Discover and respect .python-version files in parent directories

Author: zanieb

crates/uv-python/src/version_files.rs

@@ -4,6 +4,7 @@ use std::path::{Path, PathBuf};
 use fs_err as fs;
 use itertools::Itertools;
 use tracing::debug;
+use uv_fs::Simplified;
 
 use crate::PythonRequest;
 
@@ -23,37 +24,54 @@ pub struct PythonVersionFile {
 }
 
 impl PythonVersionFile {
-    /// Find a Python version file in the given directory.
+    /// Find a Python version file in the given directory or any of its parents.
     pub async fn discover(
         working_directory: impl AsRef<Path>,
         // TODO(zanieb): Create a `DiscoverySettings` struct for these options
         no_config: bool,
         prefer_versions: bool,
     ) -> Result<Option<Self>, std::io::Error> {
-        let versions_path = working_directory.as_ref().join(PYTHON_VERSIONS_FILENAME);
-        let version_path = working_directory.as_ref().join(PYTHON_VERSION_FILENAME);
+        let Some(path) = Self::find_nearest(working_directory, prefer_versions) else {
+            return Ok(None);
+        };
 
         if no_config {
-            if version_path.exists() {
-                debug!("Ignoring `.python-version` file due to `--no-config`");
-            } else if versions_path.exists() {
-                debug!("Ignoring `.python-versions` file due to `--no-config`");
-            };
+            debug!(
+                "Ignoring Python version file at `{}` due to `--no-config`",
+                path.user_display()
+            );
             return Ok(None);
         }
 
-        let paths = if prefer_versions {
-            [versions_path, version_path]
-        } else {
-            [version_path, versions_path]
-        };
-        for path in paths {
-            if let Some(result) = Self::try_from_path(path).await? {
-                return Ok(Some(result));
+        // Uses `try_from_path` instead of `from_path` to avoid TOCTOU failures.
+        Self::try_from_path(path).await
+    }
+
+    fn find_nearest(working_directory: impl AsRef<Path>, prefer_versions: bool) -> Option<PathBuf> {
+        let mut current = working_directory.as_ref();
+        loop {
+            let version_path = current.join(PYTHON_VERSION_FILENAME);
+            let versions_path = current.join(PYTHON_VERSIONS_FILENAME);
+
+            let paths = if prefer_versions {
+                [versions_path, version_path]
+            } else {
+                [version_path, versions_path]
             };
+            for path in paths {
+                if path.exists() {
+                    return Some(path);
+                }
+            }
+
+            if let Some(parent) = current.parent() {
+                current = parent;
+            } else {
+                break;
+            }
         }
 
-        Ok(None)
+        None
     }
 
     /// Try to read a Python version file at the given path.

crates/uv/tests/python_find.rs

@@ -197,6 +197,38 @@ fn python_find_pin() {
 
     ----- stderr -----
     "###);
+
+    let child_dir = context.temp_dir.child("child");
+    child_dir.create_dir_all().unwrap();
+
+    // We should also find pinned versions in the parent directory
+    uv_snapshot!(context.filters(), context.python_find().current_dir(&child_dir), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    [PYTHON-3.12]
+
+    ----- stderr -----
+    "###);
+
+    uv_snapshot!(context.filters(), context.python_pin().arg("3.11").current_dir(&child_dir), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Updated `.python-version` from `3.12` -> `3.11`
+
+    ----- stderr -----
+    "###);
+
+    // Unless the child directory also has a pin
+    uv_snapshot!(context.filters(), context.python_find().current_dir(&child_dir), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    [PYTHON-3.11]
+
+    ----- stderr -----
+    "###);
 }
 
 #[test]