Add hint when Python downloads are disabled (#14522)

Follow-up to https://github.com/astral-sh/uv/pull/14509 to provide the
_reason_ downloads are disabled and surface it as a hint rather than a
debug log.

e.g.,

```
❯ cargo run -q -- run --no-managed-python -p 3.13.4 python
error: No interpreter found for Python 3.13.4 in virtual environments or search path

hint: A managed Python download is available for Python 3.13.4, but the Python preference is set to 'only system'
```

crates/uv-python/src/installation.rs

@@ -107,18 +107,9 @@ impl PythonInstallation {
             Err(err) => err,
         };
 
-        let downloads_enabled = preference.allows_managed()
-            && python_downloads.is_automatic()
-            && client_builder.connectivity.is_online();
-
-        if !downloads_enabled {
-            debug!("Python downloads are disabled. Skipping check for available downloads...");
-            return Err(err);
-        }
-
         match err {
             // If Python is missing, we should attempt a download
-            Error::MissingPython(_) => {}
+            Error::MissingPython(..) => {}
             // If we raised a non-critical error, we should attempt a download
             Error::Discovery(ref err) if !err.is_critical() => {}
             // Otherwise, this is fatal
@@ -126,40 +117,109 @@ impl PythonInstallation {
         }
 
         // If we can't convert the request to a download, throw the original error
-        let Some(request) = PythonDownloadRequest::from_request(request) else {
+        let Some(download_request) = PythonDownloadRequest::from_request(request) else {
             return Err(err);
         };
 
-        debug!("Requested Python not found, checking for available download...");
-        match Self::fetch(
-            request.fill()?,
+        let downloads_enabled = preference.allows_managed()
+            && python_downloads.is_automatic()
+            && client_builder.connectivity.is_online();
+
+        let download = download_request.clone().fill().map(|request| {
+            ManagedPythonDownload::from_request(&request, python_downloads_json_url)
+        });
+
+        // Regardless of whether downloads are enabled, we want to determine if the download is
+        // available to power error messages. However, if downloads aren't enabled, we don't want to
+        // report any errors related to them.
+        let download = match download {
+            Ok(Ok(download)) => Some(download),
+            // If the download cannot be found, return the _original_ discovery error
+            Ok(Err(downloads::Error::NoDownloadFound(_))) => {
+                if downloads_enabled {
+                    debug!("No downloads are available for {request}");
+                    return Err(err);
+                }
+                None
+            }
+            Err(err) | Ok(Err(err)) => {
+                if downloads_enabled {
+                    // We failed to determine the platform information
+                    return Err(err.into());
+                }
+                None
+            }
+        };
+
+        let Some(download) = download else {
+            // N.B. We should only be in this case when downloads are disabled; when downloads are
+            // enabled, we should fail eagerly when something goes wrong with the download.
+            debug_assert!(!downloads_enabled);
+            return Err(err);
+        };
+
+        // If the download is available, but not usable, we attach a hint to the original error.
+        if !downloads_enabled {
+            let for_request = match request {
+                PythonRequest::Default | PythonRequest::Any => String::new(),
+                _ => format!(" for {request}"),
+            };
+
+            match python_downloads {
+                PythonDownloads::Automatic => {}
+                PythonDownloads::Manual => {
+                    return Err(err.with_missing_python_hint(format!(
+                        "A managed Python download is available{for_request}, but Python downloads are set to 'manual', use `uv python install {}` to install the required version",
+                        request.to_canonical_string(),
+                    )));
+                }
+                PythonDownloads::Never => {
+                    return Err(err.with_missing_python_hint(format!(
+                        "A managed Python download is available{for_request}, but Python downloads are set to 'never'"
+                    )));
+                }
+            }
+
+            match preference {
+                PythonPreference::OnlySystem => {
+                    return Err(err.with_missing_python_hint(format!(
+                        "A managed Python download is available{for_request}, but the Python preference is set to 'only system'"
+                    )));
+                }
+                PythonPreference::Managed
+                | PythonPreference::OnlyManaged
+                | PythonPreference::System => {}
+            }
+
+            if !client_builder.connectivity.is_online() {
+                return Err(err.with_missing_python_hint(format!(
+                    "A managed Python download is available{for_request}, but uv is set to offline mode"
+                )));
+            }
+
+            return Err(err);
+        }
+
+        Self::fetch(
+            download,
             client_builder,
             cache,
             reporter,
             python_install_mirror,
             pypy_install_mirror,
-            python_downloads_json_url,
             preview,
         )
         .await
-        {
-            Ok(installation) => Ok(installation),
-            // Throw the original error if we couldn't find a download
-            Err(Error::Download(downloads::Error::NoDownloadFound(_))) => Err(err),
-            // But if the download failed, throw that error
-            Err(err) => Err(err),
-        }
     }
 
