+///
 /// 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_file() -> Result<Option<PythonRequest>, std::io::Error> {
-    if let Some(version) = read_version_file().await? {
+pub async fn request_from_version_file(
+    directory: Option<&Path>,
+) -> Result<Option<PythonRequest>, std::io::Error> {
+    if let Some(version) = read_version_file(directory).await? {
         Ok(Some(PythonRequest::parse(&version)))
-    } else if let Some(versions) = read_versions_file().await? {
+    } else if let Some(versions) = read_versions_file(directory).await? {
         Ok(versions
             .into_iter()
             .next()
@@ -52,10 +61,17 @@ pub async fn write_version_file(version: &str) -> Result<(), std::io::Error> {
     fs::tokio::write(PYTHON_VERSION_FILENAME, format!("{version}\n")).await
 }
 
-async fn read_versions_file() -> Result<Option<Vec<String>>, std::io::Error> {
-    match fs::tokio::read_to_string(PYTHON_VERSIONS_FILENAME).await {
+async fn read_versions_file(
+    directory: Option<&Path>,
+) -> Result<Option<Vec<String>>, std::io::Error> {
+    let file_path = directory.map(|pth| pth.join(PYTHON_VERSIONS_FILENAME));
+    let path = file_path
+        .as_deref()
+        .unwrap_or(Path::new(PYTHON_VERSIONS_FILENAME));
+
+    match fs::tokio::read_to_string(path).await {
         Ok(content) => {
-            debug!("Reading requests from `{PYTHON_VERSIONS_FILENAME}`");
+            debug!("Reading requests from `{}`", path.display());
             Ok(Some(
                 content
                     .lines()
@@ -73,10 +89,15 @@ async fn read_versions_file() -> Result<Option<Vec<String>>, std::io::Error> {
     }
 }
 
-async fn read_version_file() -> Result<Option<String>, std::io::Error> {
-    match fs::tokio::read_to_string(PYTHON_VERSION_FILENAME).await {
+async fn read_version_file(directory: Option<&Path>) -> Result<Option<String>, std::io::Error> {
+    let file_path = directory.map(|pth| pth.join(PYTHON_VERSION_FILENAME));
+    let path = file_path
+        .as_deref()
+        .unwrap_or(Path::new(PYTHON_VERSION_FILENAME));
+
+    match fs::tokio::read_to_string(path).await {
         Ok(content) => {
-            debug!("Reading requests from `{PYTHON_VERSION_FILENAME}`");
+            debug!("Reading requests from `{}`", path.display());
             Ok(content
                 .lines()
                 .find(|line| {

crates/uv/src/commands/project/mod.rs

@@ -161,7 +161,9 @@ impl FoundInterpreter {
         let python_request = if let Some(request) = python_request {
             Some(request)
             // (2) Request from `.python-version`
-        } else if let Some(request) = request_from_version_file().await? {
+        } else if let Some(request) =
+            request_from_version_file(Some(workspace.install_path())).await?
+        {
             Some(request)
             // (3) `Requires-Python` in `pyproject.toml`
         } else {

crates/uv/src/commands/project/run.rs

@@ -102,7 +102,7 @@ pub(crate) async fn run(
             let python_request = if let Some(request) = python.as_deref() {
                 Some(PythonRequest::parse(request))
                 // (2) Request from `.python-version`
-            } else if let Some(request) = request_from_version_file().await? {
+            } else if let Some(request) = request_from_version_file(None).await? {
                 Some(request)
                 // (3) `Requires-Python` in `pyproject.toml`
             } else {

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

@@ -55,7 +55,7 @@ pub(crate) async fn install(
             }
             None
         } else {
-            requests_from_version_file().await?
+            requests_from_version_file(None).await?
         };
         version_file_requests.unwrap_or_else(|| vec![PythonRequest::Any])
     } else {

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

@@ -49,7 +49,7 @@ pub(crate) async fn pin(
 
     let Some(request) = request else {
         // Display the current pinned Python version
-        if let Some(pins) = requests_from_version_file().await? {
+        if let Some(pins) = requests_from_version_file(None).await? {
             for pin in pins {
                 writeln!(printer.stdout(), "{}", pin.to_canonical_string())?;
                 if let Some(virtual_project) = &virtual_project {
@@ -126,7 +126,7 @@ pub(crate) async fn pin(
         request.to_canonical_string()
     };
 
-    let existing = request_from_version_file().await.ok().flatten();
+    let existing = request_from_version_file(None).await.ok().flatten();
     write_version_file(&output).await?;
 
     if let Some(existing) = existing

crates/uv/src/commands/venv.rs

@@ -140,7 +140,7 @@ async fn venv_impl(
 
     let mut interpreter_request = python_request.map(PythonRequest::parse);
     if preview.is_enabled() && interpreter_request.is_none() {
-        interpreter_request = request_from_version_file().await.into_diagnostic()?;
+        interpreter_request = request_from_version_file(None).await.into_diagnostic()?;
     }
     if preview.is_disabled() && relocatable {
         warn_user_once!("`--relocatable` is experimental and may change without warning");