Do not create a virtual environment when locking (#4147)

Closes https://github.com/astral-sh/uv/issues/4141

crates/uv-distribution/src/workspace.rs

@@ -200,6 +200,11 @@ impl Workspace {
         &self.root
     }
 
+    /// The path to the workspace virtual environment.
+    pub fn venv(&self) -> PathBuf {
+        self.root.join(".venv")
+    }
+
     /// The members of the workspace.
     pub fn packages(&self) -> &BTreeMap<PackageName, WorkspaceMember> {
         &self.packages

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

@@ -14,7 +14,7 @@ use uv_git::GitResolver;
 use uv_normalize::PackageName;
 use uv_requirements::upgrade::{read_lockfile, LockedRequirements};
 use uv_resolver::{ExcludeNewer, FlatIndex, InMemoryIndex, Lock, OptionsBuilder, RequiresPython};
-use uv_toolchain::PythonEnvironment;
+use uv_toolchain::{Interpreter, Toolchain};
 use uv_types::{BuildIsolation, EmptyInstalledPackages, HashStrategy, InFlight};
 use uv_warnings::warn_user;
 
@@ -39,8 +39,14 @@ pub(crate) async fn lock(
     // Find the project requirements.
     let workspace = Workspace::discover(&std::env::current_dir()?, None).await?;
 
-    // Discover or create the virtual environment.
-    let venv = project::init_environment(&workspace, preview, cache, printer)?;
+    // Find an interpreter for the project
+    let interpreter = match project::find_environment(&workspace, cache) {
+        Ok(environment) => environment.into_interpreter(),
+        Err(uv_toolchain::Error::NotFound(_)) => {
+            Toolchain::find_default(PreviewMode::Enabled, cache)?.into_interpreter()
+        }
+        Err(err) => return Err(err.into()),
+    };
 
     // Perform the lock operation.
     let root_project_name = workspace.root_member().and_then(|member| {
@@ -53,7 +59,7 @@ pub(crate) async fn lock(
     match do_lock(
         root_project_name,
         &workspace,
-        &venv,
+        &interpreter,
         &index_locations,
         upgrade,
         exclude_newer,
@@ -81,7 +87,7 @@ pub(crate) async fn lock(
 pub(super) async fn do_lock(
     root_project_name: Option<PackageName>,
     workspace: &Workspace,
-    venv: &PythonEnvironment,
+    interpreter: &Interpreter,
     index_locations: &IndexLocations,
     upgrade: Upgrade,
     exclude_newer: Option<ExcludeNewer>,
@@ -119,7 +125,7 @@ pub(super) async fn do_lock(
         requires_python
     } else {
         let requires_python =
-            RequiresPython::greater_than_equal_version(venv.interpreter().python_minor_version());
+            RequiresPython::greater_than_equal_version(interpreter.python_minor_version());
         if let Some(root_project_name) = root_project_name.as_ref() {
             warn_user!(
                 "No `requires-python` field found in `{root_project_name}`. Defaulting to `{requires_python}`.",
@@ -132,17 +138,16 @@ pub(super) async fn do_lock(
         requires_python
     };
 
-    // Determine the tags, markers, and interpreter to use for resolution.
-    let interpreter = venv.interpreter();
-    let tags = venv.interpreter().tags()?;
-    let markers = venv.interpreter().markers();
+    // Determine the tags and markers to use for resolution.
+    let tags = interpreter.tags()?;
+    let markers = interpreter.markers();
 
     // Initialize the registry client.
     // TODO(zanieb): Support client options e.g. offline, tls, etc.
     let client = RegistryClientBuilder::new(cache.clone())
         .index_urls(index_locations.index_urls())
         .markers(markers)
-        .platform(venv.interpreter().platform())
+        .platform(interpreter.platform())
         .build();
 
     // TODO(charlie): Respect project configuration.

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

@@ -67,6 +67,14 @@ pub(crate) enum ProjectError {
     RequiresPython(#[from] uv_resolver::RequiresPythonError),
 }
 
+/// Find the virtual environment for the current project.
+pub(crate) fn find_environment(
+    workspace: &Workspace,
+    cache: &Cache,
+) -> Result<PythonEnvironment, uv_toolchain::Error> {
+    PythonEnvironment::from_root(workspace.venv(), cache)
+}
+
 /// Initialize a virtual environment for the current project.
 pub(crate) fn init_environment(
     workspace: &Workspace,
@@ -74,11 +82,9 @@ pub(crate) fn init_environment(
     cache: &Cache,
     printer: Printer,
 ) -> Result<PythonEnvironment, ProjectError> {
-    let venv = workspace.root().join(".venv");
-
     // Discover or create the virtual environment.
     // TODO(charlie): If the environment isn't compatible with `--python`, recreate it.
-    match PythonEnvironment::from_root(&venv, cache) {
+    match find_environment(workspace, cache) {
         Ok(venv) => Ok(venv),
         Err(uv_toolchain::Error::NotFound(_)) => {
             // TODO(charlie): Respect `--python`; if unset, respect `Requires-Python`.
@@ -91,6 +97,7 @@ pub(crate) fn init_environment(
                 interpreter.sys_executable().user_display().cyan()
             )?;
 
+            let venv = workspace.venv();
             writeln!(
                 printer.stderr(),
                 "Creating virtualenv at: {}",

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

@@ -73,7 +73,7 @@ pub(crate) async fn run(
         let lock = project::lock::do_lock(
             root_project_name,
             project.workspace(),
-            &venv,
+            venv.interpreter(),
             &index_locations,
             upgrade,
             exclude_newer,