     /// Download and install the requested installation.
     pub async fn fetch(
-        request: PythonDownloadRequest,
+        download: &'static ManagedPythonDownload,
         client_builder: &BaseClientBuilder<'_>,
         cache: &Cache,
         reporter: Option<&dyn Reporter>,
         python_install_mirror: Option<&str>,
         pypy_install_mirror: Option<&str>,
-        python_downloads_json_url: Option<&str>,
         preview: PreviewMode,
     ) -> Result<Self, Error> {
         let installations = ManagedPythonInstallations::from_settings(None)?.init()?;
@@ -167,7 +227,6 @@ impl PythonInstallation {
         let scratch_dir = installations.scratch();
         let _lock = installations.lock().await?;
 
-        let download = ManagedPythonDownload::from_request(&request, python_downloads_json_url)?;
         let client = client_builder.build();
 
         info!("Fetching requested Python...");

crates/uv-python/src/lib.rs

@@ -1,4 +1,5 @@
 //! Find requested Python interpreters and query interpreters for information.
+use owo_colors::OwoColorize;
 use thiserror::Error;
 
 #[cfg(test)]
@@ -93,8 +94,8 @@ pub enum Error {
     #[error(transparent)]
     KeyError(#[from] installation::PythonInstallationKeyError),
 
-    #[error(transparent)]
-    MissingPython(#[from] PythonNotFound),
+    #[error("{}{}", .0, if let Some(hint) = .1 { format!("\n\n{}{} {hint}", "hint".bold().cyan(), ":".bold()) } else { String::new() })]
+    MissingPython(PythonNotFound, Option<String>),
 
     #[error(transparent)]
     MissingEnvironment(#[from] environment::EnvironmentNotFound),
@@ -103,6 +104,21 @@ pub enum Error {
     InvalidEnvironment(#[from] environment::InvalidEnvironment),
 }
 
+impl Error {
+    pub(crate) fn with_missing_python_hint(self, hint: String) -> Self {
+        match self {
+            Error::MissingPython(err, _) => Error::MissingPython(err, Some(hint)),
+            _ => self,
+        }
+    }
+}
+
+impl From<PythonNotFound> for Error {
+    fn from(err: PythonNotFound) -> Self {
+        Error::MissingPython(err, None)
+    }
+}
+
 // The mock interpreters are not valid on Windows so we don't have unit test coverage there
 // TODO(zanieb): We should write a mock interpreter script that works on Windows
 #[cfg(all(test, unix))]

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

@@ -129,7 +129,8 @@ pub(crate) async fn pin(
     {
         Ok(python) => Some(python),
         // If no matching Python version is found, don't fail unless `resolved` was requested
-        Err(uv_python::Error::MissingPython(err)) if !resolved => {
+        Err(uv_python::Error::MissingPython(err, ..)) if !resolved => {
+            // N.B. We omit the hint and just show the inner error message
             warn_user_once!("{err}");
             None
         }

