Ignore virtual environments in parent directories when choosing Python version for new projects (#9075)

`uv init` shouldn't have been using `EnvironmentPreference::Any` for
discovery of a Python interpreter, it seems like an oversight that it
was reading from virtual environments. I changed it to
`EnvironmentPreference::OnlySystem` so we'll use the first Python on the
`PATH` instead. However, I think we actually do want to respect a
virtual environment's Python version if it's in the target project
directory already, so I've implemented that as well.

Closes https://github.com/astral-sh/uv/issues/9072
Closes https://github.com/astral-sh/uv/issues/8092

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

@@ -17,8 +17,9 @@ use uv_git::GIT;
 use uv_pep440::Version;
 use uv_pep508::PackageName;
 use uv_python::{
-    EnvironmentPreference, PythonDownloads, PythonInstallation, PythonPreference, PythonRequest,
+    EnvironmentPreference, PythonDownloads, PythonEnvironment, PythonInstallation,
-    PythonVariant, PythonVersionFile, VersionFileDiscoveryOptions, VersionRequest,
+    PythonPreference, PythonRequest, PythonVariant, PythonVersionFile, VersionFileDiscoveryOptions,
+    VersionRequest,
 };
 use uv_resolver::RequiresPython;
 use uv_scripts::{Pep723Script, ScriptTag};
@@ -409,7 +410,7 @@ async fn init_project(
                 } else {
                     let interpreter = PythonInstallation::find_or_download(
                         Some(python_request),
-                        EnvironmentPreference::Any,
+                        EnvironmentPreference::OnlySystem,
                         python_preference,
                         python_downloads,
                         &client_builder,
@@ -431,7 +432,7 @@ async fn init_project(
             python_request => {
                 let interpreter = PythonInstallation::find_or_download(
                     Some(&python_request),
-                    EnvironmentPreference::Any,
+                    EnvironmentPreference::OnlySystem,
                     python_preference,
                     python_downloads,
                     &client_builder,
@@ -457,8 +458,29 @@ async fn init_project(
                 (requires_python, python_request)
             }
         }
+    } else if let Ok(virtualenv) = PythonEnvironment::from_root(path.join(".venv"), cache) {
+        // (2) An existing Python environment in the target directory
+        debug!("Using Python version from existing virtual environment in project");
+        let interpreter = virtualenv.into_interpreter();
+
+        let requires_python =
+            RequiresPython::greater_than_equal_version(&interpreter.python_minor_version());
+
+        // Pin to the minor version.
+        let python_request = if no_pin_python {
+            None
+        } else {
+            Some(PythonRequest::Version(VersionRequest::MajorMinor(
+                interpreter.python_major(),
+                interpreter.python_minor(),
+                PythonVariant::Default,
+            )))
+        };
+
+        (requires_python, python_request)
     } else if let Some(requires_python) = workspace.as_ref().and_then(find_requires_python) {
-        // (2) `requires-python` from the workspace
+        // (3) `requires-python` from the workspace
+        debug!("Using Python version from project workspace");
         let python_request = PythonRequest::Version(VersionRequest::Range(
             requires_python.specifiers().clone(),
             PythonVariant::Default,
@@ -470,7 +492,7 @@ async fn init_project(
         } else {
             let interpreter = PythonInstallation::find_or_download(
                 Some(&python_request),
-                EnvironmentPreference::Any,
+                EnvironmentPreference::OnlySystem,
                 python_preference,
                 python_downloads,
                 &client_builder,
@@ -489,10 +511,10 @@ async fn init_project(
 
         (requires_python, python_request)
     } else {
-        // (3) Default to the system Python
+        // (4) Default to the system Python
         let interpreter = PythonInstallation::find_or_download(
             None,
-            EnvironmentPreference::Any,
+            EnvironmentPreference::OnlySystem,
             python_preference,
             python_downloads,
             &client_builder,

crates/uv/tests/it/init.rs

@@ -2042,6 +2042,103 @@ fn init_requires_python_version_file() -> Result<()> {
     Ok(())
 }
 
+/// Run `uv init`, inferring the Python version from an existing `.venv`
+#[test]
+fn init_existing_environment() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.8", "3.12"]);
+
+    let child = context.temp_dir.child("foo");
+    child.create_dir_all()?;
+
+    // Create a new virtual environment in the directory
+    uv_snapshot!(context.filters(), context.venv().current_dir(&child).arg("--python").arg("3.12"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using CPython 3.12.[X] interpreter at: [PYTHON-3.12]
+    Creating virtual environment at: .venv
+    Activate with: source .venv/[BIN]/activate
+    "###);
+
+    uv_snapshot!(context.filters(), context.init().current_dir(&context.temp_dir).arg(child.as_os_str()), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Initialized project `foo` at `[TEMP_DIR]/foo`
+    "###);
+
+    let pyproject_toml = fs_err::read_to_string(child.join("pyproject.toml"))?;
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "foo"
+        version = "0.1.0"
+        description = "Add your description here"
+        readme = "README.md"
+        requires-python = ">=3.12"
+        dependencies = []
+        "###
+        );
+    });
+
+    Ok(())
+}
+
+/// Run `uv init`, it should ignore a the Python version from a parent `.venv`
+#[test]
+fn init_existing_environment_parent() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.8", "3.12"]);
+
+    // Create a new virtual environment in the parent directory
+    uv_snapshot!(context.filters(), context.venv().current_dir(&context.temp_dir).arg("--python").arg("3.12"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using CPython 3.12.[X] interpreter at: [PYTHON-3.12]
+    Creating virtual environment at: .venv
+    Activate with: source .venv/[BIN]/activate
+    "###);
+
+    let child = context.temp_dir.child("foo");
+
+    uv_snapshot!(context.filters(), context.init().current_dir(&context.temp_dir).arg(child.as_os_str()), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Initialized project `foo` at `[TEMP_DIR]/foo`
+    "###);
+
+    let pyproject_toml = fs_err::read_to_string(child.join("pyproject.toml"))?;
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "foo"
+        version = "0.1.0"
+        description = "Add your description here"
+        readme = "README.md"
+        requires-python = ">=3.8"
+        dependencies = []
+        "###
+        );
+    });
+
+    Ok(())
+}
+
 /// Run `uv init` from within an unmanaged project.
 #[test]
 fn init_unmanaged() -> Result<()> {