Show non-critical Python discovery errors if no other interpreter is found (#10716)

Previously, these errors would only be visible in the debug logs as
"Skipping bad interpreter ..." which can lead us to making some
ridiculous claims like "There is no virtual environment" or "Python is
not installed" when really we just failed to query the interpreter for
some reason.

We show the first error, sort of arbitrarily — but I think it matches
user expectation, i.e., this would be the first Python on your PATH.

Related to #10713

crates/uv-python/src/discovery.rs

@@ -977,9 +977,16 @@ pub(crate) fn find_python_installation(
 ) -> Result<FindPythonResult, Error> {
     let installations = find_python_installations(request, environments, preference, cache);
     let mut first_prerelease = None;
+    let mut first_error = None;
     for result in installations {
         // Iterate until the first critical error or happy result
         if !result.as_ref().err().map_or(true, Error::is_critical) {
+            // Track the first non-critical error
+            if first_error.is_none() {
+                if let Err(err) = result {
+                    first_error = Some(err);
+                }
+            }
             continue;
         }
 
@@ -1032,6 +1039,12 @@ pub(crate) fn find_python_installation(
         return Ok(Ok(installation));
     }
 
+    // If we found a Python, but it was unusable for some reason, report that instead of saying we
+    // couldn't find any Python interpreters.
+    if let Some(err) = first_error {
+        return Err(err);
+    }
+
     Ok(Err(PythonNotFound {
         request: request.clone(),
         environment_preference: environments,

crates/uv-python/src/lib.rs

@@ -111,7 +111,7 @@ mod tests {
 
     use crate::{
         discovery::{
-            find_best_python_installation, find_python_installation, EnvironmentPreference,
+            self, find_best_python_installation, find_python_installation, EnvironmentPreference,
         },
         PythonPreference,
     };
@@ -589,11 +589,10 @@ mod tests {
                 PythonPreference::default(),
                 &context.cache,
             )
-        })?;
+        });
         assert!(
-            matches!(result, Err(PythonNotFound { .. })),
+            matches!(result, Err(discovery::Error::Query(..))),
-            // TODO(zanieb): We could improve the error handling to hint this to the user
+            "If only Python 2 is available, we should report the interpreter query error; got {result:?}"
-            "If only Python 2 is available, we should not find a python; got {result:?}"
         );
 
         Ok(())

crates/uv/tests/it/common/mod.rs

@@ -636,7 +636,7 @@ impl TestContext {
             .env(EnvVars::UV_PREVIEW, "1")
             .env(EnvVars::UV_PYTHON_INSTALL_DIR, "")
             .current_dir(&self.temp_dir);
-        self.add_shared_args(&mut command, true);
+        self.add_shared_args(&mut command, false);
         command
     }
 

crates/uv/tests/it/pip_sync.rs

@@ -51,7 +51,10 @@ fn missing_requirements_txt() {
 
 #[test]
 fn missing_venv() -> Result<()> {
-    let context = TestContext::new("3.12");
+    let context = TestContext::new("3.12")
+        .with_filtered_virtualenv_bin()
+        .with_filtered_python_names();
+
     let requirements = context.temp_dir.child("requirements.txt");
     requirements.write_str("anyio")?;
     fs::remove_dir_all(&context.venv)?;
@@ -61,6 +64,19 @@ fn missing_venv() -> Result<()> {
     exit_code: 2
     ----- stdout -----
 
+    ----- stderr -----
+    error: Failed to inspect Python interpreter from active virtual environment at `.venv/[BIN]/python`
+      Caused by: Python interpreter not found at `[VENV]/[BIN]/python`
+    "###);
+
+    assert!(predicates::path::missing().eval(&context.venv));
+
+    // If not "active", we hint to create one
+    uv_snapshot!(context.filters(), context.pip_sync().arg("requirements.txt").env_remove("VIRTUAL_ENV"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
     ----- stderr -----
     error: No virtual environment found; run `uv venv` to create an environment, or pass `--system` to install into a non-virtual environment
     "###);

crates/uv/tests/it/python_find.rs

@@ -569,7 +569,7 @@ fn python_find_venv_invalid() {
         .with_filtered_virtualenv_bin();
 
     // We find the virtual environment
-    uv_snapshot!(context.filters(), context.python_find(), @r###"
+    uv_snapshot!(context.filters(), context.python_find().env(EnvVars::VIRTUAL_ENV, context.venv.as_os_str()), @r###"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -581,7 +581,7 @@ fn python_find_venv_invalid() {
     // If the binaries are missing from a virtual environment, we fail
     fs_err::remove_dir_all(venv_bin_path(&context.venv)).unwrap();
 
-    uv_snapshot!(context.filters(), context.python_find(), @r###"
+    uv_snapshot!(context.filters(), context.python_find().env(EnvVars::VIRTUAL_ENV, context.venv.as_os_str()), @r###"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -592,7 +592,7 @@ fn python_find_venv_invalid() {
     "###);
 
     // Unless the virtual environment is not active
-    uv_snapshot!(context.filters(), context.python_find().env_remove(EnvVars::VIRTUAL_ENV), @r###"
+    uv_snapshot!(context.filters(), context.python_find(), @r###"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -604,7 +604,7 @@ fn python_find_venv_invalid() {
     // If there's not a `pyvenv.cfg` file, it's also non-fatal, we ignore the environment
     fs_err::remove_file(context.venv.join("pyvenv.cfg")).unwrap();
 
-    uv_snapshot!(context.filters(), context.python_find(), @r###"
+    uv_snapshot!(context.filters(), context.python_find().env(EnvVars::VIRTUAL_ENV, context.venv.as_os_str()), @r###"
     success: true
     exit_code: 0
     ----- stdout -----