Use specialized error message for invalid Python install / uninstall requests (#5171)

## Summary

Closes https://github.com/astral-sh/uv/issues/4819.

crates/uv-python/src/downloads.rs

@@ -68,8 +68,6 @@ pub enum Error {
     NameError(String),
     #[error("Failed to parse request part")]
     InvalidRequestPlatform(#[from] platform::Error),
-    #[error("Cannot download managed Python for request: {0}")]
-    InvalidRequestKind(PythonRequest),
     // TODO(zanieb): Implement display for `PythonDownloadRequest`
     #[error("No download found for request: {0:?}")]
     NoDownloadFound(PythonDownloadRequest),
@@ -142,26 +140,23 @@ impl PythonDownloadRequest {
     ///
     /// Returns [`None`] if the request kind is not compatible with a download, e.g., it is
     /// a request for a specific directory or executable name.
-    pub fn try_from_request(request: &PythonRequest) -> Option<Self> {
-        Self::from_request(request).ok()
-    }
-
-    /// Construct a new [`PythonDownloadRequest`] from a [`PythonRequest`].
-    pub fn from_request(request: &PythonRequest) -> Result<Self, Error> {
+    pub fn from_request(request: &PythonRequest) -> Option<Self> {
         match request {
-            PythonRequest::Version(version) => Ok(Self::default().with_version(version.clone())),
+            PythonRequest::Version(version) => Some(Self::default().with_version(version.clone())),
             PythonRequest::Implementation(implementation) => {
-                Ok(Self::default().with_implementation(*implementation))
+                Some(Self::default().with_implementation(*implementation))
             }
-            PythonRequest::ImplementationVersion(implementation, version) => Ok(Self::default()
-                .with_implementation(*implementation)
-                .with_version(version.clone())),
-            PythonRequest::Key(request) => Ok(request.clone()),
-            PythonRequest::Any => Ok(Self::default()),
+            PythonRequest::ImplementationVersion(implementation, version) => Some(
+                Self::default()
+                    .with_implementation(*implementation)
+                    .with_version(version.clone()),
+            ),
+            PythonRequest::Key(request) => Some(request.clone()),
+            PythonRequest::Any => Some(Self::default()),
             // We can't download a managed installation for these request kinds
             PythonRequest::Directory(_)
             | PythonRequest::ExecutableName(_)
-            | PythonRequest::File(_) => Err(Error::InvalidRequestKind(request.clone())),
+            | PythonRequest::File(_) => None,
         }
     }
 

crates/uv-python/src/installation.rs

@@ -90,7 +90,7 @@ impl PythonInstallation {
 
         // Perform a fetch aggressively if managed Python is preferred
         if matches!(preference, PythonPreference::Managed) && python_fetch.is_automatic() {
-            if let Some(request) = PythonDownloadRequest::try_from_request(&request) {
+            if let Some(request) = PythonDownloadRequest::from_request(&request) {
                 return Self::fetch(request.fill(), client_builder, cache, reporter).await;
             }
         }
@@ -104,7 +104,7 @@ impl PythonInstallation {
                     && python_fetch.is_automatic()
                     && client_builder.connectivity.is_online() =>
             {
-                if let Some(request) = PythonDownloadRequest::try_from_request(&request) {
+                if let Some(request) = PythonDownloadRequest::from_request(&request) {
                     debug!("Requested Python not found, checking for available download...");
                     match Self::fetch(request.fill(), client_builder, cache, reporter).await {
                         Ok(installation) => Ok(installation),

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

@@ -12,7 +12,7 @@ use uv_cache::Cache;
 use uv_client::Connectivity;
 use uv_configuration::PreviewMode;
 use uv_fs::Simplified;
-use uv_python::downloads::{self, DownloadResult, ManagedPythonDownload, PythonDownloadRequest};
+use uv_python::downloads::{DownloadResult, ManagedPythonDownload, PythonDownloadRequest};
 use uv_python::managed::{ManagedPythonInstallation, ManagedPythonInstallations};
 use uv_python::{
     requests_from_version_file, PythonRequest, PYTHON_VERSIONS_FILENAME, PYTHON_VERSION_FILENAME,
@@ -67,8 +67,12 @@ pub(crate) async fn install(
 
     let download_requests = requests
         .iter()
-        .map(PythonDownloadRequest::from_request)
-        .collect::<Result<Vec<_>, downloads::Error>>()?;
+        .map(|request| {
+            PythonDownloadRequest::from_request(request).ok_or_else(|| {
+                anyhow::anyhow!("Cannot download managed Python for request: {request}")
+            })
+        })
+        .collect::<Result<Vec<_>>>()?;
 
     let installed_installations: Vec<_> = installations.find_all()?.collect();
     let mut unfilled_requests = Vec::new();

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

@@ -7,7 +7,7 @@ use itertools::Itertools;
 use owo_colors::OwoColorize;
 
 use uv_configuration::PreviewMode;
-use uv_python::downloads::{self, PythonDownloadRequest};
+use uv_python::downloads::PythonDownloadRequest;
 use uv_python::managed::ManagedPythonInstallations;
 use uv_python::PythonRequest;
 use uv_warnings::warn_user_once;
@@ -43,8 +43,12 @@ pub(crate) async fn uninstall(
 
     let download_requests = requests
         .iter()
-        .map(PythonDownloadRequest::from_request)
-        .collect::<Result<Vec<_>, downloads::Error>>()?;
+        .map(|request| {
+            PythonDownloadRequest::from_request(request).ok_or_else(|| {
+                anyhow::anyhow!("Cannot uninstall managed Python for request: {request}")
+            })
+        })
+        .collect::<Result<Vec<_>>>()?;
 
     let installed_installations: Vec<_> = installations.find_all()?.collect();
     let mut matching_installations = BTreeSet::default();