Add `--python` support to `uv run` (#3189)

crates/uv/src/cli.rs

@@ -1706,6 +1706,20 @@ pub(crate) struct RunArgs {
     #[arg(long)]
     pub(crate) with: Vec<String>,
 
+    /// The Python interpreter to use to build the run environment.
+    ///
+    /// By default, `uv` uses the virtual environment in the current working directory or any parent
+    /// directory, falling back to searching for a Python executable in `PATH`. The `--python`
+    /// option allows you to specify a different interpreter.
+    ///
+    /// Supported formats:
+    /// - `3.10` looks for an installed Python 3.10 using `py --list-paths` on Windows, or
+    ///   `python3.10` on Linux and macOS.
+    /// - `python3.10` or `python.exe` looks for a binary with the given name in `PATH`.
+    /// - `/home/ferris/.local/bin/python3.10` uses the exact Python at the given path.
+    #[arg(long, short, verbatim_doc_comment, group = "discovery")]
+    pub(crate) python: Option<String>,
+
     /// Run without the current workspace installed.
     #[arg(long)]
     pub(crate) no_workspace: bool,

crates/uv/src/commands/run.rs

@@ -45,6 +45,7 @@ pub(crate) async fn run(
     target: Option<String>,
     mut args: Vec<OsString>,
     mut requirements: Vec<RequirementsSource>,
+    python: Option<String>,
     isolated: bool,
     no_workspace: bool,
     preview: PreviewMode,
@@ -86,7 +87,15 @@ pub(crate) async fn run(
     // Detect the current Python interpreter.
     // TODO(zanieb): Create ephemeral environments
     // TODO(zanieb): Accept `--python`
-    let run_env = environment_for_run(&requirements, &overrides, isolated, cache, printer).await?;
+    let run_env = environment_for_run(
+        &requirements,
+        &overrides,
+        python.as_deref(),
+        isolated,
+        cache,
+        printer,
+    )
+    .await?;
     let python_env = run_env.python;
 
     // Construct the command
@@ -164,6 +173,7 @@ fn find_workspace_requirements() -> Result<Option<Vec<RequirementsSource>>> {
 async fn environment_for_run(
     requirements: &[RequirementsSource],
     overrides: &[RequirementsSource],
+    python: Option<&str>,
     isolated: bool,
     cache: &Cache,
     printer: Printer,
@@ -194,8 +204,19 @@ async fn environment_for_run(
     )
     .await?;
 
+    // Determine an interpreter to use
+    let python_env = if let Some(python) = python {
+        PythonEnvironment::from_requested_python(python, cache)?
+    } else {
+        PythonEnvironment::from_default_python(cache)?
+    };
+
     // Check if the current environment satisfies the requirements
     if let Some(venv) = current_venv {
+        // Ensure it matches the selected interpreter
+        // TODO(zanieb): We should check if a version was requested and see if the environment meets that
+        //               too but this can wait until we refactor interpreter discovery
+        if venv.root() == python_env.root() {
             // Determine the set of installed packages.
             let site_packages = SitePackages::from_executable(&venv)?;
 
@@ -203,7 +224,11 @@ async fn environment_for_run(
             // enough to let us remove this check. But right now, for large environments, it's an order of
             // magnitude faster to validate the environment than to resolve the requirements.
             if spec.source_trees.is_empty()
-            && site_packages.satisfies(&spec.requirements, &spec.editables, &spec.constraints)?
+                && site_packages.satisfies(
+                    &spec.requirements,
+                    &spec.editables,
+                    &spec.constraints,
+                )?
             {
                 debug!("Current environment satisfies requirements");
                 return Ok(RunEnvironment {
@@ -212,17 +237,9 @@ async fn environment_for_run(
                 });
             }
         }
+    }
     // Otherwise, we need a new environment
 
-    // Find an interpreter to use
-    // TODO(zanieb): Populate `python` from the user
-    let python = None;
-    let python_env = if let Some(python) = python {
-        PythonEnvironment::from_requested_python(python, cache)?
-    } else {
-        PythonEnvironment::from_default_python(cache)?
-    };
-
     // Create a virtual environment
     // TODO(zanieb): Move this path derivation elsewhere
     let uv_state_path = std::env::current_dir()?.join(".uv");

crates/uv/src/main.rs

@@ -507,6 +507,7 @@ async fn run() -> Result<ExitStatus> {
                 args.target,
                 args.args,
                 requirements,
+                args.python,
                 args.isolated,
                 args.no_workspace,
                 globals.preview,

crates/uv/src/settings.rs

@@ -88,6 +88,7 @@ pub(crate) struct RunSettings {
     pub(crate) isolated: bool,
     pub(crate) with: Vec<String>,
     pub(crate) no_workspace: bool,
+    pub(crate) python: Option<String>,
 }
 
 impl RunSettings {
@@ -100,6 +101,7 @@ impl RunSettings {
             isolated,
             with,
             no_workspace,
+            python,
         } = args;
 
         Self {
@@ -109,6 +111,7 @@ impl RunSettings {
             isolated,
             with,
             no_workspace,
+            python,
         }
     }
 }