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

@@ -195,6 +195,12 @@ impl TestContext {
             "managed installations, search path, or registry".to_string(),
             "[PYTHON SOURCES]".to_string(),
         ));
+        self.filters.push((
+            "registry or search path".to_string(),
+            "[PYTHON SOURCES]".to_string(),
+        ));
+        self.filters
+            .push(("search path".to_string(), "[PYTHON SOURCES]".to_string()));
         self
     }
 

crates/uv/tests/it/lock.rs

@@ -4318,14 +4318,16 @@ fn lock_requires_python() -> Result<()> {
 
     // Install from the lockfile.
     // Note we need to disable Python fetches or we'll just download 3.12
-    uv_snapshot!(context_unsupported.filters(), context_unsupported.sync().arg("--frozen").arg("--no-python-downloads"), @r###"
+    uv_snapshot!(context_unsupported.filters(), context_unsupported.sync().arg("--frozen").arg("--no-python-downloads"), @r"
     success: false
     exit_code: 2
     ----- stdout -----
 
     ----- stderr -----
     error: No interpreter found for Python >=3.12 in [PYTHON SOURCES]
-    "###);
+
+    hint: A managed Python download is available for Python >=3.12, but Python downloads are set to 'never'
+    ");
 
     Ok(())
 }

crates/uv/tests/it/python_find.rs

@@ -873,6 +873,8 @@ fn python_find_script_python_not_found() {
 
     ----- stderr -----
     No interpreter found in [PYTHON SOURCES]
+
+    hint: A managed Python download is available, but Python downloads are set to 'never'
     ");
 }
 

crates/uv/tests/it/python_install.rs

@@ -196,14 +196,16 @@ fn python_install_automatic() {
     uv_snapshot!(context.filters(), context.run()
         .env_remove("VIRTUAL_ENV")
         .arg("--no-python-downloads")
-        .arg("python").arg("-c").arg("import sys; print(sys.version_info[:2])"), @r###"
+        .arg("python").arg("-c").arg("import sys; print(sys.version_info[:2])"), @r"
     success: false
     exit_code: 2
     ----- stdout -----
 
     ----- stderr -----
     error: No interpreter found in [PYTHON SOURCES]
-    "###);
+
+    hint: A managed Python download is available, but Python downloads are set to 'never'
+    ");
 
     // Otherwise, we should fetch the latest Python version
     uv_snapshot!(context.filters(), context.run()

crates/uv/tests/it/python_pin.rs

@@ -164,7 +164,7 @@ fn python_pin() {
     // (skip on Windows because the snapshot is different and the behavior is not platform dependent)
     #[cfg(unix)]
     {
-        uv_snapshot!(context.filters(), context.python_pin().arg("pypy"), @r###"
+        uv_snapshot!(context.filters(), context.python_pin().arg("pypy"), @r"
         success: true
         exit_code: 0
         ----- stdout -----
@@ -172,7 +172,7 @@ fn python_pin() {
 
         ----- stderr -----
         warning: No interpreter found for PyPy in managed installations or search path
-        "###);
+        ");
 
         let python_version = context.read(PYTHON_VERSION_FILENAME);
         assert_snapshot!(python_version, @r###"
@@ -361,7 +361,7 @@ fn python_pin_global_creates_parent_dirs() {
 fn python_pin_no_python() {
     let context: TestContext = TestContext::new_with_versions(&[]);
 
-    uv_snapshot!(context.filters(), context.python_pin().arg("3.12"), @r###"
+    uv_snapshot!(context.filters(), context.python_pin().arg("3.12"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -369,7 +369,7 @@ fn python_pin_no_python() {
 
     ----- stderr -----
     warning: No interpreter found for Python 3.12 in managed installations or search path
-    "###);
+    ");
 }
 
 #[test]
@@ -448,7 +448,7 @@ fn python_pin_compatible_with_requires_python() -> Result<()> {
     "###);
 
     // Request a version that is compatible and uses a Python variant
-    uv_snapshot!(context.filters(), context.python_pin().arg("3.13t"), @r###"
+    uv_snapshot!(context.filters(), context.python_pin().arg("3.13t"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -456,7 +456,7 @@ fn python_pin_compatible_with_requires_python() -> Result<()> {
 
     ----- stderr -----
     warning: No interpreter found for Python 3.13t in [PYTHON SOURCES]
-    "###);
+    ");
 
