Error when discovered Python is incompatible with --isolated workspace (#6885)

## Summary

We should have consistent errors with and without `--isolated`.

crates/uv/src/commands/project/mod.rs

@@ -146,6 +146,46 @@ pub(crate) fn find_requires_python(
     }))
 }
 
+/// Returns an error if the [`Interpreter`] does not satisfy the [`Workspace`] `requires-python`.
+#[allow(clippy::result_large_err)]
+pub(crate) fn validate_requires_python(
+    interpreter: &Interpreter,
+    workspace: &Workspace,
+    requires_python: &RequiresPython,
+) -> Result<(), ProjectError> {
+    if !requires_python.contains(interpreter.python_version()) {
+        // If the Python version is compatible with one of the workspace _members_, raise
+        // a dedicated error. For example, if the workspace root requires Python >=3.12, but
+        // a library in the workspace is compatible with Python >=3.8, the user may attempt
+        // to sync on Python 3.8. This will fail, but we should provide a more helpful error
+        // message.
+        for (name, member) in workspace.packages() {
+            let Some(project) = member.pyproject_toml().project.as_ref() else {
+                continue;
+            };
+            let Some(specifiers) = project.requires_python.as_ref() else {
+                continue;
+            };
+            if specifiers.contains(interpreter.python_version()) {
+                return Err(ProjectError::RequestedMemberPythonIncompatibility(
+                    interpreter.python_version().clone(),
+                    requires_python.clone(),
+                    name.clone(),
+                    specifiers.clone(),
+                    member.root().clone(),
+                ));
+            }
+        }
+
+        return Err(ProjectError::RequestedPythonIncompatibility(
+            interpreter.python_version().clone(),
+            requires_python.clone(),
+        ));
+    }
+
+    Ok(())
+}
+
 /// Find the virtual environment for the current project.
 fn find_environment(
     workspace: &Workspace,
@@ -297,35 +337,7 @@ impl FoundInterpreter {
         }
 
         if let Some(requires_python) = requires_python.as_ref() {
-            if !requires_python.contains(interpreter.python_version()) {
-                // If the Python version is compatible with one of the workspace _members_, raise
-                // a dedicated error. For example, if the workspace root requires Python >=3.12, but
-                // a library in the workspace is compatible with Python >=3.8, the user may attempt
-                // to sync on Python 3.8. This will fail, but we should provide a more helpful error
-                // message.
-                for (name, member) in workspace.packages() {
-                    let Some(project) = member.pyproject_toml().project.as_ref() else {
-                        continue;
-                    };
-                    let Some(specifiers) = project.requires_python.as_ref() else {
-                        continue;
-                    };
-                    if specifiers.contains(interpreter.python_version()) {
-                        return Err(ProjectError::RequestedMemberPythonIncompatibility(
-                            interpreter.python_version().clone(),
-                            requires_python.clone(),
-                            name.clone(),
-                            specifiers.clone(),
-                            member.root().clone(),
-                        ));
-                    }
-                }
-
-                return Err(ProjectError::RequestedPythonIncompatibility(
-                    interpreter.python_version().clone(),
-                    requires_python.clone(),
-                ));
-            }
+            validate_requires_python(&interpreter, workspace, requires_python)?;
         }
 
         Ok(Self::Interpreter(interpreter))

crates/uv/src/commands/project/run.rs

@@ -34,7 +34,7 @@ use crate::commands::pip::loggers::{
 use crate::commands::pip::operations;
 use crate::commands::pip::operations::Modifications;
 use crate::commands::project::environment::CachedEnvironment;
-use crate::commands::project::{ProjectError, WorkspacePython};
+use crate::commands::project::{validate_requires_python, ProjectError, WorkspacePython};
 use crate::commands::reporters::PythonDownloadReporter;
 use crate::commands::{project, ExitStatus, SharedState};
 use crate::printer::Printer;
@@ -352,30 +352,35 @@ pub(crate) async fn run(
 
                 // If we're isolating the environment, use an ephemeral virtual environment as the
                 // base environment for the project.
-                let interpreter = {
-                    let client_builder = BaseClientBuilder::new()
-                        .connectivity(connectivity)
-                        .native_tls(native_tls);
+                let client_builder = BaseClientBuilder::new()
+                    .connectivity(connectivity)
+                    .native_tls(native_tls);
 
-                    // Resolve the Python request and requirement for the workspace.
-                    let WorkspacePython { python_request, .. } = WorkspacePython::from_request(
-                        python.as_deref().map(PythonRequest::parse),
-                        project.workspace(),
-                    )
-                    .await?;
+                // Resolve the Python request and requirement for the workspace.
+                let WorkspacePython {
+                    python_request,
+                    requires_python,
+                } = WorkspacePython::from_request(
+                    python.as_deref().map(PythonRequest::parse),
+                    project.workspace(),
+                )
+                .await?;
 
-                    PythonInstallation::find_or_download(
-                        python_request.as_ref(),
-                        EnvironmentPreference::Any,
-                        python_preference,
-                        python_downloads,
-                        &client_builder,
-                        cache,
-                        Some(&download_reporter),
-                    )
-                    .await?
-                    .into_interpreter()
-                };
+                let interpreter = PythonInstallation::find_or_download(
+                    python_request.as_ref(),
+                    EnvironmentPreference::Any,
+                    python_preference,
+                    python_downloads,
+                    &client_builder,
+                    cache,
+                    Some(&download_reporter),
+                )
+                .await?
+                .into_interpreter();
+
+                if let Some(requires_python) = requires_python.as_ref() {
+                    validate_requires_python(&interpreter, project.workspace(), requires_python)?;
+                }
 
                 // Create a virtual environment
                 temp_dir = cache.environment()?;

crates/uv/tests/run.rs

@@ -1617,3 +1617,57 @@ fn run_project_toml_error() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn run_isolated_incompatible_python() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.8", "3.11"]);
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! { r#"
+        [project]
+        name = "foo"
+        version = "1.0.0"
+        requires-python = ">=3.12"
+        dependencies = ["iniconfig"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+        "#
+    })?;
+
+    let python_version = context.temp_dir.child(PYTHON_VERSION_FILENAME);
+    python_version.write_str("3.8")?;
+
+    let test_script = context.temp_dir.child("main.py");
+    test_script.write_str(indoc! { r#"
+        import iniconfig
+
+        x: str | int = "hello"
+        print(x)
+       "#
+    })?;
+
+    // We should reject Python 3.8...
+    uv_snapshot!(context.filters(), context.run().arg("main.py"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.8.[X] interpreter at: [PYTHON-3.8]
+    error: The requested Python interpreter (3.8.[X]) is incompatible with the project Python requirement: `>=3.12`
+    "###);
+
+    // ...even if `--isolated` is provided.
+    uv_snapshot!(context.filters(), context.run().arg("--isolated").arg("main.py"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: The requested Python interpreter (3.8.[X]) is incompatible with the project Python requirement: `>=3.12`
+    "###);
+
+    Ok(())
+}