Move version file reading to uv-toolchain (#4340)

In preparation for using this in other commands.

crates/uv-toolchain/src/lib.rs

@@ -13,6 +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::virtualenv::{Error as VirtualEnvError, PyVenvConfiguration, VirtualEnvironment};
 
 mod discovery;
@@ -28,6 +29,7 @@ mod py_launcher;
 mod python_version;
 mod target;
 mod toolchain;
+mod version_files;
 mod virtualenv;
 
 #[cfg(not(test))]

crates/uv-toolchain/src/version_files.rs

@@ -0,0 +1,67 @@
+use fs_err as fs;
+use std::{io, path::PathBuf};
+use tracing::debug;
+
+use crate::ToolchainRequest;
+
+/// Read [`ToolchainRequest`]s from a version file, if present.
+///
+/// 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> {
+    if let Some(versions) = read_versions_file().await? {
+        Ok(Some(
+            versions
+                .into_iter()
+                .map(|version| ToolchainRequest::parse(&version))
+                .collect(),
+        ))
+    } else if let Some(version) = read_version_file().await? {
+        Ok(Some(vec![ToolchainRequest::parse(&version)]))
+    } else {
+        Ok(None)
+    }
+}
+
+/// Read a [`ToolchainRequest`] from a version file, if present.
+///
+/// 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> {
+    if let Some(version) = read_version_file().await? {
+        Ok(Some(ToolchainRequest::parse(&version)))
+    } else if let Some(versions) = read_versions_file().await? {
+        Ok(versions
+            .into_iter()
+            .next()
+            .inspect(|_| debug!("Using the first version from `.python-versions`"))
+            .map(|version| ToolchainRequest::parse(&version)))
+    } else {
+        Ok(None)
+    }
+}
+
+async fn read_versions_file() -> Result<Option<Vec<String>>, io::Error> {
+    if !PathBuf::from(".python-versions").try_exists()? {
+        return Ok(None);
+    }
+    debug!("Reading requests from `.python-versions`");
+    let lines: Vec<String> = fs::tokio::read_to_string(".python-versions")
+        .await?
+        .lines()
+        .map(ToString::to_string)
+        .collect();
+    Ok(Some(lines))
+}
+
+async fn read_version_file() -> Result<Option<String>, io::Error> {
+    if !PathBuf::from(".python-version").try_exists()? {
+        return Ok(None);
+    }
+    debug!("Reading requests from `.python-version`");
+    Ok(fs::tokio::read_to_string(".python-version")
+        .await?
+        .lines()
+        .next()
+        .map(ToString::to_string))
+}

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

@@ -1,16 +1,14 @@
 use anyhow::Result;
-use fs_err as fs;
 use futures::StreamExt;
 use itertools::Itertools;
 use std::fmt::Write;
-use std::path::PathBuf;
 use uv_cache::Cache;
 use uv_client::Connectivity;
 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::ToolchainRequest;
+use uv_toolchain::{requests_from_version_files, ToolchainRequest};
 use uv_warnings::warn_user;
 
 use crate::commands::ExitStatus;
@@ -37,15 +35,10 @@ pub(crate) async fn install(
     let toolchain_dir = toolchains.root();
 
     let requests: Vec<_> = if targets.is_empty() {
-        if let Some(versions) = read_versions_file().await? {
-            versions
-                .into_iter()
-                .map(|version| ToolchainRequest::parse(&version))
-                .collect()
-        } else if let Some(version) = read_version_file().await? {
-            vec![ToolchainRequest::parse(&version)]
+        if let Some(requests) = requests_from_version_files().await? {
+            requests
         } else {
-            vec![ToolchainRequest::default()]
+            vec![ToolchainRequest::Any]
         }
     } else {
         targets
@@ -162,26 +155,3 @@ pub(crate) async fn install(
 
     Ok(ExitStatus::Success)
 }
-
-async fn read_versions_file() -> Result<Option<Vec<String>>> {
-    if !PathBuf::from(".python-versions").try_exists()? {
-        return Ok(None);
-    }
-    let lines: Vec<String> = fs::tokio::read_to_string(".python-versions")
-        .await?
-        .lines()
-        .map(ToString::to_string)
-        .collect();
-    Ok(Some(lines))
-}
-
-async fn read_version_file() -> Result<Option<String>> {
-    if !PathBuf::from(".python-version").try_exists()? {
-        return Ok(None);
-    }
-    Ok(fs::tokio::read_to_string(".python-version")
-        .await?
-        .lines()
-        .next()
-        .map(ToString::to_string))
-}