     // Request a implementation version that is compatible
     uv_snapshot!(context.filters(), context.python_pin().arg("cpython@3.11"), @r###"
@@ -587,27 +587,17 @@ fn warning_pinned_python_version_not_installed() -> Result<()> {
 /// We do need a Python interpreter for `--resolved` pins
 #[test]
 fn python_pin_resolve_no_python() {
-    let context: TestContext = TestContext::new_with_versions(&[]);
+    let context: TestContext = TestContext::new_with_versions(&[]).with_filtered_python_sources();
+    uv_snapshot!(context.filters(), context.python_pin().arg("--resolved").arg("3.12"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
 
-    if cfg!(windows) {
-        uv_snapshot!(context.filters(), context.python_pin().arg("--resolved").arg("3.12"), @r###"
-        success: false
-        exit_code: 2
-        ----- stdout -----
+    ----- stderr -----
+    error: No interpreter found for Python 3.12 in [PYTHON SOURCES]
 
-        ----- stderr -----
-        error: No interpreter found for Python 3.12 in managed installations, search path, or registry
-        "###);
-    } else {
-        uv_snapshot!(context.filters(), context.python_pin().arg("--resolved").arg("3.12"), @r###"
-        success: false
-        exit_code: 2
-        ----- stdout -----
-
-        ----- stderr -----
-        error: No interpreter found for Python 3.12 in managed installations or search path
-        "###);
-    }
+    hint: A managed Python download is available for Python 3.12, but Python downloads are set to 'never'
+    ");
 }
 
 #[test]
@@ -741,14 +731,16 @@ fn python_pin_resolve() {
     // Request an implementation that is not installed
     // (skip on Windows because the snapshot is different and the behavior is not platform dependent)
     #[cfg(unix)]
-    uv_snapshot!(context.filters(), context.python_pin().arg("--resolved").arg("pypy"), @r###"
+    uv_snapshot!(context.filters(), context.python_pin().arg("--resolved").arg("pypy"), @r"
     success: false
     exit_code: 2
     ----- stdout -----
 
     ----- stderr -----
     error: No interpreter found for PyPy in managed installations or search path
-    "###);
+
+    hint: A managed Python download is available for PyPy, but Python downloads are set to 'never'
+    ");
 
     let python_version = context.read(PYTHON_VERSION_FILENAME);
     insta::with_settings!({

crates/uv/tests/it/tool_run.rs

@@ -2049,6 +2049,54 @@ fn tool_run_python_at_version() {
     "###);
 }
 
+#[test]
+fn tool_run_hint_version_not_available() {
+    let context = TestContext::new_with_versions(&[])
+        .with_filtered_counts()
+        .with_filtered_python_sources();
+
+    uv_snapshot!(context.filters(), context.tool_run()
+        .arg("python@3.12")
+        .env(EnvVars::UV_PYTHON_DOWNLOADS, "never"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: No interpreter found for Python 3.12 in [PYTHON SOURCES]
+
+    hint: A managed Python download is available for Python 3.12, but Python downloads are set to 'never'
+    ");
+
+    uv_snapshot!(context.filters(), context.tool_run()
+        .arg("python@3.12")
+        .env(EnvVars::UV_PYTHON_DOWNLOADS, "auto")
+        .env(EnvVars::UV_OFFLINE, "true"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: No interpreter found for Python 3.12 in [PYTHON SOURCES]
+
+    hint: A managed Python download is available for Python 3.12, but uv is set to offline mode
+    ");
+
+    uv_snapshot!(context.filters(), context.tool_run()
+        .arg("python@3.12")
+        .env(EnvVars::UV_PYTHON_DOWNLOADS, "auto")
+        .env(EnvVars::UV_NO_MANAGED_PYTHON, "true"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: No interpreter found for Python 3.12 in [PYTHON SOURCES]
+
+    hint: A managed Python download is available for Python 3.12, but the Python preference is set to 'only system'
+    ");
+}
+
 #[test]
 fn tool_run_python_from() {
     let context = TestContext::new_with_versions(&["3.12", "3.11"])