Respect `.python-version` in `--isolated` runs (#5741)

Closes https://github.com/astral-sh/uv/issues/5740.

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

@@ -154,17 +154,23 @@ pub(crate) enum FoundInterpreter {
     Environment(PythonEnvironment),
 }
 
-impl FoundInterpreter {
-    /// Discover the interpreter to use in the current [`Workspace`].
-    pub(crate) async fn discover(
-        workspace: &Workspace,
+/// The resolved Python request and requirement for a [`Workspace`].
+#[derive(Debug, Clone)]
+pub(crate) struct WorkspacePython {
+    /// The resolved Python request, computed by considering (1) any explicit request from the user
+    /// via `--python`, (2) any implicit request from the user via `.python-version`, and (3) any
+    /// `Requires-Python` specifier in the `pyproject.toml`.
+    python_request: Option<PythonRequest>,
+    /// The resolved Python requirement for the project, computed by taking the intersection of all
+    /// `Requires-Python` specifiers in the workspace.
+    requires_python: Option<RequiresPython>,
+}
+
+impl WorkspacePython {
+    /// Determine the [`WorkspacePython`] for the current [`Workspace`].
+    pub(crate) async fn from_request(
         python_request: Option<PythonRequest>,
-        python_preference: PythonPreference,
-        python_fetch: PythonFetch,
-        connectivity: Connectivity,
-        native_tls: bool,
-        cache: &Cache,
-        printer: Printer,
+        workspace: &Workspace,
     ) -> Result<Self, ProjectError> {
         let requires_python = find_requires_python(workspace)?;
 
@@ -182,6 +188,31 @@ impl FoundInterpreter {
                 .map(|specifiers| PythonRequest::Version(VersionRequest::Range(specifiers.clone())))
         };
 
+        Ok(Self {
+            python_request,
+            requires_python,
+        })
+    }
+}
+
+impl FoundInterpreter {
+    /// Discover the interpreter to use in the current [`Workspace`].
+    pub(crate) async fn discover(
+        workspace: &Workspace,
+        python_request: Option<PythonRequest>,
+        python_preference: PythonPreference,
+        python_fetch: PythonFetch,
+        connectivity: Connectivity,
+        native_tls: bool,
+        cache: &Cache,
+        printer: Printer,
+    ) -> Result<Self, ProjectError> {
+        // Resolve the Python request and requirement for the workspace.
+        let WorkspacePython {
+            python_request,
+            requires_python,
+        } = WorkspacePython::from_request(python_request, workspace).await?;
+
         // Read from the virtual environment first.
         match find_environment(workspace, cache) {
             Ok(venv) => {

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

@@ -27,7 +27,7 @@ use uv_workspace::{DiscoveryOptions, VirtualProject, Workspace, WorkspaceError};
 
 use crate::commands::pip::operations::Modifications;
 use crate::commands::project::environment::CachedEnvironment;
-use crate::commands::project::ProjectError;
+use crate::commands::project::{ProjectError, WorkspacePython};
 use crate::commands::reporters::PythonDownloadReporter;
 use crate::commands::{pip, project, ExitStatus, SharedState};
 use crate::printer::Printer;
@@ -206,9 +206,16 @@ pub(crate) async fn run(
                         .connectivity(connectivity)
                         .native_tls(native_tls);
 
-                    // Note we force preview on during `uv run` for now since the entire interface is in preview
-                    PythonInstallation::find_or_fetch(
+                    // 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?;
+
+                    // Note we force preview on during `uv run` for now since the entire interface is in preview.
+                    PythonInstallation::find_or_fetch(
+                        python_request,
                         EnvironmentPreference::Any,
                         python_preference,
                         python_fetch,

crates/uv/tests/run.rs

@@ -915,3 +915,102 @@ fn run_without_output() -> Result<()> {
 
     Ok(())
 }
+
+/// Ensure that we can import from the root project when layering `--with` requirements.
+#[test]
+fn run_isolated_python_version() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.8", "3.12"]);
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! { r#"
+        [build-system]
+        requires = ["hatchling"]
+        build-backend = "hatchling.build"
+
+        [project]
+        name = "foo"
+        version = "1.0.0"
+        requires-python = ">=3.8"
+        dependencies = ["anyio"]
+        "#
+    })?;
+
+    let src = context.temp_dir.child("src").child("foo");
+    src.create_dir_all()?;
+
+    let init = src.child("__init__.py");
+    init.touch()?;
+
+    let main = context.temp_dir.child("main.py");
+    main.write_str(indoc! { r"
+        import sys
+
+        print((sys.version_info.major, sys.version_info.minor))
+       "
+    })?;
+
+    uv_snapshot!(context.filters(), context.run().arg("main.py"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    (3, 8)
+
+    ----- stderr -----
+    warning: `uv run` is experimental and may change without warning
+    Using Python 3.8.[X] interpreter at: [PYTHON-3.8]
+    Creating virtualenv at: .venv
+    Resolved 6 packages in [TIME]
+    Prepared 6 packages in [TIME]
+    Installed 6 packages in [TIME]
+     + anyio==4.3.0
+     + exceptiongroup==1.2.0
+     + foo==1.0.0 (from file://[TEMP_DIR]/)
+     + idna==3.6
+     + sniffio==1.3.1
+     + typing-extensions==4.10.0
+    "###);
+
+    uv_snapshot!(context.filters(), context.run().arg("--isolated").arg("main.py"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    (3, 8)
+
+    ----- stderr -----
+    warning: `uv run` is experimental and may change without warning
+    Resolved 6 packages in [TIME]
+    Prepared 5 packages in [TIME]
+    Installed 6 packages in [TIME]
+     + anyio==4.3.0
+     + exceptiongroup==1.2.0
+     + foo==1.0.0 (from file://[TEMP_DIR]/)
+     + idna==3.6
+     + sniffio==1.3.1
+     + typing-extensions==4.10.0
+    "###);
+
+    // Set the `.python-version` to `3.12`.
+    context
+        .temp_dir
+        .child(PYTHON_VERSION_FILENAME)
+        .write_str("3.12")?;
+
+    uv_snapshot!(context.filters(), context.run().arg("--isolated").arg("main.py"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    (3, 12)
+
+    ----- stderr -----
+    warning: `uv run` is experimental and may change without warning
+    Resolved 6 packages in [TIME]
+    Prepared 3 packages in [TIME]
+    Installed 4 packages in [TIME]
+     + anyio==4.3.0
+     + foo==1.0.0 (from file://[TEMP_DIR]/)
+     + idna==3.6
+     + sniffio==1.3.1
+    "###);
+
+    Ok(())
+}