Detect python version from python project by default in uv venv (#5592

)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

`uv venv` should support adopting python version specified in
`requires-python` from `pyproject.toml`. This allows customization on
the venv setup when syncing from python project.

Closes #5552.

It also serves as a workaround to close
#5258.

## Test Plan

<!-- How was it tested? -->

1. Run `uv venv` in folder with `pyroject.toml` specifying
`requries-python = "<3.10"`. Python 3.9 is selected for venv.
2. Change to `requries-python = "<3.11"` and run `uv venv` again. Python
3.10 is selected now.
3. Switch to a folder without `pyproject.toml` then run `uv venv`.
Python 3.12 is selected now.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv/src/commands/venv.rs

@@ -20,16 +20,18 @@ use uv_configuration::{
     NoBuild, PreviewMode, SetupPyStrategy,
 };
 use uv_dispatch::BuildDispatch;
-use uv_fs::Simplified;
+use uv_fs::{Simplified, CWD};
 use uv_python::{
     request_from_version_file, EnvironmentPreference, PythonFetch, PythonInstallation,
-    PythonPreference, PythonRequest,
+    PythonPreference, PythonRequest, VersionRequest,
 };
-use uv_resolver::{ExcludeNewer, FlatIndex};
+use uv_resolver::{ExcludeNewer, FlatIndex, RequiresPython};
 use uv_shell::Shell;
 use uv_types::{BuildContext, BuildIsolation, HashStrategy};
 use uv_warnings::warn_user_once;
+use uv_workspace::{DiscoveryOptions, VirtualProject, WorkspaceError};
 
+use crate::commands::project::find_requires_python;
 use crate::commands::reporters::PythonDownloadReporter;
 use crate::commands::{pip, ExitStatus, SharedState};
 use crate::printer::Printer;
@@ -130,23 +132,45 @@ async fn venv_impl(
     printer: Printer,
     relocatable: bool,
 ) -> miette::Result<ExitStatus> {
+    if preview.is_disabled() && relocatable {
+        warn_user_once!("`--relocatable` is experimental and may change without warning");
+    }
+
     let client_builder = BaseClientBuilder::default()
         .connectivity(connectivity)
         .native_tls(native_tls);
 
-    let client_builder_clone = client_builder.clone();
-
     let reporter = PythonDownloadReporter::single(printer);
 
+    // (1) Explicit request from user
     let mut interpreter_request = python_request.map(PythonRequest::parse);
+
+    // (2) Request from `.python-version`
     if preview.is_enabled() && interpreter_request.is_none() {
         interpreter_request =
             request_from_version_file(&std::env::current_dir().into_diagnostic()?)
                 .await
                 .into_diagnostic()?;
     }
-    if preview.is_disabled() && relocatable {
-        warn_user_once!("`--relocatable` is experimental and may change without warning");
+
+    // (3) `Requires-Python` in `pyproject.toml`
+    if preview.is_enabled() && interpreter_request.is_none() {
+        let project = match VirtualProject::discover(&CWD, &DiscoveryOptions::default()).await {
+            Ok(project) => Some(project),
+            Err(WorkspaceError::MissingPyprojectToml) => None,
+            Err(WorkspaceError::NonWorkspace(_)) => None,
+            Err(err) => return Err(err).into_diagnostic(),
+        };
+
+        if let Some(project) = project {
+            interpreter_request = find_requires_python(project.workspace())
+                .into_diagnostic()?
+                .as_ref()
+                .map(RequiresPython::specifiers)
+                .map(|specifiers| {
+                    PythonRequest::Version(VersionRequest::Range(specifiers.clone()))
+                });
+        }
     }
 
     // Locate the Python interpreter to use in the environment
@@ -217,7 +241,7 @@ async fn venv_impl(
         }
 
         // Instantiate a client.
-        let client = RegistryClientBuilder::from(client_builder_clone)
+        let client = RegistryClientBuilder::from(client_builder)
             .cache(cache.clone())
             .index_urls(index_locations.index_urls())
             .index_strategy(index_strategy)

crates/uv/tests/venv.rs

@@ -3,6 +3,7 @@
 use anyhow::Result;
 use assert_cmd::prelude::*;
 use assert_fs::prelude::*;
+use indoc::indoc;
 use uv_python::{PYTHON_VERSIONS_FILENAME, PYTHON_VERSION_FILENAME};
 
 use crate::common::{uv_snapshot, TestContext};
@@ -171,6 +172,184 @@ fn create_venv_reads_request_from_python_versions_file() {
     context.venv.assert(predicates::path::is_dir());
 }
 
+#[test]
+fn create_venv_respects_pyproject_requires_python() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.11", "3.9", "3.10", "3.12"]);
+
+    // Without a Python requirement, we use the first on the PATH
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "#
+    );
+
+    // With `requires-python = "<3.11"`, we prefer the first available version
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! { r#"
+        [project]
+        name = "foo"
+        version = "1.0.0"
+        requires-python = "<3.11"
+        dependencies = []
+        "#
+    })?;
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.9.[X] interpreter at: [PYTHON-3.9]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "###
+    );
+
+    // With `requires-python = "==3.11.*"`, we prefer exact version (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 = "==3.11.*"
+        dependencies = []
+        "#
+    })?;
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "#
+    );
+
+    // With `requires-python = ">=3.11,<3.12"`, we prefer exact version (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 = ">=3.11,<3.12"
+        dependencies = []
+        "#
+    })?;
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "#
+    );
+
+    // With `requires-python = ">=3.10"`, we prefer first compatible version (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 = ">=3.11"
+        dependencies = []
+        "#
+    })?;
+
+    // With `requires-python = ">=3.11"`, we prefer first compatible version (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 = ">=3.11"
+        dependencies = []
+        "#
+    })?;
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "#
+    );
+
+    // With `requires-python = ">3.11"`, we prefer first compatible version (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 = ">3.11"
+        dependencies = []
+        "#
+    })?;
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.11.[X] interpreter at: [PYTHON-3.11]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "#
+    );
+
+    // With `requires-python = ">=3.12"`, we prefer first compatible version (3.12)
+    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 = []
+        "#
+    })?;
+
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--preview"), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.12.[X] interpreter at: [PYTHON-3.12]
+    Creating virtualenv at: .venv
+    Activate with: source .venv/bin/activate
+    "#
+    );
+
+    context.venv.assert(predicates::path::is_dir());
+
+    Ok(())
+}
+
 #[test]
 fn create_venv_explicit_request_takes_priority_over_python_version_file() {
     let context = TestContext::new_with_versions(&["3.11", "3.12"]);